// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package modload

// This file contains the module-mode package loader, as well as some accessory
// functions pertaining to the package import graph.
//
// There are two exported entry points into package loading — LoadPackages and
// ImportFromFiles — both implemented in terms of loadFromRoots, which itself
// manipulates an instance of the loader struct.
//
// Although most of the loading state is maintained in the loader struct,
// one key piece - the build list - is a global, so that it can be modified
// separate from the loading operation, such as during "go get"
// upgrades/downgrades or in "go mod" operations.
// TODO(#40775): It might be nice to make the loader take and return
// a buildList rather than hard-coding use of the global.
//
// Loading is an iterative process. On each iteration, we try to load the
// requested packages and their transitive imports, then try to resolve modules
// for any imported packages that are still missing.
//
// The first step of each iteration identifies a set of “root” packages.
// Normally the root packages are exactly those matching the named pattern
// arguments. However, for the "all" meta-pattern, the final set of packages is
// computed from the package import graph, and therefore cannot be an initial
// input to loading that graph. Instead, the root packages for the "all" pattern
// are those contained in the main module, and allPatternIsRoot parameter to the
// loader instructs it to dynamically expand those roots to the full "all"
// pattern as loading progresses.
//
// The pkgInAll flag on each loadPkg instance tracks whether that
// package is known to match the "all" meta-pattern.
// A package matches the "all" pattern if:
// 	- it is in the main module, or
// 	- it is imported by any test in the main module, or
// 	- it is imported by another package in "all", or
// 	- the main module specifies a go version ≤ 1.15, and the package is imported
// 	  by a *test of* another package in "all".
//
// When graph pruning is in effect, we want to spot-check the graph-pruning
// invariants — which depend on which packages are known to be in "all" — even
// when we are only loading individual packages, so we set the pkgInAll flag
// regardless of the whether the "all" pattern is a root.
// (This is necessary to maintain the “import invariant” described in
// https://golang.org/design/36460-lazy-module-loading.)
//
// Because "go mod vendor" prunes out the tests of vendored packages, the
// behavior of the "all" pattern with -mod=vendor in Go 1.11–1.15 is the same
// as the "all" pattern (regardless of the -mod flag) in 1.16+.
// The loader uses the GoVersion parameter to determine whether the "all"
// pattern should close over tests (as in Go 1.11–1.15) or stop at only those
// packages transitively imported by the packages and tests in the main module
// ("all" in Go 1.16+ and "go mod vendor" in Go 1.11+).
//
// Note that it is possible for a loaded package NOT to be in "all" even when we
// are loading the "all" pattern. For example, packages that are transitive
// dependencies of other roots named on the command line must be loaded, but are
// not in "all". (The mod_notall test illustrates this behavior.)
// Similarly, if the LoadTests flag is set but the "all" pattern does not close
// over test dependencies, then when we load the test of a package that is in
// "all" but outside the main module, the dependencies of that test will not
// necessarily themselves be in "all". (That configuration does not arise in Go
// 1.11–1.15, but it will be possible in Go 1.16+.)
//
// Loading proceeds from the roots, using a parallel work-queue with a limit on
// the amount of active work (to avoid saturating disks, CPU cores, and/or
// network connections). Each package is added to the queue the first time it is
// imported by another package. When we have finished identifying the imports of
// a package, we add the test for that package if it is needed. A test may be
// needed if:
// 	- the package matches a root pattern and tests of the roots were requested, or
// 	- the package is in the main module and the "all" pattern is requested
// 	  (because the "all" pattern includes the dependencies of tests in the main
// 	  module), or
// 	- the package is in "all" and the definition of "all" we are using includes
// 	  dependencies of tests (as is the case in Go ≤1.15).
//
// After all available packages have been loaded, we examine the results to
// identify any requested or imported packages that are still missing, and if
// so, which modules we could add to the module graph in order to make the
// missing packages available. We add those to the module graph and iterate,
// until either all packages resolve successfully or we cannot identify any
// module that would resolve any remaining missing package.
//
// If the main module is “tidy” (that is, if "go mod tidy" is a no-op for it)
// and all requested packages are in "all", then loading completes in a single
// iteration.
// TODO(bcmills): We should also be able to load in a single iteration if the
// requested packages all come from modules that are themselves tidy, regardless
// of whether those packages are in "all". Today, that requires two iterations
// if those packages are not found in existing dependencies of the main module.

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"go/build"
	"io/fs"
	"os"
	"path"
	pathpkg "path"
	"path/filepath"
	"reflect"
	"runtime"
	"sort"
	"strings"
	"sync"
	"sync/atomic"

	"cmd/go/internal/base"
	"cmd/go/internal/cfg"
	"cmd/go/internal/fsys"
	"cmd/go/internal/imports"
	"cmd/go/internal/modfetch"
	"cmd/go/internal/mvs"
	"cmd/go/internal/par"
	"cmd/go/internal/search"
	"cmd/go/internal/str"

	"golang.org/x/mod/module"
	"golang.org/x/mod/semver"
)

// loaded is the most recently-used package loader.
// It holds details about individual packages.
//
// This variable should only be accessed directly in top-level exported
// functions. All other functions that require or produce a *loader should pass
// or return it as an explicit parameter.
var loaded *loader

// PackageOpts control the behavior of the LoadPackages function.
type PackageOpts struct {
	// GoVersion is the Go version to which the go.mod file should be updated
	// after packages have been loaded.
	//
	// An empty GoVersion means to use the Go version already specified in the
	// main module's go.mod file, or the latest Go version if there is no main
	// module.
	GoVersion string

	// Tags are the build tags in effect (as interpreted by the
	// cmd/go/internal/imports package).
	// If nil, treated as equivalent to imports.Tags().
	Tags map[string]bool

	// Tidy, if true, requests that the build list and go.sum file be reduced to
	// the minimial dependencies needed to reproducibly reload the requested
	// packages.
	Tidy bool

	// TidyCompatibleVersion is the oldest Go version that must be able to
	// reproducibly reload the requested packages.
	//
	// If empty, the compatible version is the Go version immediately prior to the
	// 'go' version listed in the go.mod file.
	TidyCompatibleVersion string

	// VendorModulesInGOROOTSrc indicates that if we are within a module in
	// GOROOT/src, packages in the module's vendor directory should be resolved as
	// actual module dependencies (instead of standard-library packages).
	VendorModulesInGOROOTSrc bool

	// ResolveMissingImports indicates that we should attempt to add module
	// dependencies as needed to resolve imports of packages that are not found.
	//
	// For commands that support the -mod flag, resolving imports may still fail
	// if the flag is set to "readonly" (the default) or "vendor".
	ResolveMissingImports bool

	// AssumeRootsImported indicates that the transitive dependencies of the root
	// packages should be treated as if those roots will be imported by the main
	// module.
	AssumeRootsImported bool

	// AllowPackage, if non-nil, is called after identifying the module providing
	// each package. If AllowPackage returns a non-nil error, that error is set
	// for the package, and the imports and test of that package will not be
	// loaded.
	//
	// AllowPackage may be invoked concurrently by multiple goroutines,
	// and may be invoked multiple times for a given package path.
	AllowPackage func(ctx context.Context, path string, mod module.Version) error

	// LoadTests loads the test dependencies of each package matching a requested
	// pattern. If ResolveMissingImports is also true, test dependencies will be
	// resolved if missing.
	LoadTests bool

	// UseVendorAll causes the "all" package pattern to be interpreted as if
	// running "go mod vendor" (or building with "-mod=vendor").
	//
	// This is a no-op for modules that declare 'go 1.16' or higher, for which this
	// is the default (and only) interpretation of the "all" pattern in module mode.
	UseVendorAll bool

	// AllowErrors indicates that LoadPackages should not terminate the process if
	// an error occurs.
	AllowErrors bool

	// SilencePackageErrors indicates that LoadPackages should not print errors
	// that occur while matching or loading packages, and should not terminate the
	// process if such an error occurs.
	//
	// Errors encountered in the module graph will still be reported.
	//
	// The caller may retrieve the silenced package errors using the Lookup
	// function, and matching errors are still populated in the Errs field of the
	// associated search.Match.)
	SilencePackageErrors bool

	// SilenceMissingStdImports indicates that LoadPackages should not print
	// errors or terminate the process if an imported package is missing, and the
	// import path looks like it might be in the standard library (perhaps in a
	// future version).
	SilenceMissingStdImports bool

	// SilenceNoGoErrors indicates that LoadPackages should not print
	// imports.ErrNoGo errors.
	// This allows the caller to invoke LoadPackages (and report other errors)
	// without knowing whether the requested packages exist for the given tags.
	//
	// Note that if a requested package does not exist *at all*, it will fail
	// during module resolution and the error will not be suppressed.
	SilenceNoGoErrors bool

	// SilenceUnmatchedWarnings suppresses the warnings normally emitted for
	// patterns that did not match any packages.
	SilenceUnmatchedWarnings bool
}

// LoadPackages identifies the set of packages matching the given patterns and
// loads the packages in the import graph rooted at that set.
func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (matches []*search.Match, loadedPackages []string) {
	if opts.Tags == nil {
		opts.Tags = imports.Tags()
	}

	patterns = search.CleanPatterns(patterns)
	matches = make([]*search.Match, 0, len(patterns))
	allPatternIsRoot := false
	for _, pattern := range patterns {
		matches = append(matches, search.NewMatch(pattern))
		if pattern == "all" {
			allPatternIsRoot = true
		}
	}

	updateMatches := func(rs *Requirements, ld *loader) {
		for _, m := range matches {
			switch {
			case m.IsLocal():
				// Evaluate list of file system directories on first iteration.
				if m.Dirs == nil {
					matchLocalDirs(ctx, m, rs)
				}

				// Make a copy of the directory list and translate to import paths.
				// Note that whether a directory corresponds to an import path
				// changes as the build list is updated, and a directory can change
				// from not being in the build list to being in it and back as
				// the exact version of a particular module increases during
				// the loader iterations.
				m.Pkgs = m.Pkgs[:0]
				for _, dir := range m.Dirs {
					pkg, err := resolveLocalPackage(ctx, dir, rs)
					if err != nil {
						if !m.IsLiteral() && (err == errPkgIsBuiltin || err == errPkgIsGorootSrc) {
							continue // Don't include "builtin" or GOROOT/src in wildcard patterns.
						}

						// If we're outside of a module, ensure that the failure mode
						// indicates that.
						if !HasModRoot() {
							die()
						}

						if ld != nil {
							m.AddError(err)
						}
						continue
					}
					m.Pkgs = append(m.Pkgs, pkg)
				}

			case m.IsLiteral():
				m.Pkgs = []string{m.Pattern()}

			case strings.Contains(m.Pattern(), "..."):
				m.Errs = m.Errs[:0]
				mg, err := rs.Graph(ctx)
				if err != nil {
					// The module graph is (or may be) incomplete — perhaps we failed to
					// load the requirements of some module. This is an error in matching
					// the patterns to packages, because we may be missing some packages
					// or we may erroneously match packages in the wrong versions of
					// modules. However, for cases like 'go list -e', the error should not
					// necessarily prevent us from loading the packages we could find.
					m.Errs = append(m.Errs, err)
				}
				matchPackages(ctx, m, opts.Tags, includeStd, mg.BuildList())

			case m.Pattern() == "all":
				if ld == nil {
					// The initial roots are the packages in the main module.
					// loadFromRoots will expand that to "all".
					m.Errs = m.Errs[:0]
					matchPackages(ctx, m, opts.Tags, omitStd, MainModules.Versions())
				} else {
					// Starting with the packages in the main module,
					// enumerate the full list of "all".
					m.Pkgs = ld.computePatternAll()
				}

			case m.Pattern() == "std" || m.Pattern() == "cmd":
				if m.Pkgs == nil {
					m.MatchPackages() // Locate the packages within GOROOT/src.
				}

			default:
				panic(fmt.Sprintf("internal error: modload missing case for pattern %s", m.Pattern()))
			}
		}
	}

	initialRS := LoadModFile(ctx)

	ld := loadFromRoots(ctx, loaderParams{
		PackageOpts:  opts,
		requirements: initialRS,

		allPatternIsRoot: allPatternIsRoot,

		listRoots: func(rs *Requirements) (roots []string) {
			updateMatches(rs, nil)
			for _, m := range matches {
				roots = append(roots, m.Pkgs...)
			}
			return roots
		},
	})

	// One last pass to finalize wildcards.
	updateMatches(ld.requirements, ld)

	// List errors in matching patterns (such as directory permission
	// errors for wildcard patterns).
	if !ld.SilencePackageErrors {
		for _, match := range matches {
			for _, err := range match.Errs {
				ld.errorf("%v\n", err)
			}
		}
	}
	base.ExitIfErrors()

	if !opts.SilenceUnmatchedWarnings {
		search.WarnUnmatched(matches)
	}

	if opts.Tidy {
		if cfg.BuildV {
			mg, _ := ld.requirements.Graph(ctx)

			for _, m := range initialRS.rootModules {
				var unused bool
				if ld.requirements.pruning == unpruned {
					// m is unused if it was dropped from the module graph entirely. If it
					// was only demoted from direct to indirect, it may still be in use via
					// a transitive import.
					unused = mg.Selected(m.Path) == "none"
				} else {
					// m is unused if it was dropped from the roots. If it is still present
					// as a transitive dependency, that transitive dependency is not needed
					// by any package or test in the main module.
					_, ok := ld.requirements.rootSelected(m.Path)
					unused = !ok
				}
				if unused {
					fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
				}
			}
		}

		keep := keepSums(ctx, ld, ld.requirements, loadedZipSumsOnly)
		if compatDepth := pruningForGoVersion(ld.TidyCompatibleVersion); compatDepth != ld.requirements.pruning {
			compatRS := newRequirements(compatDepth, ld.requirements.rootModules, ld.requirements.direct)
			ld.checkTidyCompatibility(ctx, compatRS)

			for m := range keepSums(ctx, ld, compatRS, loadedZipSumsOnly) {
				keep[m] = true
			}
		}

		if !ExplicitWriteGoMod {
			modfetch.TrimGoSum(keep)

			// commitRequirements below will also call WriteGoSum, but the "keep" map
			// we have here could be strictly larger: commitRequirements only commits
			// loaded.requirements, but here we may have also loaded (and want to
			// preserve checksums for) additional entities from compatRS, which are
			// only needed for compatibility with ld.TidyCompatibleVersion.
			if err := modfetch.WriteGoSum(keep, mustHaveCompleteRequirements()); err != nil {
				base.Fatalf("go: %v", err)
			}
		}

		// Update the go.mod file's Go version if necessary.
		modFile := MainModules.ModFile(MainModules.mustGetSingleMainModule())
		if ld.GoVersion != "" {
			modFile.AddGoStmt(ld.GoVersion)
		}
	}

	// Success! Update go.mod and go.sum (if needed) and return the results.
	// We'll skip updating if ExplicitWriteGoMod is true (the caller has opted
	// to call WriteGoMod itself) or if ResolveMissingImports is false (the
	// command wants to examine the package graph as-is).
	loaded = ld
	requirements = loaded.requirements

	for _, pkg := range ld.pkgs {
		if !pkg.isTest() {
			loadedPackages = append(loadedPackages, pkg.path)
		}
	}
	sort.Strings(loadedPackages)

	if !ExplicitWriteGoMod && opts.ResolveMissingImports {
		if err := commitRequirements(ctx); err != nil {
			base.Fatalf("go: %v", err)
		}
	}

	return matches, loadedPackages
}

// matchLocalDirs is like m.MatchDirs, but tries to avoid scanning directories
// outside of the standard library and active modules.
func matchLocalDirs(ctx context.Context, m *search.Match, rs *Requirements) {
	if !m.IsLocal() {
		panic(fmt.Sprintf("internal error: resolveLocalDirs on non-local pattern %s", m.Pattern()))
	}

	if i := strings.Index(m.Pattern(), "..."); i >= 0 {
		// The pattern is local, but it is a wildcard. Its packages will
		// only resolve to paths if they are inside of the standard
		// library, the main module, or some dependency of the main
		// module. Verify that before we walk the filesystem: a filesystem
		// walk in a directory like /var or /etc can be very expensive!
		dir := filepath.Dir(filepath.Clean(m.Pattern()[:i+3]))
		absDir := dir
		if !filepath.IsAbs(dir) {
			absDir = filepath.Join(base.Cwd(), dir)
		}

		modRoot := findModuleRoot(absDir)
		found := false
		for _, mod := range MainModules.Versions() {
			if MainModules.ModRoot(mod) == modRoot {
				found = true
				break
			}
		}
		if !found && search.InDir(absDir, cfg.GOROOTsrc) == "" && pathInModuleCache(ctx, absDir, rs) == "" {
			m.Dirs = []string{}
			m.AddError(fmt.Errorf("directory prefix %s outside available modules", base.ShortPath(absDir)))
			return
		}
	}

	m.MatchDirs(modRoots)
}

// resolveLocalPackage resolves a filesystem path to a package path.
func resolveLocalPackage(ctx context.Context, dir string, rs *Requirements) (string, error) {
	var absDir string
	if filepath.IsAbs(dir) {
		absDir = filepath.Clean(dir)
	} else {
		absDir = filepath.Join(base.Cwd(), dir)
	}

	bp, err := cfg.BuildContext.ImportDir(absDir, 0)
	if err != nil && (bp == nil || len(bp.IgnoredGoFiles) == 0) {
		// golang.org/issue/32917: We should resolve a relative path to a
		// package path only if the relative path actually contains the code
		// for that package.
		//
		// If the named directory does not exist or contains no Go files,
		// the package does not exist.
		// Other errors may affect package loading, but not resolution.
		if _, err := fsys.Stat(absDir); err != nil {
			if os.IsNotExist(err) {
				// Canonicalize OS-specific errors to errDirectoryNotFound so that error
				// messages will be easier for users to search for.
				return "", &fs.PathError{Op: "stat", Path: absDir, Err: errDirectoryNotFound}
			}
			return "", err
		}
		if _, noGo := err.(*build.NoGoError); noGo {
			// A directory that does not contain any Go source files — even ignored
			// ones! — is not a Go package, and we can't resolve it to a package
			// path because that path could plausibly be provided by some other
			// module.
			//
			// Any other error indicates that the package “exists” (at least in the
			// sense that it cannot exist in any other module), but has some other
			// problem (such as a syntax error).
			return "", err
		}
	}

	for _, mod := range MainModules.Versions() {
		modRoot := MainModules.ModRoot(mod)
		if modRoot != "" && absDir == modRoot {
			if absDir == cfg.GOROOTsrc {
				return "", errPkgIsGorootSrc
			}
			return MainModules.PathPrefix(mod), nil
		}
	}

	// Note: The checks for @ here are just to avoid misinterpreting
	// the module cache directories (formerly GOPATH/src/mod/foo@v1.5.2/bar).
	// It's not strictly necessary but helpful to keep the checks.
	var pkgNotFoundErr error
	pkgNotFoundLongestPrefix := ""
	for _, mainModule := range MainModules.Versions() {
		modRoot := MainModules.ModRoot(mainModule)
		if modRoot != "" && strings.HasPrefix(absDir, modRoot+string(filepath.Separator)) && !strings.Contains(absDir[len(modRoot):], "@") {
			suffix := filepath.ToSlash(absDir[len(modRoot):])
			if strings.HasPrefix(suffix, "/vendor/") {
				if cfg.BuildMod != "vendor" {
					return "", fmt.Errorf("without -mod=vendor, directory %s has no package path", absDir)
				}

				readVendorList(mainModule)
				pkg := strings.TrimPrefix(suffix, "/vendor/")
				if _, ok := vendorPkgModule[pkg]; !ok {
					return "", fmt.Errorf("directory %s is not a package listed in vendor/modules.txt", absDir)
				}
				return pkg, nil
			}

			mainModulePrefix := MainModules.PathPrefix(mainModule)
			if mainModulePrefix == "" {
				pkg := strings.TrimPrefix(suffix, "/")
				if pkg == "builtin" {
					// "builtin" is a pseudo-package with a real source file.
					// It's not included in "std", so it shouldn't resolve from "."
					// within module "std" either.
					return "", errPkgIsBuiltin
				}
				return pkg, nil
			}

			pkg := mainModulePrefix + suffix
			if _, ok, err := dirInModule(pkg, mainModulePrefix, modRoot, true); err != nil {
				return "", err
			} else if !ok {
				// This main module could contain the directory but doesn't. Other main
				// modules might contain the directory, so wait till we finish the loop
				// to see if another main module contains directory. But if not,
				// return an error.
				if len(mainModulePrefix) > len(pkgNotFoundLongestPrefix) {
					pkgNotFoundLongestPrefix = mainModulePrefix
					pkgNotFoundErr = &PackageNotInModuleError{MainModules: []module.Version{mainModule}, Pattern: pkg}
				}
				continue
			}
			return pkg, nil
		}
	}
	if pkgNotFoundErr != nil {
		return "", pkgNotFoundErr
	}

	if sub := search.InDir(absDir, cfg.GOROOTsrc); sub != "" && sub != "." && !strings.Contains(sub, "@") {
		pkg := filepath.ToSlash(sub)
		if pkg == "builtin" {
			return "", errPkgIsBuiltin
		}
		return pkg, nil
	}

	pkg := pathInModuleCache(ctx, absDir, rs)
	if pkg == "" {
		return "", fmt.Errorf("directory %s outside available modules", base.ShortPath(absDir))
	}
	return pkg, nil
}

var (
	errDirectoryNotFound = errors.New("directory not found")
	errPkgIsGorootSrc    = errors.New("GOROOT/src is not an importable package")
	errPkgIsBuiltin      = errors.New(`"builtin" is a pseudo-package, not an importable package`)
)

// pathInModuleCache returns the import path of the directory dir,
// if dir is in the module cache copy of a module in our build list.
func pathInModuleCache(ctx context.Context, dir string, rs *Requirements) string {
	tryMod := func(m module.Version) (string, bool) {
		var root string
		var err error
		if repl := Replacement(m); repl.Path != "" && repl.Version == "" {
			root = repl.Path
			if !filepath.IsAbs(root) {
				root = filepath.Join(replaceRelativeTo(), root)
			}
		} else if repl.Path != "" {
			root, err = modfetch.DownloadDir(repl)
		} else {
			root, err = modfetch.DownloadDir(m)
		}
		if err != nil {
			return "", false
		}

		sub := search.InDir(dir, root)
		if sub == "" {
			return "", false
		}
		sub = filepath.ToSlash(sub)
		if strings.Contains(sub, "/vendor/") || strings.HasPrefix(sub, "vendor/") || strings.Contains(sub, "@") {
			return "", false
		}

		return path.Join(m.Path, filepath.ToSlash(sub)), true
	}

	if rs.pruning == pruned {
		for _, m := range rs.rootModules {
			if v, _ := rs.rootSelected(m.Path); v != m.Version {
				continue // m is a root, but we have a higher root for the same path.
			}
			if importPath, ok := tryMod(m); ok {
				// checkMultiplePaths ensures that a module can be used for at most one
				// requirement, so this must be it.
				return importPath
			}
		}
	}

	// None of the roots contained dir, or the graph is unpruned (so we don't want
	// to distinguish between roots and transitive dependencies). Either way,
	// check the full graph to see if the directory is a non-root dependency.
	//
	// If the roots are not consistent with the full module graph, the selected
	// versions of root modules may differ from what we already checked above.
	// Re-check those paths too.

	mg, _ := rs.Graph(ctx)
	var importPath string
	for _, m := range mg.BuildList() {
		var found bool
		importPath, found = tryMod(m)
		if found {
			break
		}
	}
	return importPath
}

// ImportFromFiles adds modules to the build list as needed
// to satisfy the imports in the named Go source files.
//
// Errors in missing dependencies are silenced.
//
// TODO(bcmills): Silencing errors seems off. Take a closer look at this and
// figure out what the error-reporting actually ought to be.
func ImportFromFiles(ctx context.Context, gofiles []string) {
	rs := LoadModFile(ctx)

	tags := imports.Tags()
	imports, testImports, err := imports.ScanFiles(gofiles, tags)
	if err != nil {
		base.Fatalf("go: %v", err)
	}

	loaded = loadFromRoots(ctx, loaderParams{
		PackageOpts: PackageOpts{
			Tags:                  tags,
			ResolveMissingImports: true,
			SilencePackageErrors:  true,
		},
		requirements: rs,
		listRoots: func(*Requirements) (roots []string) {
			roots = append(roots, imports...)
			roots = append(roots, testImports...)
			return roots
		},
	})
	requirements = loaded.requirements
}

// DirImportPath returns the effective import path for dir,
// provided it is within a main module, or else returns ".".
func (mms *MainModuleSet) DirImportPath(ctx context.Context, dir string) (path string, m module.Version) {
	if !HasModRoot() {
		return ".", module.Version{}
	}
	LoadModFile(ctx) // Sets targetPrefix.

	if !filepath.IsAbs(dir) {
		dir = filepath.Join(base.Cwd(), dir)
	} else {
		dir = filepath.Clean(dir)
	}

	var longestPrefix string
	var longestPrefixPath string
	var longestPrefixVersion module.Version
	for _, v := range mms.Versions() {
		modRoot := mms.ModRoot(v)
		if dir == modRoot {
			return mms.PathPrefix(v), v
		}
		if strings.HasPrefix(dir, modRoot+string(filepath.Separator)) {
			pathPrefix := MainModules.PathPrefix(v)
			if pathPrefix > longestPrefix {
				longestPrefix = pathPrefix
				longestPrefixVersion = v
				suffix := filepath.ToSlash(dir[len(modRoot):])
				if strings.HasPrefix(suffix, "/vendor/") {
					longestPrefixPath = strings.TrimPrefix(suffix, "/vendor/")
				}
				longestPrefixPath = mms.PathPrefix(v) + suffix
			}
		}
	}
	if len(longestPrefix) > 0 {
		return longestPrefixPath, longestPrefixVersion
	}

	return ".", module.Version{}
}

// ImportMap returns the actual package import path
// for an import path found in source code.
// If the given import path does not appear in the source code
// for the packages that have been loaded, ImportMap returns the empty string.
func ImportMap(path string) string {
	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
	if !ok {
		return ""
	}
	return pkg.path
}

// PackageDir returns the directory containing the source code
// for the package named by the import path.
func PackageDir(path string) string {
	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
	if !ok {
		return ""
	}
	return pkg.dir
}

// PackageModule returns the module providing the package named by the import path.
func PackageModule(path string) module.Version {
	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
	if !ok {
		return module.Version{}
	}
	return pkg.mod
}

// Lookup returns the source directory, import path, and any loading error for
// the package at path as imported from the package in parentDir.
// Lookup requires that one of the Load functions in this package has already
// been called.
func Lookup(parentPath string, parentIsStd bool, path string) (dir, realPath string, err error) {
	if path == "" {
		panic("Lookup called with empty package path")
	}

	if parentIsStd {
		path = loaded.stdVendor(parentPath, path)
	}
	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
	if !ok {
		// The loader should have found all the relevant paths.
		// There are a few exceptions, though:
		//	- during go list without -test, the p.Resolve calls to process p.TestImports and p.XTestImports
		//	  end up here to canonicalize the import paths.
		//	- during any load, non-loaded packages like "unsafe" end up here.
		//	- during any load, build-injected dependencies like "runtime/cgo" end up here.
		//	- because we ignore appengine/* in the module loader,
		//	  the dependencies of any actual appengine/* library end up here.
		dir := findStandardImportPath(path)
		if dir != "" {
			return dir, path, nil
		}
		return "", "", errMissing
	}
	return pkg.dir, pkg.path, pkg.err
}

// A loader manages the process of loading information about
// the required packages for a particular build,
// checking that the packages are available in the module set,
// and updating the module set if needed.
type loader struct {
	loaderParams

	// allClosesOverTests indicates whether the "all" pattern includes
	// dependencies of tests outside the main module (as in Go 1.11–1.15).
	// (Otherwise — as in Go 1.16+ — the "all" pattern includes only the packages
	// transitively *imported by* the packages and tests in the main module.)
	allClosesOverTests bool

	work *par.Queue

	// reset on each iteration
	roots    []*loadPkg
	pkgCache *par.Cache // package path (string) → *loadPkg
	pkgs     []*loadPkg // transitive closure of loaded packages and tests; populated in buildStacks
}

// loaderParams configure the packages loaded by, and the properties reported
// by, a loader instance.
type loaderParams struct {
	PackageOpts
	requirements *Requirements

	allPatternIsRoot bool // Is the "all" pattern an additional root?

	listRoots func(rs *Requirements) []string
}

func (ld *loader) reset() {
	select {
	case <-ld.work.Idle():
	default:
		panic("loader.reset when not idle")
	}

	ld.roots = nil
	ld.pkgCache = new(par.Cache)
	ld.pkgs = nil
}

// errorf reports an error via either os.Stderr or base.Errorf,
// according to whether ld.AllowErrors is set.
func (ld *loader) errorf(format string, args ...interface{}) {
	if ld.AllowErrors {
		fmt.Fprintf(os.Stderr, format, args...)
	} else {
		base.Errorf(format, args...)
	}
}

// A loadPkg records information about a single loaded package.
type loadPkg struct {
	// Populated at construction time:
	path   string // import path
	testOf *loadPkg

	// Populated at construction time and updated by (*loader).applyPkgFlags:
	flags atomicLoadPkgFlags

	// Populated by (*loader).load:
	mod         module.Version // module providing package
	dir         string         // directory containing source code
	err         error          // error loading package
	imports     []*loadPkg     // packages imported by this one
	testImports []string       // test-only imports, saved for use by pkg.test.
	inStd       bool
	altMods     []module.Version // modules that could have contained the package but did not

	// Populated by (*loader).pkgTest:
	testOnce sync.Once
	test     *loadPkg

	// Populated by postprocessing in (*loader).buildStacks:
	stack *loadPkg // package importing this one in minimal import stack for this pkg
}

// loadPkgFlags is a set of flags tracking metadata about a package.
type loadPkgFlags int8

const (
	// pkgInAll indicates that the package is in the "all" package pattern,
	// regardless of whether we are loading the "all" package pattern.
	//
	// When the pkgInAll flag and pkgImportsLoaded flags are both set, the caller
	// who set the last of those flags must propagate the pkgInAll marking to all
	// of the imports of the marked package.
	//
	// A test is marked with pkgInAll if that test would promote the packages it
	// imports to be in "all" (such as when the test is itself within the main
	// module, or when ld.allClosesOverTests is true).
	pkgInAll loadPkgFlags = 1 << iota

	// pkgIsRoot indicates that the package matches one of the root package
	// patterns requested by the caller.
	//
	// If LoadTests is set, then when pkgIsRoot and pkgImportsLoaded are both set,
	// the caller who set the last of those flags must populate a test for the
	// package (in the pkg.test field).
	//
	// If the "all" pattern is included as a root, then non-test packages in "all"
	// are also roots (and must be marked pkgIsRoot).
	pkgIsRoot

	// pkgFromRoot indicates that the package is in the transitive closure of
	// imports starting at the roots. (Note that every package marked as pkgIsRoot
	// is also trivially marked pkgFromRoot.)
	pkgFromRoot

	// pkgImportsLoaded indicates that the imports and testImports fields of a
	// loadPkg have been populated.
	pkgImportsLoaded
)

// has reports whether all of the flags in cond are set in f.
func (f loadPkgFlags) has(cond loadPkgFlags) bool {
	return f&cond == cond
}

// An atomicLoadPkgFlags stores a loadPkgFlags for which individual flags can be
// added atomically.
type atomicLoadPkgFlags struct {
	bits int32
}

// update sets the given flags in af (in addition to any flags already set).
//
// update returns the previous flag state so that the caller may determine which
// flags were newly-set.
func (af *atomicLoadPkgFlags) update(flags loadPkgFlags) (old loadPkgFlags) {
	for {
		old := atomic.LoadInt32(&af.bits)
		new := old | int32(flags)
		if new == old || atomic.CompareAndSwapInt32(&af.bits, old, new) {
			return loadPkgFlags(old)
		}
	}
}

// has reports whether all of the flags in cond are set in af.
func (af *atomicLoadPkgFlags) has(cond loadPkgFlags) bool {
	return loadPkgFlags(atomic.LoadInt32(&af.bits))&cond == cond
}

// isTest reports whether pkg is a test of another package.
func (pkg *loadPkg) isTest() bool {
	return pkg.testOf != nil
}

// fromExternalModule reports whether pkg was loaded from a module other than
// the main module.
func (pkg *loadPkg) fromExternalModule() bool {
	if pkg.mod.Path == "" {
		return false // loaded from the standard library, not a module
	}
	return !MainModules.Contains(pkg.mod.Path)
}

var errMissing = errors.New("cannot find package")

// loadFromRoots attempts to load the build graph needed to process a set of
// root packages and their dependencies.
//
// The set of root packages is returned by the params.listRoots function, and
// expanded to the full set of packages by tracing imports (and possibly tests)
// as needed.
func loadFromRoots(ctx context.Context, params loaderParams) *loader {
	ld := &loader{
		loaderParams: params,
		work:         par.NewQueue(runtime.GOMAXPROCS(0)),
	}

	if ld.GoVersion == "" {
		ld.GoVersion = MainModules.GoVersion()

		if ld.Tidy && semver.Compare("v"+ld.GoVersion, "v"+LatestGoVersion()) > 0 {
			ld.errorf("go: go.mod file indicates go %s, but maximum version supported by tidy is %s\n", ld.GoVersion, LatestGoVersion())
			base.ExitIfErrors()
		}
	}

	if ld.Tidy {
		if ld.TidyCompatibleVersion == "" {
			ld.TidyCompatibleVersion = priorGoVersion(ld.GoVersion)
		} else if semver.Compare("v"+ld.TidyCompatibleVersion, "v"+ld.GoVersion) > 0 {
			// Each version of the Go toolchain knows how to interpret go.mod and
			// go.sum files produced by all previous versions, so a compatibility
			// version higher than the go.mod version adds nothing.
			ld.TidyCompatibleVersion = ld.GoVersion
		}
	}

	if semver.Compare("v"+ld.GoVersion, narrowAllVersionV) < 0 && !ld.UseVendorAll {
		// The module's go version explicitly predates the change in "all" for graph
		// pruning, so continue to use the older interpretation.
		ld.allClosesOverTests = true
	}

	var err error
	ld.requirements, err = convertPruning(ctx, ld.requirements, pruningForGoVersion(ld.GoVersion))
	if err != nil {
		ld.errorf("go: %v\n", err)
	}

	if ld.requirements.pruning == unpruned {
		// If the module graph does not support pruning, we assume that we will need
		// the full module graph in order to load package dependencies.
		//
		// This might not be strictly necessary, but it matches the historical
		// behavior of the 'go' command and keeps the go.mod file more consistent in
		// case of erroneous hand-edits — which are less likely to be detected by
		// spot-checks in modules that do not maintain the expanded go.mod
		// requirements needed for graph pruning.
		var err error
		ld.requirements, _, err = expandGraph(ctx, ld.requirements)
		if err != nil {
			ld.errorf("go: %v\n", err)
		}
	}

	for {
		ld.reset()

		// Load the root packages and their imports.
		// Note: the returned roots can change on each iteration,
		// since the expansion of package patterns depends on the
		// build list we're using.
		rootPkgs := ld.listRoots(ld.requirements)

		if ld.requirements.pruning == pruned && cfg.BuildMod == "mod" {
			// Before we start loading transitive imports of packages, locate all of
			// the root packages and promote their containing modules to root modules
			// dependencies. If their go.mod files are tidy (the common case) and the
			// set of root packages does not change then we can select the correct
			// versions of all transitive imports on the first try and complete
			// loading in a single iteration.
			changedBuildList := ld.preloadRootModules(ctx, rootPkgs)
			if changedBuildList {
				// The build list has changed, so the set of root packages may have also
				// changed. Start over to pick up the changes. (Preloading roots is much
				// cheaper than loading the full import graph, so we would rather pay
				// for an extra iteration of preloading than potentially end up
				// discarding the result of a full iteration of loading.)
				continue
			}
		}

		inRoots := map[*loadPkg]bool{}
		for _, path := range rootPkgs {
			root := ld.pkg(ctx, path, pkgIsRoot)
			if !inRoots[root] {
				ld.roots = append(ld.roots, root)
				inRoots[root] = true
			}
		}

		// ld.pkg adds imported packages to the work queue and calls applyPkgFlags,
		// which adds tests (and test dependencies) as needed.
		//
		// When all of the work in the queue has completed, we'll know that the
		// transitive closure of dependencies has been loaded.
		<-ld.work.Idle()

		ld.buildStacks()

		changed, err := ld.updateRequirements(ctx)
		if err != nil {
			ld.errorf("go: %v\n", err)
			break
		}
		if changed {
			// Don't resolve missing imports until the module graph have stabilized.
			// If the roots are still changing, they may turn out to specify a
			// requirement on the missing package(s), and we would rather use a
			// version specified by a new root than add a new dependency on an
			// unrelated version.
			continue
		}

		if !ld.ResolveMissingImports || (!HasModRoot() && !allowMissingModuleImports) {
			// We've loaded as much as we can without resolving missing imports.
			break
		}

		modAddedBy := ld.resolveMissingImports(ctx)
		if len(modAddedBy) == 0 {
			// The roots are stable, and we've resolved all of the missing packages
			// that we can.
			break
		}

		toAdd := make([]module.Version, 0, len(modAddedBy))
		for m, _ := range modAddedBy {
			toAdd = append(toAdd, m)
		}
		module.Sort(toAdd) // to make errors deterministic

		// We ran updateRequirements before resolving missing imports and it didn't
		// make any changes, so we know that the requirement graph is already
		// consistent with ld.pkgs: we don't need to pass ld.pkgs to updateRoots
		// again. (That would waste time looking for changes that we have already
		// applied.)
		var noPkgs []*loadPkg
		// We also know that we're going to call updateRequirements again next
		// iteration so we don't need to also update it here. (That would waste time
		// computing a "direct" map that we'll have to recompute later anyway.)
		direct := ld.requirements.direct
		rs, err := updateRoots(ctx, direct, ld.requirements, noPkgs, toAdd, ld.AssumeRootsImported)
		if err != nil {
			// If an error was found in a newly added module, report the package
			// import stack instead of the module requirement stack. Packages
			// are more descriptive.
			if err, ok := err.(*mvs.BuildListError); ok {
				if pkg := modAddedBy[err.Module()]; pkg != nil {
					ld.errorf("go: %s: %v\n", pkg.stackText(), err.Err)
					break
				}
			}
			ld.errorf("go: %v\n", err)
			break
		}
		if reflect.DeepEqual(rs.rootModules, ld.requirements.rootModules) {
			// Something is deeply wrong. resolveMissingImports gave us a non-empty
			// set of modules to add to the graph, but adding those modules had no
			// effect — either they were already in the graph, or updateRoots did not
			// add them as requested.
			panic(fmt.Sprintf("internal error: adding %v to module graph had no effect on root requirements (%v)", toAdd, rs.rootModules))
		}
		ld.requirements = rs
	}
	base.ExitIfErrors() // TODO(bcmills): Is this actually needed?

	// Tidy the build list, if applicable, before we report errors.
	// (The process of tidying may remove errors from irrelevant dependencies.)
	if ld.Tidy {
		rs, err := tidyRoots(ctx, ld.requirements, ld.pkgs)
		if err != nil {
			ld.errorf("go: %v\n", err)
		}

		if ld.requirements.pruning == pruned {
			// We continuously add tidy roots to ld.requirements during loading, so at
			// this point the tidy roots should be a subset of the roots of
			// ld.requirements, ensuring that no new dependencies are brought inside
			// the graph-pruning horizon.
			// If that is not the case, there is a bug in the loading loop above.
			for _, m := range rs.rootModules {
				if v, ok := ld.requirements.rootSelected(m.Path); !ok || v != m.Version {
					ld.errorf("go: internal error: a requirement on %v is needed but was not added during package loading\n", m)
					base.ExitIfErrors()
				}
			}
		}
		ld.requirements = rs
	}

	// Report errors, if any.
	for _, pkg := range ld.pkgs {
		if pkg.err == nil {
			continue
		}

		// Add importer information to checksum errors.
		if sumErr := (*ImportMissingSumError)(nil); errors.As(pkg.err, &sumErr) {
			if importer := pkg.stack; importer != nil {
				sumErr.importer = importer.path
				sumErr.importerVersion = importer.mod.Version
				sumErr.importerIsTest = importer.testOf != nil
			}
		}

		if ld.SilencePackageErrors {
			continue
		}
		if stdErr := (*ImportMissingError)(nil); errors.As(pkg.err, &stdErr) &&
			stdErr.isStd && ld.SilenceMissingStdImports {
			continue
		}
		if ld.SilenceNoGoErrors && errors.Is(pkg.err, imports.ErrNoGo) {
			continue
		}

		ld.errorf("%s: %v\n", pkg.stackText(), pkg.err)
	}

	ld.checkMultiplePaths()
	return ld
}

// updateRequirements ensures that ld.requirements is consistent with the
// information gained from ld.pkgs.
//
// In particular:
//
// 	- Modules that provide packages directly imported from the main module are
// 	  marked as direct, and are promoted to explicit roots. If a needed root
// 	  cannot be promoted due to -mod=readonly or -mod=vendor, the importing
// 	  package is marked with an error.
//
// 	- If ld scanned the "all" pattern independent of build constraints, it is
// 	  guaranteed to have seen every direct import. Module dependencies that did
// 	  not provide any directly-imported package are then marked as indirect.
//
// 	- Root dependencies are updated to their selected versions.
//
// The "changed" return value reports whether the update changed the selected
// version of any module that either provided a loaded package or may now
// provide a package that was previously unresolved.
func (ld *loader) updateRequirements(ctx context.Context) (changed bool, err error) {
	rs := ld.requirements

	// direct contains the set of modules believed to provide packages directly
	// imported by the main module.
	var direct map[string]bool

	// If we didn't scan all of the imports from the main module, or didn't use
	// imports.AnyTags, then we didn't necessarily load every package that
	// contributes “direct” imports — so we can't safely mark existing direct
	// dependencies in ld.requirements as indirect-only. Propagate them as direct.
	loadedDirect := ld.allPatternIsRoot && reflect.DeepEqual(ld.Tags, imports.AnyTags())
	if loadedDirect {
		direct = make(map[string]bool)
	} else {
		// TODO(bcmills): It seems like a shame to allocate and copy a map here when
		// it will only rarely actually vary from rs.direct. Measure this cost and
		// maybe avoid the copy.
		direct = make(map[string]bool, len(rs.direct))
		for mPath := range rs.direct {
			direct[mPath] = true
		}
	}

	for _, pkg := range ld.pkgs {
		if pkg.mod.Version != "" || !MainModules.Contains(pkg.mod.Path) {
			continue
		}
		for _, dep := range pkg.imports {
			if !dep.fromExternalModule() {
				continue
			}

			if pkg.err == nil && cfg.BuildMod != "mod" {
				if v, ok := rs.rootSelected(dep.mod.Path); !ok || v != dep.mod.Version {
					// dep.mod is not an explicit dependency, but needs to be.
					// Because we are not in "mod" mode, we will not be able to update it.
					// Instead, mark the importing package with an error.
					//
					// TODO(#41688): The resulting error message fails to include the file
					// position of the import statement (because that information is not
					// tracked by the module loader). Figure out how to plumb the import
					// position through.
					pkg.err = &DirectImportFromImplicitDependencyError{
						ImporterPath: pkg.path,
						ImportedPath: dep.path,
						Module:       dep.mod,
					}
					// cfg.BuildMod does not allow us to change dep.mod to be a direct
					// dependency, so don't mark it as such.
					continue
				}
			}

			// dep is a package directly imported by a package or test in the main
			// module and loaded from some other module (not the standard library).
			// Mark its module as a direct dependency.
			direct[dep.mod.Path] = true
		}
	}

	var addRoots []module.Version
	if ld.Tidy {
		// When we are tidying a module with a pruned dependency graph, we may need
		// to add roots to preserve the versions of indirect, test-only dependencies
		// that are upgraded above or otherwise missing from the go.mod files of
		// direct dependencies. (For example, the direct dependency might be a very
		// stable codebase that predates modules and thus lacks a go.mod file, or
		// the author of the direct dependency may have forgotten to commit a change
		// to the go.mod file, or may have made an erroneous hand-edit that causes
		// it to be untidy.)
		//
		// Promoting an indirect dependency to a root adds the next layer of its
		// dependencies to the module graph, which may increase the selected
		// versions of other modules from which we have already loaded packages.
		// So after we promote an indirect dependency to a root, we need to reload
		// packages, which means another iteration of loading.
		//
		// As an extra wrinkle, the upgrades due to promoting a root can cause
		// previously-resolved packages to become unresolved. For example, the
		// module providing an unstable package might be upgraded to a version
		// that no longer contains that package. If we then resolve the missing
		// package, we might add yet another root that upgrades away some other
		// dependency. (The tests in mod_tidy_convergence*.txt illustrate some
		// particularly worrisome cases.)
		//
		// To ensure that this process of promoting, adding, and upgrading roots
		// eventually terminates, during iteration we only ever add modules to the
		// root set — we only remove irrelevant roots at the very end of
		// iteration, after we have already added every root that we plan to need
		// in the (eventual) tidy root set.
		//
		// Since we do not remove any roots during iteration, even if they no
		// longer provide any imported packages, the selected versions of the
		// roots can only increase and the set of roots can only expand. The set
		// of extant root paths is finite and the set of versions of each path is
		// finite, so the iteration *must* reach a stable fixed-point.
		tidy, err := tidyRoots(ctx, rs, ld.pkgs)
		if err != nil {
			return false, err
		}
		addRoots = tidy.rootModules
	}

	rs, err = updateRoots(ctx, direct, rs, ld.pkgs, addRoots, ld.AssumeRootsImported)
	if err != nil {
		// We don't actually know what even the root requirements are supposed to be,
		// so we can't proceed with loading. Return the error to the caller
		return false, err
	}

	if rs != ld.requirements && !reflect.DeepEqual(rs.rootModules, ld.requirements.rootModules) {
		// The roots of the module graph have changed in some way (not just the
		// "direct" markings). Check whether the changes affected any of the loaded
		// packages.
		mg, err := rs.Graph(ctx)
		if err != nil {
			return false, err
		}
		for _, pkg := range ld.pkgs {
			if pkg.fromExternalModule() && mg.Selected(pkg.mod.Path) != pkg.mod.Version {
				changed = true
				break
			}
			if pkg.err != nil {
				// Promoting a module to a root may resolve an import that was
				// previously missing (by pulling in a previously-prune dependency that
				// provides it) or ambiguous (by promoting exactly one of the
				// alternatives to a root and ignoring the second-level alternatives) or
				// otherwise errored out (by upgrading from a version that cannot be
				// fetched to one that can be).
				//
				// Instead of enumerating all of the possible errors, we'll just check
				// whether importFromModules returns nil for the package.
				// False-positives are ok: if we have a false-positive here, we'll do an
				// extra iteration of package loading this time, but we'll still
				// converge when the root set stops changing.
				//
				// In some sense, we can think of this as ‘upgraded the module providing
				// pkg.path from "none" to a version higher than "none"’.
				if _, _, _, err = importFromModules(ctx, pkg.path, rs, nil); err == nil {
					changed = true
					break
				}
			}
		}
	}

	ld.requirements = rs
	return changed, nil
}

// resolveMissingImports returns a set of modules that could be added as
// dependencies in order to resolve missing packages from pkgs.
//
// The newly-resolved packages are added to the addedModuleFor map, and
// resolveMissingImports returns a map from each new module version to
// the first missing package that module would resolve.
func (ld *loader) resolveMissingImports(ctx context.Context) (modAddedBy map[module.Version]*loadPkg) {
	type pkgMod struct {
		pkg *loadPkg
		mod *module.Version
	}
	var pkgMods []pkgMod
	for _, pkg := range ld.pkgs {
		if pkg.err == nil {
			continue
		}
		if pkg.isTest() {
			// If we are missing a test, we are also missing its non-test version, and
			// we should only add the missing import once.
			continue
		}
		if !errors.As(pkg.err, new(*ImportMissingError)) {
			// Leave other errors for Import or load.Packages to report.
			continue
		}

		pkg := pkg
		var mod module.Version
		ld.work.Add(func() {
			var err error
			mod, err = queryImport(ctx, pkg.path, ld.requirements)
			if err != nil {
				var ime *ImportMissingError
				if errors.As(err, &ime) {
					for curstack := pkg.stack; curstack != nil; curstack = curstack.stack {
						if MainModules.Contains(curstack.mod.Path) {
							ime.ImportingMainModule = curstack.mod
							break
						}
					}
				}
				// pkg.err was already non-nil, so we can reasonably attribute the error
				// for pkg to either the original error or the one returned by
				// queryImport. The existing error indicates only that we couldn't find
				// the package, whereas the query error also explains why we didn't fix
				// the problem — so we prefer the latter.
				pkg.err = err
			}

			// err is nil, but we intentionally leave pkg.err non-nil and pkg.mod
			// unset: we still haven't satisfied other invariants of a
			// successfully-loaded package, such as scanning and loading the imports
			// of that package. If we succeed in resolving the new dependency graph,
			// the caller can reload pkg and update the error at that point.
			//
			// Even then, the package might not be loaded from the version we've
			// identified here. The module may be upgraded by some other dependency,
			// or by a transitive dependency of mod itself, or — less likely — the
			// package may be rejected by an AllowPackage hook or rendered ambiguous
			// by some other newly-added or newly-upgraded dependency.
		})

		pkgMods = append(pkgMods, pkgMod{pkg: pkg, mod: &mod})
	}
	<-ld.work.Idle()

	modAddedBy = map[module.Version]*loadPkg{}
	for _, pm := range pkgMods {
		pkg, mod := pm.pkg, *pm.mod
		if mod.Path == "" {
			continue
		}

		fmt.Fprintf(os.Stderr, "go: found %s in %s %s\n", pkg.path, mod.Path, mod.Version)
		if modAddedBy[mod] == nil {
			modAddedBy[mod] = pkg
		}
	}

	return modAddedBy
}

// pkg locates the *loadPkg for path, creating and queuing it for loading if
// needed, and updates its state to reflect the given flags.
//
// The imports of the returned *loadPkg will be loaded asynchronously in the
// ld.work queue, and its test (if requested) will also be populated once
// imports have been resolved. When ld.work goes idle, all transitive imports of
// the requested package (and its test, if requested) will have been loaded.
func (ld *loader) pkg(ctx context.Context, path string, flags loadPkgFlags) *loadPkg {
	if flags.has(pkgImportsLoaded) {
		panic("internal error: (*loader).pkg called with pkgImportsLoaded flag set")
	}

	pkg := ld.pkgCache.Do(path, func() interface{} {
		pkg := &loadPkg{
			path: path,
		}
		ld.applyPkgFlags(ctx, pkg, flags)

		ld.work.Add(func() { ld.load(ctx, pkg) })
		return pkg
	}).(*loadPkg)

	ld.applyPkgFlags(ctx, pkg, flags)
	return pkg
}

// applyPkgFlags updates pkg.flags to set the given flags and propagate the
// (transitive) effects of those flags, possibly loading or enqueueing further
// packages as a result.
func (ld *loader) applyPkgFlags(ctx context.Context, pkg *loadPkg, flags loadPkgFlags) {
	if flags == 0 {
		return
	}

	if flags.has(pkgInAll) && ld.allPatternIsRoot && !pkg.isTest() {
		// This package matches a root pattern by virtue of being in "all".
		flags |= pkgIsRoot
	}
	if flags.has(pkgIsRoot) {
		flags |= pkgFromRoot
	}

	old := pkg.flags.update(flags)
	new := old | flags
	if new == old || !new.has(pkgImportsLoaded) {
		// We either didn't change the state of pkg, or we don't know anything about
		// its dependencies yet. Either way, we can't usefully load its test or
		// update its dependencies.
		return
	}

	if !pkg.isTest() {
		// Check whether we should add (or update the flags for) a test for pkg.
		// ld.pkgTest is idempotent and extra invocations are inexpensive,
		// so it's ok if we call it more than is strictly necessary.
		wantTest := false
		switch {
		case ld.allPatternIsRoot && MainModules.Contains(pkg.mod.Path):
			// We are loading the "all" pattern, which includes packages imported by
			// tests in the main module. This package is in the main module, so we
			// need to identify the imports of its test even if LoadTests is not set.
			//
			// (We will filter out the extra tests explicitly in computePatternAll.)
			wantTest = true

		case ld.allPatternIsRoot && ld.allClosesOverTests && new.has(pkgInAll):
			// This variant of the "all" pattern includes imports of tests of every
			// package that is itself in "all", and pkg is in "all", so its test is
			// also in "all" (as above).
			wantTest = true

		case ld.LoadTests && new.has(pkgIsRoot):
			// LoadTest explicitly requests tests of “the root packages”.
			wantTest = true
		}

		if wantTest {
			var testFlags loadPkgFlags
			if MainModules.Contains(pkg.mod.Path) || (ld.allClosesOverTests && new.has(pkgInAll)) {
				// Tests of packages in the main module are in "all", in the sense that
				// they cause the packages they import to also be in "all". So are tests
				// of packages in "all" if "all" closes over test dependencies.
				testFlags |= pkgInAll
			}
			ld.pkgTest(ctx, pkg, testFlags)
		}
	}

	if new.has(pkgInAll) && !old.has(pkgInAll|pkgImportsLoaded) {
		// We have just marked pkg with pkgInAll, or we have just loaded its
		// imports, or both. Now is the time to propagate pkgInAll to the imports.
		for _, dep := range pkg.imports {
			ld.applyPkgFlags(ctx, dep, pkgInAll)
		}
	}

	if new.has(pkgFromRoot) && !old.has(pkgFromRoot|pkgImportsLoaded) {
		for _, dep := range pkg.imports {
			ld.applyPkgFlags(ctx, dep, pkgFromRoot)
		}
	}
}

// preloadRootModules loads the module requirements needed to identify the
// selected version of each module providing a package in rootPkgs,
// adding new root modules to the module graph if needed.
func (ld *loader) preloadRootModules(ctx context.Context, rootPkgs []string) (changedBuildList bool) {
	needc := make(chan map[module.Version]bool, 1)
	needc <- map[module.Version]bool{}
	for _, path := range rootPkgs {
		path := path
		ld.work.Add(func() {
			// First, try to identify the module containing the package using only roots.
			//
			// If the main module is tidy and the package is in "all" — or if we're
			// lucky — we can identify all of its imports without actually loading the
			// full module graph.
			m, _, _, err := importFromModules(ctx, path, ld.requirements, nil)
			if err != nil {
				var missing *ImportMissingError
				if errors.As(err, &missing) && ld.ResolveMissingImports {
					// This package isn't provided by any selected module.
					// If we can find it, it will be a new root dependency.
					m, err = queryImport(ctx, path, ld.requirements)
				}
				if err != nil {
					// We couldn't identify the root module containing this package.
					// Leave it unresolved; we will report it during loading.
					return
				}
			}
			if m.Path == "" {
				// The package is in std or cmd. We don't need to change the root set.
				return
			}

			v, ok := ld.requirements.rootSelected(m.Path)
			if !ok || v != m.Version {
				// We found the requested package in m, but m is not a root, so
				// loadModGraph will not load its requirements. We need to promote the
				// module to a root to ensure that any other packages this package
				// imports are resolved from correct dependency versions.
				//
				// (This is the “argument invariant” from
				// https://golang.org/design/36460-lazy-module-loading.)
				need := <-needc
				need[m] = true
				needc <- need
			}
		})
	}
	<-ld.work.Idle()

	need := <-needc
	if len(need) == 0 {
		return false // No roots to add.
	}

	toAdd := make([]module.Version, 0, len(need))
	for m := range need {
		toAdd = append(toAdd, m)
	}
	module.Sort(toAdd)

	rs, err := updateRoots(ctx, ld.requirements.direct, ld.requirements, nil, toAdd, ld.AssumeRootsImported)
	if err != nil {
		// We are missing some root dependency, and for some reason we can't load
		// enough of the module dependency graph to add the missing root. Package
		// loading is doomed to fail, so fail quickly.
		ld.errorf("go: %v\n", err)
		base.ExitIfErrors()
		return false
	}
	if reflect.DeepEqual(rs.rootModules, ld.requirements.rootModules) {
		// Something is deeply wrong. resolveMissingImports gave us a non-empty
		// set of modules to add to the graph, but adding those modules had no
		// effect — either they were already in the graph, or updateRoots did not
		// add them as requested.
		panic(fmt.Sprintf("internal error: adding %v to module graph had no effect on root requirements (%v)", toAdd, rs.rootModules))
	}

	ld.requirements = rs
	return true
}

// load loads an individual package.
func (ld *loader) load(ctx context.Context, pkg *loadPkg) {
	if strings.Contains(pkg.path, "@") {
		// Leave for error during load.
		return
	}
	if build.IsLocalImport(pkg.path) || filepath.IsAbs(pkg.path) {
		// Leave for error during load.
		// (Module mode does not allow local imports.)
		return
	}

	if search.IsMetaPackage(pkg.path) {
		pkg.err = &invalidImportError{
			importPath: pkg.path,
			err:        fmt.Errorf("%q is not an importable package; see 'go help packages'", pkg.path),
		}
		return
	}

	var mg *ModuleGraph
	if ld.requirements.pruning == unpruned {
		var err error
		mg, err = ld.requirements.Graph(ctx)
		if err != nil {
			// We already checked the error from Graph in loadFromRoots and/or
			// updateRequirements, so we ignored the error on purpose and we should
			// keep trying to push past it.
			//
			// However, because mg may be incomplete (and thus may select inaccurate
			// versions), we shouldn't use it to load packages. Instead, we pass a nil
			// *ModuleGraph, which will cause mg to first try loading from only the
			// main module and root dependencies.
			mg = nil
		}
	}

	pkg.mod, pkg.dir, pkg.altMods, pkg.err = importFromModules(ctx, pkg.path, ld.requirements, mg)
	if pkg.dir == "" {
		return
	}
	if MainModules.Contains(pkg.mod.Path) {
		// Go ahead and mark pkg as in "all". This provides the invariant that a
		// package that is *only* imported by other packages in "all" is always
		// marked as such before loading its imports.
		//
		// We don't actually rely on that invariant at the moment, but it may
		// improve efficiency somewhat and makes the behavior a bit easier to reason
		// about (by reducing churn on the flag bits of dependencies), and costs
		// essentially nothing (these atomic flag ops are essentially free compared
		// to scanning source code for imports).
		ld.applyPkgFlags(ctx, pkg, pkgInAll)
	}
	if ld.AllowPackage != nil {
		if err := ld.AllowPackage(ctx, pkg.path, pkg.mod); err != nil {
			pkg.err = err
		}
	}

	pkg.inStd = (search.IsStandardImportPath(pkg.path) && search.InDir(pkg.dir, cfg.GOROOTsrc) != "")

	var imports, testImports []string

	if cfg.BuildContext.Compiler == "gccgo" && pkg.inStd {
		// We can't scan standard packages for gccgo.
	} else {
		var err error
		imports, testImports, err = scanDir(pkg.dir, ld.Tags)
		if err != nil {
			pkg.err = err
			return
		}
	}

	pkg.imports = make([]*loadPkg, 0, len(imports))
	var importFlags loadPkgFlags
	if pkg.flags.has(pkgInAll) {
		importFlags = pkgInAll
	}
	for _, path := range imports {
		if pkg.inStd {
			// Imports from packages in "std" and "cmd" should resolve using
			// GOROOT/src/vendor even when "std" is not the main module.
			path = ld.stdVendor(pkg.path, path)
		}
		pkg.imports = append(pkg.imports, ld.pkg(ctx, path, importFlags))
	}
	pkg.testImports = testImports

	ld.applyPkgFlags(ctx, pkg, pkgImportsLoaded)
}

// pkgTest locates the test of pkg, creating it if needed, and updates its state
// to reflect the given flags.
//
// pkgTest requires that the imports of pkg have already been loaded (flagged
// with pkgImportsLoaded).
func (ld *loader) pkgTest(ctx context.Context, pkg *loadPkg, testFlags loadPkgFlags) *loadPkg {
	if pkg.isTest() {
		panic("pkgTest called on a test package")
	}

	createdTest := false
	pkg.testOnce.Do(func() {
		pkg.test = &loadPkg{
			path:   pkg.path,
			testOf: pkg,
			mod:    pkg.mod,
			dir:    pkg.dir,
			err:    pkg.err,
			inStd:  pkg.inStd,
		}
		ld.applyPkgFlags(ctx, pkg.test, testFlags)
		createdTest = true
	})

	test := pkg.test
	if createdTest {
		test.imports = make([]*loadPkg, 0, len(pkg.testImports))
		var importFlags loadPkgFlags
		if test.flags.has(pkgInAll) {
			importFlags = pkgInAll
		}
		for _, path := range pkg.testImports {
			if pkg.inStd {
				path = ld.stdVendor(test.path, path)
			}
			test.imports = append(test.imports, ld.pkg(ctx, path, importFlags))
		}
		pkg.testImports = nil
		ld.applyPkgFlags(ctx, test, pkgImportsLoaded)
	} else {
		ld.applyPkgFlags(ctx, test, testFlags)
	}

	return test
}

// stdVendor returns the canonical import path for the package with the given
// path when imported from the standard-library package at parentPath.
func (ld *loader) stdVendor(parentPath, path string) string {
	if search.IsStandardImportPath(path) {
		return path
	}

	if str.HasPathPrefix(parentPath, "cmd") {
		if !ld.VendorModulesInGOROOTSrc || !MainModules.Contains("cmd") {
			vendorPath := pathpkg.Join("cmd", "vendor", path)

			if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
				return vendorPath
			}
		}
	} else if !ld.VendorModulesInGOROOTSrc || !MainModules.Contains("std") || str.HasPathPrefix(parentPath, "vendor") {
		// If we are outside of the 'std' module, resolve imports from within 'std'
		// to the vendor directory.
		//
		// Do the same for importers beginning with the prefix 'vendor/' even if we
		// are *inside* of the 'std' module: the 'vendor/' packages that resolve
		// globally from GOROOT/src/vendor (and are listed as part of 'go list std')
		// are distinct from the real module dependencies, and cannot import
		// internal packages from the real module.
		//
		// (Note that although the 'vendor/' packages match the 'std' *package*
		// pattern, they are not part of the std *module*, and do not affect
		// 'go mod tidy' and similar module commands when working within std.)
		vendorPath := pathpkg.Join("vendor", path)
		if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
			return vendorPath
		}
	}

	// Not vendored: resolve from modules.
	return path
}

// computePatternAll returns the list of packages matching pattern "all",
// starting with a list of the import paths for the packages in the main module.
func (ld *loader) computePatternAll() (all []string) {
	for _, pkg := range ld.pkgs {
		if pkg.flags.has(pkgInAll) && !pkg.isTest() {
			all = append(all, pkg.path)
		}
	}
	sort.Strings(all)
	return all
}

// checkMultiplePaths verifies that a given module path is used as itself
// or as a replacement for another module, but not both at the same time.
//
// (See https://golang.org/issue/26607 and https://golang.org/issue/34650.)
func (ld *loader) checkMultiplePaths() {
	mods := ld.requirements.rootModules
	if cached := ld.requirements.graph.Load(); cached != nil {
		if mg := cached.(cachedGraph).mg; mg != nil {
			mods = mg.BuildList()
		}
	}

	firstPath := map[module.Version]string{}
	for _, mod := range mods {
		src := resolveReplacement(mod)
		if prev, ok := firstPath[src]; !ok {
			firstPath[src] = mod.Path
		} else if prev != mod.Path {
			ld.errorf("go: %s@%s used for two different module paths (%s and %s)\n", src.Path, src.Version, prev, mod.Path)
		}
	}
}

// checkTidyCompatibility emits an error if any package would be loaded from a
// different module under rs than under ld.requirements.
func (ld *loader) checkTidyCompatibility(ctx context.Context, rs *Requirements) {
	suggestUpgrade := false
	suggestEFlag := false
	suggestFixes := func() {
		if ld.AllowErrors {
			// The user is explicitly ignoring these errors, so don't bother them with
			// other options.
			return
		}

		// We print directly to os.Stderr because this information is advice about
		// how to fix errors, not actually an error itself.
		// (The actual errors should have been logged already.)

		fmt.Fprintln(os.Stderr)

		goFlag := ""
		if ld.GoVersion != MainModules.GoVersion() {
			goFlag = " -go=" + ld.GoVersion
		}

		compatFlag := ""
		if ld.TidyCompatibleVersion != priorGoVersion(ld.GoVersion) {
			compatFlag = " -compat=" + ld.TidyCompatibleVersion
		}
		if suggestUpgrade {
			eDesc := ""
			eFlag := ""
			if suggestEFlag {
				eDesc = ", leaving some packages unresolved"
				eFlag = " -e"
			}
			fmt.Fprintf(os.Stderr, "To upgrade to the versions selected by go %s%s:\n\tgo mod tidy%s -go=%s && go mod tidy%s -go=%s%s\n", ld.TidyCompatibleVersion, eDesc, eFlag, ld.TidyCompatibleVersion, eFlag, ld.GoVersion, compatFlag)
		} else if suggestEFlag {
			// If some packages are missing but no package is upgraded, then we
			// shouldn't suggest upgrading to the Go 1.16 versions explicitly — that
			// wouldn't actually fix anything for Go 1.16 users, and *would* break
			// something for Go 1.17 users.
			fmt.Fprintf(os.Stderr, "To proceed despite packages unresolved in go %s:\n\tgo mod tidy -e%s%s\n", ld.TidyCompatibleVersion, goFlag, compatFlag)
		}

		fmt.Fprintf(os.Stderr, "If reproducibility with go %s is not needed:\n\tgo mod tidy%s -compat=%s\n", ld.TidyCompatibleVersion, goFlag, ld.GoVersion)

		// TODO(#46141): Populate the linked wiki page.
		fmt.Fprintf(os.Stderr, "For other options, see:\n\thttps://golang.org/doc/modules/pruning\n")
	}

	mg, err := rs.Graph(ctx)
	if err != nil {
		ld.errorf("go: error loading go %s module graph: %v\n", ld.TidyCompatibleVersion, err)
		suggestFixes()
		return
	}

	// Re-resolve packages in parallel.
	//
	// We re-resolve each package — rather than just checking versions — to ensure
	// that we have fetched module source code (and, importantly, checksums for
	// that source code) for all modules that are necessary to ensure that imports
	// are unambiguous. That also produces clearer diagnostics, since we can say
	// exactly what happened to the package if it became ambiguous or disappeared
	// entirely.
	//
	// We re-resolve the packages in parallel because this process involves disk
	// I/O to check for package sources, and because the process of checking for
	// ambiguous imports may require us to download additional modules that are
	// otherwise pruned out in Go 1.17 — we don't want to block progress on other
	// packages while we wait for a single new download.
	type mismatch struct {
		mod module.Version
		err error
	}
	mismatchMu := make(chan map[*loadPkg]mismatch, 1)
	mismatchMu <- map[*loadPkg]mismatch{}
	for _, pkg := range ld.pkgs {
		if pkg.mod.Path == "" && pkg.err == nil {
			// This package is from the standard library (which does not vary based on
			// the module graph).
			continue
		}

		pkg := pkg
		ld.work.Add(func() {
			mod, _, _, err := importFromModules(ctx, pkg.path, rs, mg)
			if mod != pkg.mod {
				mismatches := <-mismatchMu
				mismatches[pkg] = mismatch{mod: mod, err: err}
				mismatchMu <- mismatches
			}
		})
	}
	<-ld.work.Idle()

	mismatches := <-mismatchMu
	if len(mismatches) == 0 {
		// Since we're running as part of 'go mod tidy', the roots of the module
		// graph should contain only modules that are relevant to some package in
		// the package graph. We checked every package in the package graph and
		// didn't find any mismatches, so that must mean that all of the roots of
		// the module graph are also consistent.
		//
		// If we're wrong, Go 1.16 in -mod=readonly mode will error out with
		// "updates to go.mod needed", which would be very confusing. So instead,
		// we'll double-check that our reasoning above actually holds — if it
		// doesn't, we'll emit an internal error and hopefully the user will report
		// it as a bug.
		for _, m := range ld.requirements.rootModules {
			if v := mg.Selected(m.Path); v != m.Version {
				fmt.Fprintln(os.Stderr)
				base.Fatalf("go: internal error: failed to diagnose selected-version mismatch for module %s: go %s selects %s, but go %s selects %s\n\tPlease report this at https://golang.org/issue.", m.Path, ld.GoVersion, m.Version, ld.TidyCompatibleVersion, v)
			}
		}
		return
	}

	// Iterate over the packages (instead of the mismatches map) to emit errors in
	// deterministic order.
	for _, pkg := range ld.pkgs {
		mismatch, ok := mismatches[pkg]
		if !ok {
			continue
		}

		if pkg.isTest() {
			// We already did (or will) report an error for the package itself,
			// so don't report a duplicate (and more vebose) error for its test.
			if _, ok := mismatches[pkg.testOf]; !ok {
				base.Fatalf("go: internal error: mismatch recorded for test %s, but not its non-test package", pkg.path)
			}
			continue
		}

		switch {
		case mismatch.err != nil:
			// pkg resolved successfully, but errors out using the requirements in rs.
			//
			// This could occur because the import is provided by a single root (and
			// is thus unambiguous in a main module with a pruned module graph) and
			// also one or more transitive dependencies (and is ambiguous with an
			// unpruned graph).
			//
			// It could also occur because some transitive dependency upgrades the
			// module that previously provided the package to a version that no
			// longer does, or to a version for which the module source code (but
			// not the go.mod file in isolation) has a checksum error.
			if missing := (*ImportMissingError)(nil); errors.As(mismatch.err, &missing) {
				selected := module.Version{
					Path:    pkg.mod.Path,
					Version: mg.Selected(pkg.mod.Path),
				}
				ld.errorf("%s loaded from %v,\n\tbut go %s would fail to locate it in %s\n", pkg.stackText(), pkg.mod, ld.TidyCompatibleVersion, selected)
			} else {
				if ambiguous := (*AmbiguousImportError)(nil); errors.As(mismatch.err, &ambiguous) {
					// TODO: Is this check needed?
				}
				ld.errorf("%s loaded from %v,\n\tbut go %s would fail to locate it:\n\t%v\n", pkg.stackText(), pkg.mod, ld.TidyCompatibleVersion, mismatch.err)
			}

			suggestEFlag = true

			// Even if we press ahead with the '-e' flag, the older version will
			// error out in readonly mode if it thinks the go.mod file contains
			// any *explicit* dependency that is not at its selected version,
			// even if that dependency is not relevant to any package being loaded.
			//
			// We check for that condition here. If all of the roots are consistent
			// the '-e' flag suffices, but otherwise we need to suggest an upgrade.
			if !suggestUpgrade {
				for _, m := range ld.requirements.rootModules {
					if v := mg.Selected(m.Path); v != m.Version {
						suggestUpgrade = true
						break
					}
				}
			}

		case pkg.err != nil:
			// pkg had an error in with a pruned module graph (presumably suppressed
			// with the -e flag), but the error went away using an unpruned graph.
			//
			// This is possible, if, say, the import is unresolved in the pruned graph
			// (because the "latest" version of each candidate module either is
			// unavailable or does not contain the package), but is resolved in the
			// unpruned graph due to a newer-than-latest dependency that is normally
			// pruned out.
			//
			// This could also occur if the source code for the module providing the
			// package in the pruned graph has a checksum error, but the unpruned
			// graph upgrades that module to a version with a correct checksum.
			//
			// pkg.err should have already been logged elsewhere — along with a
			// stack trace — so log only the import path and non-error info here.
			suggestUpgrade = true
			ld.errorf("%s failed to load from any module,\n\tbut go %s would load it from %v\n", pkg.path, ld.TidyCompatibleVersion, mismatch.mod)

		case pkg.mod != mismatch.mod:
			// The package is loaded successfully by both Go versions, but from a
			// different module in each. This could lead to subtle (and perhaps even
			// unnoticed!) variations in behavior between builds with different
			// toolchains.
			suggestUpgrade = true
			ld.errorf("%s loaded from %v,\n\tbut go %s would select %v\n", pkg.stackText(), pkg.mod, ld.TidyCompatibleVersion, mismatch.mod.Version)

		default:
			base.Fatalf("go: internal error: mismatch recorded for package %s, but no differences found", pkg.path)
		}
	}

	suggestFixes()
	base.ExitIfErrors()
}

// scanDir is like imports.ScanDir but elides known magic imports from the list,
// so that we do not go looking for packages that don't really exist.
//
// The standard magic import is "C", for cgo.
//
// The only other known magic imports are appengine and appengine/*.
// These are so old that they predate "go get" and did not use URL-like paths.
// Most code today now uses google.golang.org/appengine instead,
// but not all code has been so updated. When we mostly ignore build tags
// during "go vendor", we look into "// +build appengine" files and
// may see these legacy imports. We drop them so that the module
// search does not look for modules to try to satisfy them.
func scanDir(dir string, tags map[string]bool) (imports_, testImports []string, err error) {
	imports_, testImports, err = imports.ScanDir(dir, tags)

	filter := func(x []string) []string {
		w := 0
		for _, pkg := range x {
			if pkg != "C" && pkg != "appengine" && !strings.HasPrefix(pkg, "appengine/") &&
				pkg != "appengine_internal" && !strings.HasPrefix(pkg, "appengine_internal/") {
				x[w] = pkg
				w++
			}
		}
		return x[:w]
	}

	return filter(imports_), filter(testImports), err
}

// buildStacks computes minimal import stacks for each package,
// for use in error messages. When it completes, packages that
// are part of the original root set have pkg.stack == nil,
// and other packages have pkg.stack pointing at the next
// package up the import stack in their minimal chain.
// As a side effect, buildStacks also constructs ld.pkgs,
// the list of all packages loaded.
func (ld *loader) buildStacks() {
	if len(ld.pkgs) > 0 {
		panic("buildStacks")
	}
	for _, pkg := range ld.roots {
		pkg.stack = pkg // sentinel to avoid processing in next loop
		ld.pkgs = append(ld.pkgs, pkg)
	}
	for i := 0; i < len(ld.pkgs); i++ { // not range: appending to ld.pkgs in loop
		pkg := ld.pkgs[i]
		for _, next := range pkg.imports {
			if next.stack == nil {
				next.stack = pkg
				ld.pkgs = append(ld.pkgs, next)
			}
		}
		if next := pkg.test; next != nil && next.stack == nil {
			next.stack = pkg
			ld.pkgs = append(ld.pkgs, next)
		}
	}
	for _, pkg := range ld.roots {
		pkg.stack = nil
	}
}

// stackText builds the import stack text to use when
// reporting an error in pkg. It has the general form
//
//	root imports
//		other imports
//		other2 tested by
//		other2.test imports
//		pkg
//
func (pkg *loadPkg) stackText() string {
	var stack []*loadPkg
	for p := pkg; p != nil; p = p.stack {
		stack = append(stack, p)
	}

	var buf bytes.Buffer
	for i := len(stack) - 1; i >= 0; i-- {
		p := stack[i]
		fmt.Fprint(&buf, p.path)
		if p.testOf != nil {
			fmt.Fprint(&buf, ".test")
		}
		if i > 0 {
			if stack[i-1].testOf == p {
				fmt.Fprint(&buf, " tested by\n\t")
			} else {
				fmt.Fprint(&buf, " imports\n\t")
			}
		}
	}
	return buf.String()
}

// why returns the text to use in "go mod why" output about the given package.
// It is less ornate than the stackText but contains the same information.
func (pkg *loadPkg) why() string {
	var buf strings.Builder
	var stack []*loadPkg
	for p := pkg; p != nil; p = p.stack {
		stack = append(stack, p)
	}

	for i := len(stack) - 1; i >= 0; i-- {
		p := stack[i]
		if p.testOf != nil {
			fmt.Fprintf(&buf, "%s.test\n", p.testOf.path)
		} else {
			fmt.Fprintf(&buf, "%s\n", p.path)
		}
	}
	return buf.String()
}

// Why returns the "go mod why" output stanza for the given package,
// without the leading # comment.
// The package graph must have been loaded already, usually by LoadPackages.
// If there is no reason for the package to be in the current build,
// Why returns an empty string.
func Why(path string) string {
	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
	if !ok {
		return ""
	}
	return pkg.why()
}

// WhyDepth returns the number of steps in the Why listing.
// If there is no reason for the package to be in the current build,
// WhyDepth returns 0.
func WhyDepth(path string) int {
	n := 0
	pkg, _ := loaded.pkgCache.Get(path).(*loadPkg)
	for p := pkg; p != nil; p = p.stack {
		n++
	}
	return n
}