internal/fetch/load.go - pkgsite - Git at Google

 // Copyright 2019 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 fetch provides a way to fetch modules from a proxy.
 package fetch

 import (
 	"bytes"
 	"context"
 	"errors"
 	"fmt"
 	"go/ast"
 	"go/build"
 	"go/parser"
 	"go/token"
 	"io"
 	"io/fs"
 	"io/ioutil"
 	"net/http"
 	"os"
 	"path"
 	"sort"
 	"strings"

 	"go.opencensus.io/trace"
 	"golang.org/x/pkgsite/internal"
 	"golang.org/x/pkgsite/internal/derrors"
 	"golang.org/x/pkgsite/internal/godoc"
 	"golang.org/x/pkgsite/internal/source"
 	"golang.org/x/pkgsite/internal/stdlib"
 )

 // BadPackageError represents an error loading a package
 // because its contents do not make up a valid package.
 //
 // This can happen, for example, if the .go files fail
 // to parse or declare different package names.
 type BadPackageError struct {
 	Err error // Not nil.
 }

 func (bpe *BadPackageError) Error() string { return bpe.Err.Error() }

 // loadPackage loads a Go package by calling loadPackageWithBuildContext, trying
 // several build contexts in turn. It returns a goPackage with documentation
 // information for each build context that results in a valid package, in the
 // same order that the build contexts are listed. If none of them result in a
 // package, then loadPackage returns nil, nil.
 //
 // If a package is fine except that its documentation is too large, loadPackage
 // returns a goPackage whose err field is a non-nil error with godoc.ErrTooLarge in its chain.
 func loadPackage(ctx context.Context, contentDir fs.FS, goFilePaths []string, innerPath string,
 	sourceInfo *source.Info, modInfo *godoc.ModuleInfo) (_ *goPackage, err error) {
 	defer derrors.Wrap(&err, "loadPackage(ctx, zipGoFiles, %q, sourceInfo, modInfo)", innerPath)
 	ctx, span := trace.StartSpan(ctx, "fetch.loadPackage")
 	defer span.End()

 	// Make a map with all the zip file contents.
 	files := make(map[string][]byte)
 	for _, p := range goFilePaths {
 		_, name := path.Split(p)
 		b, err := readFSFile(contentDir, p, MaxFileSize)
 		if err != nil {
 			return nil, err
 		}
 		files[name] = b
 	}

 	modulePath := modInfo.ModulePath
 	importPath := path.Join(modulePath, innerPath)
 	if modulePath == stdlib.ModulePath {
 		importPath = innerPath
 	}
 	v1path := internal.V1Path(importPath, modulePath)

 	var pkg *goPackage
 	// Parse the package for each build context.
 	// The documentation is determined by the set of matching files, so keep
 	// track of those to avoid duplication.
 	docsByFiles := map[string]*internal.Documentation{}
 	for _, bc := range internal.BuildContexts {
 		mfiles, err := matchingFiles(bc.GOOS, bc.GOARCH, files)
 		if err != nil {
 			return nil, err
 		}
 		filesKey := mapKeyForFiles(mfiles)
 		if doc := docsByFiles[filesKey]; doc != nil {
 			// We have seen this set of files before.
 			// loadPackageWithBuildContext will produce the same outputs,
 			// so don't bother calling it. Just copy the doc.
 			doc2 := *doc
 			doc2.GOOS = bc.GOOS
 			doc2.GOARCH = bc.GOARCH
 			doc2.API = nil
 			for _, s := range doc.API {
 				s2 := *s
 				s2.Children = nil
 				s2.GOOS = bc.GOOS
 				s2.GOARCH = bc.GOARCH
 				s2.Children = append(s2.Children, s.Children...)
 				doc2.API = append(doc2.API, &s2)
 			}
 			pkg.docs = append(pkg.docs, &doc2)
 			continue
 		}
 		name, imports, synopsis, source, api, err := loadPackageForBuildContext(ctx,
 			mfiles, innerPath, sourceInfo, modInfo)
 		for _, s := range api {
 			s.GOOS = bc.GOOS
 			s.GOARCH = bc.GOARCH
 		}

 		switch {
 		case errors.Is(err, derrors.NotFound):
 			// No package for this build context.
 			continue
 		case errors.As(err, new(*BadPackageError)):
 			// This build context was bad, but maybe others aren't.
 			continue
 		case errors.Is(err, godoc.ErrTooLarge):
 			// The doc for this build context is too large. To keep things
 			// simple, return a single package with this error that will be used
 			// for all build contexts, and ignore the others.
 			return &goPackage{
 				err:     err,
 				path:    importPath,
 				v1path:  v1path,
 				name:    name,
 				imports: imports,
 				docs: []*internal.Documentation{{
 					GOOS:     internal.All,
 					GOARCH:   internal.All,
 					Synopsis: synopsis,
 					Source:   source,
 					API:      api,
 				}},
 			}, nil
 		case err != nil:
 			// Serious error. Fail.
 			return nil, err
 		default:
 			// No error.
 			if pkg == nil {
 				pkg = &goPackage{
 					path:    importPath,
 					v1path:  v1path,
 					name:    name,
 					imports: imports, // Use the imports from the first successful build context.
 				}
 			}
 			// All the build contexts should use the same package name. Although
 			// it's technically legal for different build tags to result in different
 			// package names, it's not something we support.
 			if name != pkg.name {
 				return nil, &BadPackageError{
 					Err: fmt.Errorf("more than one package name (%q and %q)", pkg.name, name),
 				}
 			}
 			doc := &internal.Documentation{
 				GOOS:     bc.GOOS,
 				GOARCH:   bc.GOARCH,
 				Synopsis: synopsis,
 				Source:   source,
 				API:      api,
 			}
 			docsByFiles[filesKey] = doc
 			pkg.docs = append(pkg.docs, doc)
 		}
 	}
 	// If all the build contexts succeeded and had the same set of files, then
 	// assume that the package doc is valid for all build contexts. Represent
 	// this with a single Documentation whose GOOS and GOARCH are both "all".
 	if len(docsByFiles) == 1 && len(pkg.docs) == len(internal.BuildContexts) {
 		pkg.docs = pkg.docs[:1]
 		pkg.docs[0].GOOS = internal.All
 		pkg.docs[0].GOARCH = internal.All
 		for _, s := range pkg.docs[0].API {
 			s.GOOS = internal.All
 			s.GOARCH = internal.All
 		}
 	}
 	return pkg, nil
 }

 // mapKeyForFiles generates a value that corresponds to the given set of file
 // names and can be used as a map key.
 // It assumes the filenames do not contain spaces.
 func mapKeyForFiles(files map[string][]byte) string {
 	var names []string
 	for n := range files {
 		names = append(names, n)
 	}
 	sort.Strings(names)
 	return strings.Join(names, " ")
 }

 // httpPost allows package fetch tests to stub out playground URL fetches.
 var httpPost = http.Post

 // loadPackageForBuildContext loads a Go package made of .go files in
 // files, which should match some build context.
 // modulePath is stdlib.ModulePath for the Go standard library and the
 // module path for all other modules. innerPath is the path of the Go package
 // directory relative to the module root. The files argument must contain only
 // .go files that have been verified to be of reasonable size and that match
 // the build context.
 //
 // It returns the package name, list of imports, the package synopsis, and the
 // serialized source (AST) for the package.
 //
 // It returns an error with NotFound in its chain if the directory doesn't
 // contain a Go package or all .go files have been excluded by constraints. A
 // *BadPackageError error is returned if the directory contains .go files but do
 // not make up a valid package.
 //
 // If it returns an error with ErrTooLarge in its chain, the other return values
 // are still valid.
 func loadPackageForBuildContext(ctx context.Context, files map[string][]byte, innerPath string, sourceInfo *source.Info, modInfo *godoc.ModuleInfo) (
 	name string, imports []string, synopsis string, source []byte, api []*internal.Symbol, err error) {
 	modulePath := modInfo.ModulePath
 	defer derrors.Wrap(&err, "loadPackageWithBuildContext(files, %q, %q, %+v)", innerPath, modulePath, sourceInfo)

 	packageName, goFiles, fset, err := loadFilesWithBuildContext(innerPath, files)
 	if err != nil {
 		return "", nil, "", nil, nil, err
 	}
 	docPkg := godoc.NewPackage(fset, modInfo.ModulePackages)
 	for _, pf := range goFiles {
 		removeNodes := true
 		// Don't strip the seemingly unexported functions from the builtin package;
 		// they are actually Go builtins like make, new, etc.
 		if modulePath == stdlib.ModulePath && innerPath == "builtin" {
 			removeNodes = false
 		}
 		docPkg.AddFile(pf, removeNodes)
 	}

 	// Encode first, because Render messes with the AST.
 	src, err := docPkg.Encode(ctx)
 	if err != nil {
 		return "", nil, "", nil, nil, err
 	}

 	synopsis, imports, api, err = docPkg.DocInfo(ctx, innerPath, sourceInfo, modInfo)
 	if err != nil {
 		return "", nil, "", nil, nil, err
 	}
 	return packageName, imports, synopsis, src, api, err
 }

 // loadFilesWithBuildContext loads all the given Go files at innerPath. It
 // returns the package name as it occurs in the source, a map of the ASTs of all
 // the Go files, and the token.FileSet used for parsing.
 // If there are no non-test Go files, it returns a NotFound error.
 func loadFilesWithBuildContext(innerPath string, files map[string][]byte) (pkgName string, fileMap map[string]*ast.File, _ *token.FileSet, _ error) {
 	// Parse .go files and add them to the goFiles slice.
 	var (
 		fset            = token.NewFileSet()
 		goFiles         = make(map[string]*ast.File)
 		numNonTestFiles int
 		packageName     string
 		packageNameFile string // Name of file where packageName came from.
 	)
 	for name, b := range files {
 		pf, err := parser.ParseFile(fset, name, b, parser.ParseComments)
 		if err != nil {
 			if pf == nil {
 				return "", nil, nil, fmt.Errorf("internal error: the source couldn't be read: %v", err)
 			}
 			return "", nil, nil, &BadPackageError{Err: err}
 		}
 		// Remember all files, including test files for their examples.
 		goFiles[name] = pf
 		if strings.HasSuffix(name, "_test.go") {
 			continue
 		}
 		// Keep track of the number of non-test files to check that the package name is the same.
 		// and also because a directory with only test files doesn't count as a
 		// Go package.
 		numNonTestFiles++
 		if numNonTestFiles == 1 {
 			packageName = pf.Name.Name
 			packageNameFile = name
 		} else if pf.Name.Name != packageName {
 			return "", nil, nil, &BadPackageError{Err: &build.MultiplePackageError{
 				Dir:      innerPath,
 				Packages: []string{packageName, pf.Name.Name},
 				Files:    []string{packageNameFile, name},
 			}}
 		}
 	}
 	if numNonTestFiles == 0 {
 		// This directory doesn't contain a package, or at least not one
 		// that matches this build context.
 		return "", nil, nil, derrors.NotFound
 	}
 	return packageName, goFiles, fset, nil
 }

 // matchingFiles returns a map from file names to their contents, read from zipGoFiles.
 // It includes only those files that match the build context determined by goos and goarch.
 func matchingFiles(goos, goarch string, allFiles map[string][]byte) (matchedFiles map[string][]byte, err error) {
 	defer derrors.Wrap(&err, "matchingFiles(%q, %q, zipGoFiles)", goos, goarch)

 	// bctx is used to make decisions about which of the .go files are included
 	// by build constraints.
 	bctx := &build.Context{
 		GOOS:        goos,
 		GOARCH:      goarch,
 		CgoEnabled:  true,
 		Compiler:    build.Default.Compiler,
 		ReleaseTags: build.Default.ReleaseTags,

 		JoinPath: path.Join,
 		OpenFile: func(name string) (io.ReadCloser, error) {
 			return ioutil.NopCloser(bytes.NewReader(allFiles[name])), nil
 		},

 		// If left nil, the default implementations of these read from disk,
 		// which we do not want. None of these functions should be used
 		// inside this function; it would be an internal error if they are.
 		// Set them to non-nil values to catch if that happens.
 		SplitPathList: func(string) []string { panic("internal error: unexpected call to SplitPathList") },
 		IsAbsPath:     func(string) bool { panic("internal error: unexpected call to IsAbsPath") },
 		IsDir:         func(string) bool { panic("internal error: unexpected call to IsDir") },
 		HasSubdir:     func(string, string) (string, bool) { panic("internal error: unexpected call to HasSubdir") },
 		ReadDir:       func(string) ([]os.FileInfo, error) { panic("internal error: unexpected call to ReadDir") },
 	}

 	// Copy the input map so we don't modify it.
 	matchedFiles = map[string][]byte{}
 	for n, c := range allFiles {
 		matchedFiles[n] = c
 	}
 	for name := range allFiles {
 		match, err := bctx.MatchFile(".", name) // This will access the file we just added to files map above.
 		if err != nil {
 			return nil, &BadPackageError{Err: fmt.Errorf(`bctx.MatchFile(".", %q): %w`, name, err)}
 		}
 		if !match {
 			delete(matchedFiles, name)
 		}
 	}
 	return matchedFiles, nil
 }

 // readFSFile reads up to limit bytes from path in fsys.
 func readFSFile(fsys fs.FS, path string, limit int64) (_ []byte, err error) {
 	defer derrors.Add(&err, "readFSFile(%q)", path)
 	f, err := fsys.Open(path)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 	return ioutil.ReadAll(io.LimitReader(f, limit))
 }
	// Copyright 2019 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 fetch provides a way to fetch modules from a proxy.
	package fetch

	import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"go/ast"
	"go/build"
	"go/parser"
	"go/token"
	"io"
	"io/fs"
	"io/ioutil"
	"net/http"
	"os"
	"path"
	"sort"
	"strings"

	"go.opencensus.io/trace"
	"golang.org/x/pkgsite/internal"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/godoc"
	"golang.org/x/pkgsite/internal/source"
	"golang.org/x/pkgsite/internal/stdlib"
	)

	// BadPackageError represents an error loading a package
	// because its contents do not make up a valid package.
	//
	// This can happen, for example, if the .go files fail
	// to parse or declare different package names.
	type BadPackageError struct {
	Err error // Not nil.
	}

	func (bpe *BadPackageError) Error() string { return bpe.Err.Error() }

	// loadPackage loads a Go package by calling loadPackageWithBuildContext, trying
	// several build contexts in turn. It returns a goPackage with documentation
	// information for each build context that results in a valid package, in the
	// same order that the build contexts are listed. If none of them result in a
	// package, then loadPackage returns nil, nil.
	//
	// If a package is fine except that its documentation is too large, loadPackage
	// returns a goPackage whose err field is a non-nil error with godoc.ErrTooLarge in its chain.
	func loadPackage(ctx context.Context, contentDir fs.FS, goFilePaths []string, innerPath string,
	sourceInfo source.Info, modInfo godoc.ModuleInfo) (_ *goPackage, err error) {
	defer derrors.Wrap(&err, "loadPackage(ctx, zipGoFiles, %q, sourceInfo, modInfo)", innerPath)
	ctx, span := trace.StartSpan(ctx, "fetch.loadPackage")
	defer span.End()

	// Make a map with all the zip file contents.
	files := make(map[string][]byte)
	for _, p := range goFilePaths {
	_, name := path.Split(p)
	b, err := readFSFile(contentDir, p, MaxFileSize)
	if err != nil {
	return nil, err
	}
	files[name] = b
	}

	modulePath := modInfo.ModulePath
	importPath := path.Join(modulePath, innerPath)
	if modulePath == stdlib.ModulePath {
	importPath = innerPath
	}
	v1path := internal.V1Path(importPath, modulePath)

	var pkg *goPackage
	// Parse the package for each build context.
	// The documentation is determined by the set of matching files, so keep
	// track of those to avoid duplication.
	docsByFiles := map[string]*internal.Documentation{}
	for _, bc := range internal.BuildContexts {
	mfiles, err := matchingFiles(bc.GOOS, bc.GOARCH, files)
	if err != nil {
	return nil, err
	}
	filesKey := mapKeyForFiles(mfiles)
	if doc := docsByFiles[filesKey]; doc != nil {
	// We have seen this set of files before.
	// loadPackageWithBuildContext will produce the same outputs,
	// so don't bother calling it. Just copy the doc.
	doc2 := *doc
	doc2.GOOS = bc.GOOS
	doc2.GOARCH = bc.GOARCH
	doc2.API = nil
	for _, s := range doc.API {
	s2 := *s
	s2.Children = nil
	s2.GOOS = bc.GOOS
	s2.GOARCH = bc.GOARCH
	s2.Children = append(s2.Children, s.Children...)
	doc2.API = append(doc2.API, &s2)
	}
	pkg.docs = append(pkg.docs, &doc2)
	continue
	}
	name, imports, synopsis, source, api, err := loadPackageForBuildContext(ctx,
	mfiles, innerPath, sourceInfo, modInfo)
	for _, s := range api {
	s.GOOS = bc.GOOS
	s.GOARCH = bc.GOARCH
	}

	switch {
	case errors.Is(err, derrors.NotFound):
	// No package for this build context.
	continue
	case errors.As(err, new(*BadPackageError)):
	// This build context was bad, but maybe others aren't.
	continue
	case errors.Is(err, godoc.ErrTooLarge):
	// The doc for this build context is too large. To keep things
	// simple, return a single package with this error that will be used
	// for all build contexts, and ignore the others.
	return &goPackage{
	err: err,
	path: importPath,
	v1path: v1path,
	name: name,
	imports: imports,
	docs: []*internal.Documentation{{
	GOOS: internal.All,
	GOARCH: internal.All,
	Synopsis: synopsis,
	Source: source,
	API: api,
	}},
	}, nil
	case err != nil:
	// Serious error. Fail.
	return nil, err
	default:
	// No error.
	if pkg == nil {
	pkg = &goPackage{
	path: importPath,
	v1path: v1path,
	name: name,
	imports: imports, // Use the imports from the first successful build context.
	}
	}
	// All the build contexts should use the same package name. Although
	// it's technically legal for different build tags to result in different
	// package names, it's not something we support.
	if name != pkg.name {
	return nil, &BadPackageError{
	Err: fmt.Errorf("more than one package name (%q and %q)", pkg.name, name),
	}
	}
	doc := &internal.Documentation{
	GOOS: bc.GOOS,
	GOARCH: bc.GOARCH,
	Synopsis: synopsis,
	Source: source,
	API: api,
	}
	docsByFiles[filesKey] = doc
	pkg.docs = append(pkg.docs, doc)
	}
	}
	// If all the build contexts succeeded and had the same set of files, then
	// assume that the package doc is valid for all build contexts. Represent
	// this with a single Documentation whose GOOS and GOARCH are both "all".
	if len(docsByFiles) == 1 && len(pkg.docs) == len(internal.BuildContexts) {
	pkg.docs = pkg.docs[:1]
	pkg.docs[0].GOOS = internal.All
	pkg.docs[0].GOARCH = internal.All
	for _, s := range pkg.docs[0].API {
	s.GOOS = internal.All
	s.GOARCH = internal.All
	}
	}
	return pkg, nil
	}

	// mapKeyForFiles generates a value that corresponds to the given set of file
	// names and can be used as a map key.
	// It assumes the filenames do not contain spaces.
	func mapKeyForFiles(files map[string][]byte) string {
	var names []string
	for n := range files {
	names = append(names, n)
	}
	sort.Strings(names)
	return strings.Join(names, " ")
	}

	// httpPost allows package fetch tests to stub out playground URL fetches.
	var httpPost = http.Post

	// loadPackageForBuildContext loads a Go package made of .go files in
	// files, which should match some build context.
	// modulePath is stdlib.ModulePath for the Go standard library and the
	// module path for all other modules. innerPath is the path of the Go package
	// directory relative to the module root. The files argument must contain only
	// .go files that have been verified to be of reasonable size and that match
	// the build context.
	//
	// It returns the package name, list of imports, the package synopsis, and the
	// serialized source (AST) for the package.
	//
	// It returns an error with NotFound in its chain if the directory doesn't
	// contain a Go package or all .go files have been excluded by constraints. A
	// *BadPackageError error is returned if the directory contains .go files but do
	// not make up a valid package.
	//
	// If it returns an error with ErrTooLarge in its chain, the other return values
	// are still valid.
	func loadPackageForBuildContext(ctx context.Context, files map[string][]byte, innerPath string, sourceInfo source.Info, modInfo godoc.ModuleInfo) (
	name string, imports []string, synopsis string, source []byte, api []*internal.Symbol, err error) {
	modulePath := modInfo.ModulePath
	defer derrors.Wrap(&err, "loadPackageWithBuildContext(files, %q, %q, %+v)", innerPath, modulePath, sourceInfo)

	packageName, goFiles, fset, err := loadFilesWithBuildContext(innerPath, files)
	if err != nil {
	return "", nil, "", nil, nil, err
	}
	docPkg := godoc.NewPackage(fset, modInfo.ModulePackages)
	for _, pf := range goFiles {
	removeNodes := true
	// Don't strip the seemingly unexported functions from the builtin package;
	// they are actually Go builtins like make, new, etc.
	if modulePath == stdlib.ModulePath && innerPath == "builtin" {
	removeNodes = false
	}
	docPkg.AddFile(pf, removeNodes)
	}

	// Encode first, because Render messes with the AST.
	src, err := docPkg.Encode(ctx)
	if err != nil {
	return "", nil, "", nil, nil, err
	}

	synopsis, imports, api, err = docPkg.DocInfo(ctx, innerPath, sourceInfo, modInfo)
	if err != nil {
	return "", nil, "", nil, nil, err
	}
	return packageName, imports, synopsis, src, api, err
	}

	// loadFilesWithBuildContext loads all the given Go files at innerPath. It
	// returns the package name as it occurs in the source, a map of the ASTs of all
	// the Go files, and the token.FileSet used for parsing.
	// If there are no non-test Go files, it returns a NotFound error.
	func loadFilesWithBuildContext(innerPath string, files map[string][]byte) (pkgName string, fileMap map[string]ast.File, _ token.FileSet, _ error) {
	// Parse .go files and add them to the goFiles slice.
	var (
	fset = token.NewFileSet()
	goFiles = make(map[string]*ast.File)
	numNonTestFiles int
	packageName string
	packageNameFile string // Name of file where packageName came from.
	)
	for name, b := range files {
	pf, err := parser.ParseFile(fset, name, b, parser.ParseComments)
	if err != nil {
	if pf == nil {
	return "", nil, nil, fmt.Errorf("internal error: the source couldn't be read: %v", err)
	}
	return "", nil, nil, &BadPackageError{Err: err}
	}
	// Remember all files, including test files for their examples.
	goFiles[name] = pf
	if strings.HasSuffix(name, "_test.go") {
	continue
	}
	// Keep track of the number of non-test files to check that the package name is the same.
	// and also because a directory with only test files doesn't count as a
	// Go package.
	numNonTestFiles++
	if numNonTestFiles == 1 {
	packageName = pf.Name.Name
	packageNameFile = name
	} else if pf.Name.Name != packageName {
	return "", nil, nil, &BadPackageError{Err: &build.MultiplePackageError{
	Dir: innerPath,
	Packages: []string{packageName, pf.Name.Name},
	Files: []string{packageNameFile, name},
	}}
	}
	}
	if numNonTestFiles == 0 {
	// This directory doesn't contain a package, or at least not one
	// that matches this build context.
	return "", nil, nil, derrors.NotFound
	}
	return packageName, goFiles, fset, nil
	}

	// matchingFiles returns a map from file names to their contents, read from zipGoFiles.
	// It includes only those files that match the build context determined by goos and goarch.
	func matchingFiles(goos, goarch string, allFiles map[string][]byte) (matchedFiles map[string][]byte, err error) {
	defer derrors.Wrap(&err, "matchingFiles(%q, %q, zipGoFiles)", goos, goarch)

	// bctx is used to make decisions about which of the .go files are included
	// by build constraints.
	bctx := &build.Context{
	GOOS: goos,
	GOARCH: goarch,
	CgoEnabled: true,
	Compiler: build.Default.Compiler,
	ReleaseTags: build.Default.ReleaseTags,

	JoinPath: path.Join,
	OpenFile: func(name string) (io.ReadCloser, error) {
	return ioutil.NopCloser(bytes.NewReader(allFiles[name])), nil
	},

	// If left nil, the default implementations of these read from disk,
	// which we do not want. None of these functions should be used
	// inside this function; it would be an internal error if they are.
	// Set them to non-nil values to catch if that happens.
	SplitPathList: func(string) []string { panic("internal error: unexpected call to SplitPathList") },
	IsAbsPath: func(string) bool { panic("internal error: unexpected call to IsAbsPath") },
	IsDir: func(string) bool { panic("internal error: unexpected call to IsDir") },
	HasSubdir: func(string, string) (string, bool) { panic("internal error: unexpected call to HasSubdir") },
	ReadDir: func(string) ([]os.FileInfo, error) { panic("internal error: unexpected call to ReadDir") },
	}

	// Copy the input map so we don't modify it.
	matchedFiles = map[string][]byte{}
	for n, c := range allFiles {
	matchedFiles[n] = c
	}
	for name := range allFiles {
	match, err := bctx.MatchFile(".", name) // This will access the file we just added to files map above.
	if err != nil {
	return nil, &BadPackageError{Err: fmt.Errorf(`bctx.MatchFile(".", %q): %w`, name, err)}
	}
	if !match {
	delete(matchedFiles, name)
	}
	}
	return matchedFiles, nil
	}

	// readFSFile reads up to limit bytes from path in fsys.
	func readFSFile(fsys fs.FS, path string, limit int64) (_ []byte, err error) {
	defer derrors.Add(&err, "readFSFile(%q)", path)
	f, err := fsys.Open(path)
	if err != nil {
	return nil, err
	}
	defer f.Close()
	return ioutil.ReadAll(io.LimitReader(f, limit))
	}