// 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 modfetch

import (
	"archive/zip"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path"
	"strings"
	"time"

	"cmd/go/internal/modfetch/codehost"
	"cmd/go/internal/modfile"
	"cmd/go/internal/module"
	"cmd/go/internal/semver"
)

// A codeRepo implements modfetch.Repo using an underlying codehost.Repo.
type codeRepo struct {
	modPath string

	// code is the repository containing this module.
	code codehost.Repo
	// codeRoot is the import path at the root of code.
	codeRoot string
	// codeDir is the directory (relative to root) at which we expect to find the module.
	// If pathMajor is non-empty and codeRoot is not the full modPath,
	// then we look in both codeDir and codeDir/pathMajor[1:].
	codeDir string

	// pathMajor is the suffix of modPath that indicates its major version,
	// or the empty string if modPath is at major version 0 or 1.
	//
	// pathMajor is typically of the form "/vN", but possibly ".vN", or
	// ".vN-unstable" for modules resolved using gopkg.in.
	pathMajor string
	// pathPrefix is the prefix of modPath that excludes pathMajor.
	// It is used only for logging.
	pathPrefix string

	// pseudoMajor is the major version prefix to require when generating
	// pseudo-versions for this module, derived from the module path. pseudoMajor
	// is empty if the module path does not include a version suffix (that is,
	// accepts either v0 or v1).
	pseudoMajor string
}

// newCodeRepo returns a Repo that reads the source code for the module with the
// given path, from the repo stored in code, with the root of the repo
// containing the path given by codeRoot.
func newCodeRepo(code codehost.Repo, codeRoot, path string) (Repo, error) {
	if !hasPathPrefix(path, codeRoot) {
		return nil, fmt.Errorf("mismatched repo: found %s for %s", codeRoot, path)
	}
	pathPrefix, pathMajor, ok := module.SplitPathVersion(path)
	if !ok {
		return nil, fmt.Errorf("invalid module path %q", path)
	}
	if codeRoot == path {
		pathPrefix = path
	}
	pseudoMajor := module.PathMajorPrefix(pathMajor)

	// Compute codeDir = bar, the subdirectory within the repo
	// corresponding to the module root.
	//
	// At this point we might have:
	//	path = github.com/rsc/foo/bar/v2
	//	codeRoot = github.com/rsc/foo
	//	pathPrefix = github.com/rsc/foo/bar
	//	pathMajor = /v2
	//	pseudoMajor = v2
	//
	// which gives
	//	codeDir = bar
	//
	// We know that pathPrefix is a prefix of path, and codeRoot is a prefix of
	// path, but codeRoot may or may not be a prefix of pathPrefix, because
	// codeRoot may be the entire path (in which case codeDir should be empty).
	// That occurs in two situations.
	//
	// One is when a go-import meta tag resolves the complete module path,
	// including the pathMajor suffix:
	//	path = nanomsg.org/go/mangos/v2
	//	codeRoot = nanomsg.org/go/mangos/v2
	//	pathPrefix = nanomsg.org/go/mangos
	//	pathMajor = /v2
	//	pseudoMajor = v2
	//
	// The other is similar: for gopkg.in only, the major version is encoded
	// with a dot rather than a slash, and thus can't be in a subdirectory.
	//	path = gopkg.in/yaml.v2
	//	codeRoot = gopkg.in/yaml.v2
	//	pathPrefix = gopkg.in/yaml
	//	pathMajor = .v2
	//	pseudoMajor = v2
	//
	codeDir := ""
	if codeRoot != path {
		if !hasPathPrefix(pathPrefix, codeRoot) {
			return nil, fmt.Errorf("repository rooted at %s cannot contain module %s", codeRoot, path)
		}
		codeDir = strings.Trim(pathPrefix[len(codeRoot):], "/")
	}

	r := &codeRepo{
		modPath:     path,
		code:        code,
		codeRoot:    codeRoot,
		codeDir:     codeDir,
		pathPrefix:  pathPrefix,
		pathMajor:   pathMajor,
		pseudoMajor: pseudoMajor,
	}

	return r, nil
}

func (r *codeRepo) ModulePath() string {
	return r.modPath
}

func (r *codeRepo) Versions(prefix string) ([]string, error) {
	// Special case: gopkg.in/macaroon-bakery.v2-unstable
	// does not use the v2 tags (those are for macaroon-bakery.v2).
	// It has no possible tags at all.
	if strings.HasPrefix(r.modPath, "gopkg.in/") && strings.HasSuffix(r.modPath, "-unstable") {
		return nil, nil
	}

	p := prefix
	if r.codeDir != "" {
		p = r.codeDir + "/" + p
	}
	tags, err := r.code.Tags(p)
	if err != nil {
		return nil, &module.ModuleError{
			Path: r.modPath,
			Err:  err,
		}
	}

	list := []string{}
	var incompatible []string
	for _, tag := range tags {
		if !strings.HasPrefix(tag, p) {
			continue
		}
		v := tag
		if r.codeDir != "" {
			v = v[len(r.codeDir)+1:]
		}
		if v == "" || v != module.CanonicalVersion(v) || IsPseudoVersion(v) {
			continue
		}
		if err := module.MatchPathMajor(v, r.pathMajor); err != nil {
			if r.codeDir == "" && r.pathMajor == "" && semver.Major(v) > "v1" {
				incompatible = append(incompatible, v)
			}
			continue
		}
		list = append(list, v)
	}

	if len(incompatible) > 0 {
		// Check for later versions that were created not following semantic import versioning,
		// as indicated by the absence of a go.mod file. Those versions can be addressed
		// by referring to them with a +incompatible suffix, as in v17.0.0+incompatible.
		files, err := r.code.ReadFileRevs(incompatible, "go.mod", codehost.MaxGoMod)
		if err != nil {
			return nil, &module.ModuleError{
				Path: r.modPath,
				Err:  err,
			}
		}
		for _, rev := range incompatible {
			f := files[rev]
			if os.IsNotExist(f.Err) {
				list = append(list, rev+"+incompatible")
			}
		}
	}

	SortVersions(list)
	return list, nil
}

func (r *codeRepo) Stat(rev string) (*RevInfo, error) {
	if rev == "latest" {
		return r.Latest()
	}
	codeRev := r.revToRev(rev)
	info, err := r.code.Stat(codeRev)
	if err != nil {
		return nil, &module.ModuleError{
			Path: r.modPath,
			Err: &module.InvalidVersionError{
				Version: rev,
				Err:     err,
			},
		}
	}
	return r.convert(info, rev)
}

func (r *codeRepo) Latest() (*RevInfo, error) {
	info, err := r.code.Latest()
	if err != nil {
		return nil, err
	}
	return r.convert(info, "")
}

// convert converts a version as reported by the code host to a version as
// interpreted by the module system.
//
// If statVers is a valid module version, it is used for the Version field.
// Otherwise, the Version is derived from the passed-in info and recent tags.
func (r *codeRepo) convert(info *codehost.RevInfo, statVers string) (*RevInfo, error) {
	info2 := &RevInfo{
		Name:  info.Name,
		Short: info.Short,
		Time:  info.Time,
	}

	// If this is a plain tag (no dir/ prefix)
	// and the module path is unversioned,
	// and if the underlying file tree has no go.mod,
	// then allow using the tag with a +incompatible suffix.
	var canUseIncompatible func() bool
	canUseIncompatible = func() bool {
		var ok bool
		if r.codeDir == "" && r.pathMajor == "" {
			_, errGoMod := r.code.ReadFile(info.Name, "go.mod", codehost.MaxGoMod)
			if errGoMod != nil {
				ok = true
			}
		}
		canUseIncompatible = func() bool { return ok }
		return ok
	}

	invalidf := func(format string, args ...interface{}) error {
		return &module.ModuleError{
			Path: r.modPath,
			Err: &module.InvalidVersionError{
				Version: info2.Version,
				Err:     fmt.Errorf(format, args...),
			},
		}
	}

	// checkGoMod verifies that the go.mod file for the module exists or does not
	// exist as required by info2.Version and the module path represented by r.
	checkGoMod := func() (*RevInfo, error) {
		// If r.codeDir is non-empty, then the go.mod file must exist: the module
		// author — not the module consumer, — gets to decide how to carve up the repo
		// into modules.
		//
		// Conversely, if the go.mod file exists, the module author — not the module
		// consumer — gets to determine the module's path
		//
		// r.findDir verifies both of these conditions. Execute it now so that
		// r.Stat will correctly return a notExistError if the go.mod location or
		// declared module path doesn't match.
		_, _, _, err := r.findDir(info2.Version)
		if err != nil {
			// TODO: It would be nice to return an error like "not a module".
			// Right now we return "missing go.mod", which is a little confusing.
			return nil, &module.ModuleError{
				Path: r.modPath,
				Err: &module.InvalidVersionError{
					Version: info2.Version,
					Err:     notExistError(err.Error()),
				},
			}
		}

		// If the version is +incompatible, then the go.mod file must not exist:
		// +incompatible is not an ongoing opt-out from semantic import versioning.
		if strings.HasSuffix(info2.Version, "+incompatible") {
			if !canUseIncompatible() {
				if r.pathMajor != "" {
					return nil, invalidf("+incompatible suffix not allowed: module path includes a major version suffix, so major version must match")
				} else {
					return nil, invalidf("+incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required")
				}
			}

			if err := module.MatchPathMajor(strings.TrimSuffix(info2.Version, "+incompatible"), r.pathMajor); err == nil {
				return nil, invalidf("+incompatible suffix not allowed: major version %s is compatible", semver.Major(info2.Version))
			}
		}

		return info2, nil
	}

	// Determine version.
	//
	// If statVers is canonical, then the original call was repo.Stat(statVers).
	// Since the version is canonical, we must not resolve it to anything but
	// itself, possibly with a '+incompatible' annotation: we do not need to do
	// the work required to look for an arbitrary pseudo-version.
	if statVers != "" && statVers == module.CanonicalVersion(statVers) {
		info2.Version = statVers

		if IsPseudoVersion(info2.Version) {
			if err := r.validatePseudoVersion(info, info2.Version); err != nil {
				return nil, err
			}
			return checkGoMod()
		}

		if err := module.MatchPathMajor(info2.Version, r.pathMajor); err != nil {
			if canUseIncompatible() {
				info2.Version += "+incompatible"
				return checkGoMod()
			} else {
				if vErr, ok := err.(*module.InvalidVersionError); ok {
					// We're going to describe why the version is invalid in more detail,
					// so strip out the existing “invalid version” wrapper.
					err = vErr.Err
				}
				return nil, invalidf("module contains a go.mod file, so major version must be compatible: %v", err)
			}
		}

		return checkGoMod()
	}

	// statVers is empty or non-canonical, so we need to resolve it to a canonical
	// version or pseudo-version.

	// Derive or verify a version from a code repo tag.
	// Tag must have a prefix matching codeDir.
	tagPrefix := ""
	if r.codeDir != "" {
		tagPrefix = r.codeDir + "/"
	}

	// tagToVersion returns the version obtained by trimming tagPrefix from tag.
	// If the tag is invalid or a pseudo-version, tagToVersion returns an empty
	// version.
	tagToVersion := func(tag string) (v string, tagIsCanonical bool) {
		if !strings.HasPrefix(tag, tagPrefix) {
			return "", false
		}
		trimmed := tag[len(tagPrefix):]
		// Tags that look like pseudo-versions would be confusing. Ignore them.
		if IsPseudoVersion(tag) {
			return "", false
		}

		v = semver.Canonical(trimmed) // Not module.Canonical: we don't want to pick up an explicit "+incompatible" suffix from the tag.
		if v == "" || !strings.HasPrefix(trimmed, v) {
			return "", false // Invalid or incomplete version (just vX or vX.Y).
		}
		if v == trimmed {
			tagIsCanonical = true
		}

		if err := module.MatchPathMajor(v, r.pathMajor); err != nil {
			if canUseIncompatible() {
				return v + "+incompatible", tagIsCanonical
			}
			return "", false
		}

		return v, tagIsCanonical
	}

	// If the VCS gave us a valid version, use that.
	if v, tagIsCanonical := tagToVersion(info.Version); tagIsCanonical {
		info2.Version = v
		return checkGoMod()
	}

	// Look through the tags on the revision for either a usable canonical version
	// or an appropriate base for a pseudo-version.
	var pseudoBase string
	for _, pathTag := range info.Tags {
		v, tagIsCanonical := tagToVersion(pathTag)
		if tagIsCanonical {
			if statVers != "" && semver.Compare(v, statVers) == 0 {
				// The user requested a non-canonical version, but the tag for the
				// canonical equivalent refers to the same revision. Use it.
				info2.Version = v
				return checkGoMod()
			} else {
				// Save the highest canonical tag for the revision. If we don't find a
				// better match, we'll use it as the canonical version.
				//
				// NOTE: Do not replace this with semver.Max. Despite the name,
				// semver.Max *also* canonicalizes its arguments, which uses
				// semver.Canonical instead of module.CanonicalVersion and thereby
				// strips our "+incompatible" suffix.
				if semver.Compare(info2.Version, v) < 0 {
					info2.Version = v
				}
			}
		} else if v != "" && semver.Compare(v, statVers) == 0 {
			// The user explicitly requested something equivalent to this tag. We
			// can't use the version from the tag directly: since the tag is not
			// canonical, it could be ambiguous. For example, tags v0.0.1+a and
			// v0.0.1+b might both exist and refer to different revisions.
			//
			// The tag is otherwise valid for the module, so we can at least use it as
			// the base of an unambiguous pseudo-version.
			//
			// If multiple tags match, tagToVersion will canonicalize them to the same
			// base version.
			pseudoBase = v
		}
	}

	// If we found any canonical tag for the revision, return it.
	// Even if we found a good pseudo-version base, a canonical version is better.
	if info2.Version != "" {
		return checkGoMod()
	}

	if pseudoBase == "" {
		var tag string
		if r.pseudoMajor != "" || canUseIncompatible() {
			tag, _ = r.code.RecentTag(info.Name, tagPrefix, r.pseudoMajor)
		} else {
			// Allow either v1 or v0, but not incompatible higher versions.
			tag, _ = r.code.RecentTag(info.Name, tagPrefix, "v1")
			if tag == "" {
				tag, _ = r.code.RecentTag(info.Name, tagPrefix, "v0")
			}
		}
		pseudoBase, _ = tagToVersion(tag) // empty if the tag is invalid
	}

	info2.Version = PseudoVersion(r.pseudoMajor, pseudoBase, info.Time, info.Short)
	return checkGoMod()
}

// validatePseudoVersion checks that version has a major version compatible with
// r.modPath and encodes a base version and commit metadata that agrees with
// info.
//
// Note that verifying a nontrivial base version in particular may be somewhat
// expensive: in order to do so, r.code.DescendsFrom will need to fetch at least
// enough of the commit history to find a path between version and its base.
// Fortunately, many pseudo-versions — such as those for untagged repositories —
// have trivial bases!
func (r *codeRepo) validatePseudoVersion(info *codehost.RevInfo, version string) (err error) {
	defer func() {
		if err != nil {
			if _, ok := err.(*module.ModuleError); !ok {
				if _, ok := err.(*module.InvalidVersionError); !ok {
					err = &module.InvalidVersionError{Version: version, Pseudo: true, Err: err}
				}
				err = &module.ModuleError{Path: r.modPath, Err: err}
			}
		}
	}()

	if err := module.MatchPathMajor(version, r.pathMajor); err != nil {
		return err
	}

	rev, err := PseudoVersionRev(version)
	if err != nil {
		return err
	}
	if rev != info.Short {
		switch {
		case strings.HasPrefix(rev, info.Short):
			return fmt.Errorf("revision is longer than canonical (%s)", info.Short)
		case strings.HasPrefix(info.Short, rev):
			return fmt.Errorf("revision is shorter than canonical (%s)", info.Short)
		default:
			return fmt.Errorf("does not match short name of revision (%s)", info.Short)
		}
	}

	t, err := PseudoVersionTime(version)
	if err != nil {
		return err
	}
	if !t.Equal(info.Time.Truncate(time.Second)) {
		return fmt.Errorf("does not match version-control timestamp (%s)", info.Time.UTC().Format(time.RFC3339))
	}

	tagPrefix := ""
	if r.codeDir != "" {
		tagPrefix = r.codeDir + "/"
	}

	// A pseudo-version should have a precedence just above its parent revisions,
	// and no higher. Otherwise, it would be possible for library authors to "pin"
	// dependency versions (and bypass the usual minimum version selection) by
	// naming an extremely high pseudo-version rather than an accurate one.
	//
	// Moreover, if we allow a pseudo-version to use any arbitrary pre-release
	// tag, we end up with infinitely many possible names for each commit. Each
	// name consumes resources in the module cache and proxies, so we want to
	// restrict them to a finite set under control of the module author.
	//
	// We address both of these issues by requiring the tag upon which the
	// pseudo-version is based to refer to some ancestor of the revision. We
	// prefer the highest such tag when constructing a new pseudo-version, but do
	// not enforce that property when resolving existing pseudo-versions: we don't
	// know when the parent tags were added, and the highest-tagged parent may not
	// have existed when the pseudo-version was first resolved.
	base, err := PseudoVersionBase(strings.TrimSuffix(version, "+incompatible"))
	if err != nil {
		return err
	}
	if base == "" {
		if r.pseudoMajor == "" && semver.Major(version) == "v1" {
			return fmt.Errorf("major version without preceding tag must be v0, not v1")
		}
		return nil
	} else {
		for _, tag := range info.Tags {
			versionOnly := strings.TrimPrefix(tag, tagPrefix)
			if versionOnly == base {
				// The base version is canonical, so if the version from the tag is
				// literally equal (not just equivalent), then the tag is canonical too.
				//
				// We allow pseudo-versions to be derived from non-canonical tags on the
				// same commit, so that tags like "v1.1.0+some-metadata" resolve as
				// close as possible to the canonical version ("v1.1.0") while still
				// enforcing a total ordering ("v1.1.1-0.[…]" with a unique suffix).
				//
				// However, canonical tags already have a total ordering, so there is no
				// reason not to use the canonical tag directly, and we know that the
				// canonical tag must already exist because the pseudo-version is
				// derived from it. In that case, referring to the revision by a
				// pseudo-version derived from its own canonical tag is just confusing.
				return fmt.Errorf("tag (%s) found on revision %s is already canonical, so should not be replaced with a pseudo-version derived from that tag", tag, rev)
			}
		}
	}

	tags, err := r.code.Tags(tagPrefix + base)
	if err != nil {
		return err
	}

	var lastTag string // Prefer to log some real tag rather than a canonically-equivalent base.
	ancestorFound := false
	for _, tag := range tags {
		versionOnly := strings.TrimPrefix(tag, tagPrefix)
		if semver.Compare(versionOnly, base) == 0 {
			lastTag = tag
			ancestorFound, err = r.code.DescendsFrom(info.Name, tag)
			if ancestorFound {
				break
			}
		}
	}

	if lastTag == "" {
		return fmt.Errorf("preceding tag (%s) not found", base)
	}

	if !ancestorFound {
		if err != nil {
			return err
		}
		rev, err := PseudoVersionRev(version)
		if err != nil {
			return fmt.Errorf("not a descendent of preceding tag (%s)", lastTag)
		}
		return fmt.Errorf("revision %s is not a descendent of preceding tag (%s)", rev, lastTag)
	}
	return nil
}

func (r *codeRepo) revToRev(rev string) string {
	if semver.IsValid(rev) {
		if IsPseudoVersion(rev) {
			r, _ := PseudoVersionRev(rev)
			return r
		}
		if semver.Build(rev) == "+incompatible" {
			rev = rev[:len(rev)-len("+incompatible")]
		}
		if r.codeDir == "" {
			return rev
		}
		return r.codeDir + "/" + rev
	}
	return rev
}

func (r *codeRepo) versionToRev(version string) (rev string, err error) {
	if !semver.IsValid(version) {
		return "", &module.ModuleError{
			Path: r.modPath,
			Err: &module.InvalidVersionError{
				Version: version,
				Err:     errors.New("syntax error"),
			},
		}
	}
	return r.revToRev(version), nil
}

// findDir locates the directory within the repo containing the module.
//
// If r.pathMajor is non-empty, this can be either r.codeDir or — if a go.mod
// file exists — r.codeDir/r.pathMajor[1:].
func (r *codeRepo) findDir(version string) (rev, dir string, gomod []byte, err error) {
	rev, err = r.versionToRev(version)
	if err != nil {
		return "", "", nil, err
	}

	// Load info about go.mod but delay consideration
	// (except I/O error) until we rule out v2/go.mod.
	file1 := path.Join(r.codeDir, "go.mod")
	gomod1, err1 := r.code.ReadFile(rev, file1, codehost.MaxGoMod)
	if err1 != nil && !os.IsNotExist(err1) {
		return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.pathPrefix, file1, rev, err1)
	}
	mpath1 := modfile.ModulePath(gomod1)
	found1 := err1 == nil && isMajor(mpath1, r.pathMajor)

	var file2 string
	if r.pathMajor != "" && r.codeRoot != r.modPath && !strings.HasPrefix(r.pathMajor, ".") {
		// Suppose pathMajor is "/v2".
		// Either go.mod should claim v2 and v2/go.mod should not exist,
		// or v2/go.mod should exist and claim v2. Not both.
		// Note that we don't check the full path, just the major suffix,
		// because of replacement modules. This might be a fork of
		// the real module, found at a different path, usable only in
		// a replace directive.
		//
		// TODO(bcmills): This doesn't seem right. Investigate further.
		// (Notably: why can't we replace foo/v2 with fork-of-foo/v3?)
		dir2 := path.Join(r.codeDir, r.pathMajor[1:])
		file2 = path.Join(dir2, "go.mod")
		gomod2, err2 := r.code.ReadFile(rev, file2, codehost.MaxGoMod)
		if err2 != nil && !os.IsNotExist(err2) {
			return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.pathPrefix, file2, rev, err2)
		}
		mpath2 := modfile.ModulePath(gomod2)
		found2 := err2 == nil && isMajor(mpath2, r.pathMajor)

		if found1 && found2 {
			return "", "", nil, fmt.Errorf("%s/%s and ...%s/go.mod both have ...%s module paths at revision %s", r.pathPrefix, file1, r.pathMajor, r.pathMajor, rev)
		}
		if found2 {
			return rev, dir2, gomod2, nil
		}
		if err2 == nil {
			if mpath2 == "" {
				return "", "", nil, fmt.Errorf("%s/%s is missing module path at revision %s", r.pathPrefix, file2, rev)
			}
			return "", "", nil, fmt.Errorf("%s/%s has non-...%s module path %q at revision %s", r.pathPrefix, file2, r.pathMajor, mpath2, rev)
		}
	}

	// Not v2/go.mod, so it's either go.mod or nothing. Which is it?
	if found1 {
		// Explicit go.mod with matching module path OK.
		return rev, r.codeDir, gomod1, nil
	}
	if err1 == nil {
		// Explicit go.mod with non-matching module path disallowed.
		suffix := ""
		if file2 != "" {
			suffix = fmt.Sprintf(" (and ...%s/go.mod does not exist)", r.pathMajor)
		}
		if mpath1 == "" {
			return "", "", nil, fmt.Errorf("%s is missing module path%s at revision %s", file1, suffix, rev)
		}
		if r.pathMajor != "" { // ".v1", ".v2" for gopkg.in
			return "", "", nil, fmt.Errorf("%s has non-...%s module path %q%s at revision %s", file1, r.pathMajor, mpath1, suffix, rev)
		}
		return "", "", nil, fmt.Errorf("%s has post-%s module path %q%s at revision %s", file1, semver.Major(version), mpath1, suffix, rev)
	}

	if r.codeDir == "" && (r.pathMajor == "" || strings.HasPrefix(r.pathMajor, ".")) {
		// Implicit go.mod at root of repo OK for v0/v1 and for gopkg.in.
		return rev, "", nil, nil
	}

	// Implicit go.mod below root of repo or at v2+ disallowed.
	// Be clear about possibility of using either location for v2+.
	if file2 != "" {
		return "", "", nil, fmt.Errorf("missing %s/go.mod and ...%s/go.mod at revision %s", r.pathPrefix, r.pathMajor, rev)
	}
	return "", "", nil, fmt.Errorf("missing %s/go.mod at revision %s", r.pathPrefix, rev)
}

func isMajor(mpath, pathMajor string) bool {
	if mpath == "" {
		return false
	}
	if pathMajor == "" {
		// mpath must NOT have version suffix.
		i := len(mpath)
		for i > 0 && '0' <= mpath[i-1] && mpath[i-1] <= '9' {
			i--
		}
		if i < len(mpath) && i >= 2 && mpath[i-1] == 'v' && mpath[i-2] == '/' {
			// Found valid suffix.
			return false
		}
		return true
	}
	// Otherwise pathMajor is ".v1", ".v2" (gopkg.in), or "/v2", "/v3" etc.
	return strings.HasSuffix(mpath, pathMajor)
}

func (r *codeRepo) GoMod(version string) (data []byte, err error) {
	if version != module.CanonicalVersion(version) {
		return nil, fmt.Errorf("version %s is not canonical", version)
	}

	if IsPseudoVersion(version) {
		// findDir ignores the metadata encoded in a pseudo-version,
		// only using the revision at the end.
		// Invoke Stat to verify the metadata explicitly so we don't return
		// a bogus file for an invalid version.
		_, err := r.Stat(version)
		if err != nil {
			return nil, err
		}
	}

	rev, dir, gomod, err := r.findDir(version)
	if err != nil {
		return nil, err
	}
	if gomod != nil {
		return gomod, nil
	}
	data, err = r.code.ReadFile(rev, path.Join(dir, "go.mod"), codehost.MaxGoMod)
	if err != nil {
		if os.IsNotExist(err) {
			return r.legacyGoMod(rev, dir), nil
		}
		return nil, err
	}
	return data, nil
}

func (r *codeRepo) legacyGoMod(rev, dir string) []byte {
	// We used to try to build a go.mod reflecting pre-existing
	// package management metadata files, but the conversion
	// was inherently imperfect (because those files don't have
	// exactly the same semantics as go.mod) and, when done
	// for dependencies in the middle of a build, impossible to
	// correct. So we stopped.
	// Return a fake go.mod that simply declares the module path.
	return []byte(fmt.Sprintf("module %s\n", modfile.AutoQuote(r.modPath)))
}

func (r *codeRepo) modPrefix(rev string) string {
	return r.modPath + "@" + rev
}

func (r *codeRepo) Zip(dst io.Writer, version string) error {
	if version != module.CanonicalVersion(version) {
		return fmt.Errorf("version %s is not canonical", version)
	}

	if IsPseudoVersion(version) {
		// findDir ignores the metadata encoded in a pseudo-version,
		// only using the revision at the end.
		// Invoke Stat to verify the metadata explicitly so we don't return
		// a bogus file for an invalid version.
		_, err := r.Stat(version)
		if err != nil {
			return err
		}
	}

	rev, dir, _, err := r.findDir(version)
	if err != nil {
		return err
	}
	dl, actualDir, err := r.code.ReadZip(rev, dir, codehost.MaxZipFile)
	if err != nil {
		return err
	}
	defer dl.Close()
	if actualDir != "" && !hasPathPrefix(dir, actualDir) {
		return fmt.Errorf("internal error: downloading %v %v: dir=%q but actualDir=%q", r.modPath, rev, dir, actualDir)
	}
	subdir := strings.Trim(strings.TrimPrefix(dir, actualDir), "/")

	// Spool to local file.
	f, err := ioutil.TempFile("", "go-codehost-")
	if err != nil {
		dl.Close()
		return err
	}
	defer os.Remove(f.Name())
	defer f.Close()
	maxSize := int64(codehost.MaxZipFile)
	lr := &io.LimitedReader{R: dl, N: maxSize + 1}
	if _, err := io.Copy(f, lr); err != nil {
		dl.Close()
		return err
	}
	dl.Close()
	if lr.N <= 0 {
		return fmt.Errorf("downloaded zip file too large")
	}
	size := (maxSize + 1) - lr.N
	if _, err := f.Seek(0, 0); err != nil {
		return err
	}

	// Translate from zip file we have to zip file we want.
	zr, err := zip.NewReader(f, size)
	if err != nil {
		return err
	}

	zw := zip.NewWriter(dst)
	if subdir != "" {
		subdir += "/"
	}
	haveLICENSE := false
	topPrefix := ""
	haveGoMod := make(map[string]bool)
	for _, zf := range zr.File {
		if topPrefix == "" {
			i := strings.Index(zf.Name, "/")
			if i < 0 {
				return fmt.Errorf("missing top-level directory prefix")
			}
			topPrefix = zf.Name[:i+1]
		}
		if !strings.HasPrefix(zf.Name, topPrefix) {
			return fmt.Errorf("zip file contains more than one top-level directory")
		}
		dir, file := path.Split(zf.Name)
		if file == "go.mod" {
			haveGoMod[dir] = true
		}
	}
	root := topPrefix + subdir
	inSubmodule := func(name string) bool {
		for {
			dir, _ := path.Split(name)
			if len(dir) <= len(root) {
				return false
			}
			if haveGoMod[dir] {
				return true
			}
			name = dir[:len(dir)-1]
		}
	}

	for _, zf := range zr.File {
		if !zf.FileInfo().Mode().IsRegular() {
			// Skip symlinks (golang.org/issue/27093).
			continue
		}

		if topPrefix == "" {
			i := strings.Index(zf.Name, "/")
			if i < 0 {
				return fmt.Errorf("missing top-level directory prefix")
			}
			topPrefix = zf.Name[:i+1]
		}
		if strings.HasSuffix(zf.Name, "/") { // drop directory dummy entries
			continue
		}
		if !strings.HasPrefix(zf.Name, topPrefix) {
			return fmt.Errorf("zip file contains more than one top-level directory")
		}
		name := strings.TrimPrefix(zf.Name, topPrefix)
		if !strings.HasPrefix(name, subdir) {
			continue
		}
		if name == ".hg_archival.txt" {
			// Inserted by hg archive.
			// Not correct to drop from other version control systems, but too bad.
			continue
		}
		name = strings.TrimPrefix(name, subdir)
		if isVendoredPackage(name) {
			continue
		}
		if inSubmodule(zf.Name) {
			continue
		}
		base := path.Base(name)
		if strings.ToLower(base) == "go.mod" && base != "go.mod" {
			return fmt.Errorf("zip file contains %s, want all lower-case go.mod", zf.Name)
		}
		if name == "LICENSE" {
			haveLICENSE = true
		}
		size := int64(zf.UncompressedSize64)
		if size < 0 || maxSize < size {
			return fmt.Errorf("module source tree too big")
		}
		maxSize -= size

		rc, err := zf.Open()
		if err != nil {
			return err
		}
		w, err := zw.Create(r.modPrefix(version) + "/" + name)
		if err != nil {
			return err
		}
		lr := &io.LimitedReader{R: rc, N: size + 1}
		if _, err := io.Copy(w, lr); err != nil {
			return err
		}
		if lr.N <= 0 {
			return fmt.Errorf("individual file too large")
		}
	}

	if !haveLICENSE && subdir != "" {
		data, err := r.code.ReadFile(rev, "LICENSE", codehost.MaxLICENSE)
		if err == nil {
			w, err := zw.Create(r.modPrefix(version) + "/LICENSE")
			if err != nil {
				return err
			}
			if _, err := w.Write(data); err != nil {
				return err
			}
		}
	}

	return zw.Close()
}

// hasPathPrefix reports whether the path s begins with the
// elements in prefix.
func hasPathPrefix(s, prefix string) bool {
	switch {
	default:
		return false
	case len(s) == len(prefix):
		return s == prefix
	case len(s) > len(prefix):
		if prefix != "" && prefix[len(prefix)-1] == '/' {
			return strings.HasPrefix(s, prefix)
		}
		return s[len(prefix)] == '/' && s[:len(prefix)] == prefix
	}
}

func isVendoredPackage(name string) bool {
	var i int
	if strings.HasPrefix(name, "vendor/") {
		i += len("vendor/")
	} else if j := strings.Index(name, "/vendor/"); j >= 0 {
		// This offset looks incorrect; this should probably be
		//
		// 	i = j + len("/vendor/")
		//
		// (See https://golang.org/issue/31562.)
		//
		// Unfortunately, we can't fix it without invalidating checksums.
		// Fortunately, the error appears to be strictly conservative: we'll retain
		// vendored packages that we should have pruned, but we won't prune
		// non-vendored packages that we should have retained.
		//
		// Since this defect doesn't seem to break anything, it's not worth fixing
		// for now.
		i += len("/vendor/")
	} else {
		return false
	}
	return strings.Contains(name[i:], "/")
}