cmd/go: make module@nonexistentversion failures reusable

CL 411398 added the -reuse flag for reusing cached JSON output
when the remote Git repository has not changed. One case that was
not yet cached is a lookup of a nonexistent version.

This CL adds caching of failed lookups of nonexistent versions,
by saving a checksum of all the heads and tags refs on the remote
server (we never consider other kinds of refs). If none of those have
changed, then we don't need to download the full server.

Fixes #53644.

Change-Id: I428bbc8ec8475bd7d03788934d643e1e2be3add0
Reviewed-on: https://go-review.googlesource.com/c/go/+/415678
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

6 files changed, 136 insertions, 21 deletions

diff --git a/src/cmd/go/internal/modfetch/cache.go b/src/cmd/go/internal/modfetch/cache.go
index 7ebe208c12..c1ed18736c 100644
--- a/src/cmd/go/internal/modfetch/cache.go
+++ b/src/cmd/go/internal/modfetch/cache.go
@@ -253,11 +253,12 @@ func (r *cachingRepo) Stat(rev string) (*RevInfo, error) {
 		return cachedInfo{info, err}
 	}).(cachedInfo)
 
-	if c.err != nil {
-		return nil, c.err
+	info := c.info
+	if info != nil {
+		copy := *info
+		info = &copy
 	}
-	info := *c.info
-	return &info, nil
+	return info, c.err
 }
 
 func (r *cachingRepo) Latest() (*RevInfo, error) {
@@ -277,11 +278,12 @@ func (r *cachingRepo) Latest() (*RevInfo, error) {
 		return cachedInfo{info, err}
 	}).(cachedInfo)
 
-	if c.err != nil {
-		return nil, c.err
+	info := c.info
+	if info != nil {
+		copy := *info
+		info = &copy
 	}
-	info := *c.info
-	return &info, nil
+	return info, c.err
 }
 
 func (r *cachingRepo) GoMod(version string) ([]byte, error) {
@@ -330,15 +332,21 @@ func InfoFile(path, version string) (*RevInfo, string, error) {
 	}
 
 	var info *RevInfo
+	var err2info map[error]*RevInfo
 	err := TryProxies(func(proxy string) error {
 		i, err := Lookup(proxy, path).Stat(version)
 		if err == nil {
 			info = i
+		} else {
+			if err2info == nil {
+				err2info = make(map[error]*RevInfo)
+			}
+			err2info[err] = info
 		}
 		return err
 	})
 	if err != nil {
-		return nil, "", err
+		return err2info[err], "", err
 	}
 
 	// Stat should have populated the disk cache for us.
diff --git a/src/cmd/go/internal/modfetch/codehost/codehost.go b/src/cmd/go/internal/modfetch/codehost/codehost.go
index 937ac6819a..8eaf254b44 100644
--- a/src/cmd/go/internal/modfetch/codehost/codehost.go
+++ b/src/cmd/go/internal/modfetch/codehost/codehost.go
@@ -110,12 +110,18 @@ type Origin struct {
 	// with a mutable meaning while Hash is a name with an immutable meaning.
 	Ref  string `json:",omitempty"`
 	Hash string `json:",omitempty"`
+
+	// If RepoSum is non-empty, then the resolution of this module version
+	// failed due to the repo being available but the version not being present.
+	// This depends on the entire state of the repo, which RepoSum summarizes.
+	// For Git, this is a hash of all the refs and their hashes.
+	RepoSum string `json:",omitempty"`
 }
 
 // Checkable reports whether the Origin contains anything that can be checked.
 // If not, the Origin is purely informational and should fail a CheckReuse call.
 func (o *Origin) Checkable() bool {
-	return o.TagSum != "" || o.Ref != "" || o.Hash != ""
+	return o.TagSum != "" || o.Ref != "" || o.Hash != "" || o.RepoSum != ""
 }
 
 // ClearCheckable clears the Origin enough to make Checkable return false.
@@ -124,6 +130,7 @@ func (o *Origin) ClearCheckable() {
 	o.TagPrefix = ""
 	o.Ref = ""
 	o.Hash = ""
+	o.RepoSum = ""
 }
 
 // A Tags describes the available tags in a code repository.
diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
index a225aaf1ed..35f77e870e 100644
--- a/src/cmd/go/internal/modfetch/codehost/git.go
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -182,12 +182,12 @@ func (r *gitRepo) CheckReuse(old *Origin, subdir string) error {
 		return fmt.Errorf("origin moved from %v %q %q to %v %q %q", old.VCS, old.URL, old.Subdir, "git", r.remoteURL, subdir)
 	}
 
-	// Note: Can have Hash with no Ref and no TagSum,
+	// Note: Can have Hash with no Ref and no TagSum and no RepoSum,
 	// meaning the Hash simply has to remain in the repo.
 	// In that case we assume it does in the absence of any real way to check.
 	// But if neither Hash nor TagSum is present, we have nothing to check,
 	// which we take to mean we didn't record enough information to be sure.
-	if old.Hash == "" && old.TagSum == "" {
+	if old.Hash == "" && old.TagSum == "" && old.RepoSum == "" {
 		return fmt.Errorf("non-specific origin")
 	}
 
@@ -214,7 +214,11 @@ func (r *gitRepo) CheckReuse(old *Origin, subdir string) error {
 			return fmt.Errorf("tags changed")
 		}
 	}
-
+	if old.RepoSum != "" {
+		if r.repoSum(r.refs) != old.RepoSum {
+			return fmt.Errorf("refs changed")
+		}
+	}
 	return nil
 }
 
@@ -307,6 +311,35 @@ func (r *gitRepo) Tags(prefix string) (*Tags, error) {
 	return tags, nil
 }
 
+// repoSum returns a checksum of the entire repo state,
+// which can be checked (as Origin.RepoSum) to cache
+// the absence of a specific module version.
+// The caller must supply refs, the result of a successful r.loadRefs.
+func (r *gitRepo) repoSum(refs map[string]string) string {
+	var list []string
+	for ref := range refs {
+		list = append(list, ref)
+	}
+	sort.Strings(list)
+	h := sha256.New()
+	for _, ref := range list {
+		fmt.Fprintf(h, "%q %s\n", ref, refs[ref])
+	}
+	return "r1:" + base64.StdEncoding.EncodeToString(h.Sum(nil))
+}
+
+// unknownRevisionInfo returns a RevInfo containing an Origin containing a RepoSum of refs,
+// for use when returning an UnknownRevisionError.
+func (r *gitRepo) unknownRevisionInfo(refs map[string]string) *RevInfo {
+	return &RevInfo{
+		Origin: &Origin{
+			VCS:     "git",
+			URL:     r.remoteURL,
+			RepoSum: r.repoSum(refs),
+		},
+	}
+}
+
 func (r *gitRepo) Latest() (*RevInfo, error) {
 	refs, err := r.loadRefs()
 	if err != nil {
@@ -418,7 +451,7 @@ func (r *gitRepo) stat(rev string) (info *RevInfo, err error) {
 			hash = rev
 		}
 	} else {
-		return nil, &UnknownRevisionError{Rev: rev}
+		return r.unknownRevisionInfo(refs), &UnknownRevisionError{Rev: rev}
 	}
 
 	defer func() {
@@ -532,7 +565,12 @@ func (r *gitRepo) fetchRefsLocked() error {
 func (r *gitRepo) statLocal(version, rev string) (*RevInfo, error) {
 	out, err := Run(r.dir, "git", "-c", "log.showsignature=false", "log", "--no-decorate", "-n1", "--format=format:%H %ct %D", rev, "--")
 	if err != nil {
-		return nil, &UnknownRevisionError{Rev: rev}
+		// Return info with Origin.RepoSum if possible to allow caching of negative lookup.
+		var info *RevInfo
+		if refs, err := r.loadRefs(); err == nil {
+			info = r.unknownRevisionInfo(refs)
+		}
+		return info, &UnknownRevisionError{Rev: rev}
 	}
 	f := strings.Fields(string(out))
 	if len(f) < 2 {
diff --git a/src/cmd/go/internal/modfetch/coderepo.go b/src/cmd/go/internal/modfetch/coderepo.go
index 86e3ee9d1c..b934e362a4 100644
--- a/src/cmd/go/internal/modfetch/coderepo.go
+++ b/src/cmd/go/internal/modfetch/coderepo.go
@@ -297,7 +297,15 @@ func (r *codeRepo) Stat(rev string) (*RevInfo, error) {
 	codeRev := r.revToRev(rev)
 	info, err := r.code.Stat(codeRev)
 	if err != nil {
-		return nil, &module.ModuleError{
+		// Note: info may be non-nil to supply Origin for caching error.
+		var revInfo *RevInfo
+		if info != nil {
+			revInfo = &RevInfo{
+				Origin:  info.Origin,
+				Version: rev,
+			}
+		}
+		return revInfo, &module.ModuleError{
 			Path: r.modPath,
 			Err: &module.InvalidVersionError{
 				Version: rev,
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index 1d2f5d5e15..01df14fca4 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -197,7 +197,7 @@ func queryProxy(ctx context.Context, proxy, path, query, current string, allowed
 				}
 			}
 			if err != nil {
-				return nil, queryErr
+				return info, queryErr
 			}
 		}
 		if err := allowed(ctx, module.Version{Path: path, Version: info.Version}); errors.Is(err, ErrDisallowed) {
diff --git a/src/cmd/go/testdata/script/reuse_git.txt b/src/cmd/go/testdata/script/reuse_git.txt
index 7d8844d932..a5a0c8a9a0 100644
--- a/src/cmd/go/testdata/script/reuse_git.txt
+++ b/src/cmd/go/testdata/script/reuse_git.txt
@@ -56,8 +56,7 @@ stdout '"Version": "latest"'
 stdout '"Error":.*no matching versions'
 ! stdout '"TagPrefix"'
 stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
-! stdout '"Ref":'
-! stdout '"Hash":'
+! stdout '"(Ref|Hash|RepoSum)":'
 
 # go mod download vcstest/hello/sub/v9 should also fail, print origin info with TagPrefix
 ! go mod download -x -json vcs-test.golang.org/git/hello.git/sub/v9@latest
@@ -66,8 +65,33 @@ stdout '"Version": "latest"'
 stdout '"Error":.*no matching versions'
 stdout '"TagPrefix": "sub/"'
 stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
-! stdout '"Ref":'
-! stdout '"Hash":'
+! stdout '"(Ref|Hash|RepoSum)":'
+
+# go mod download vcstest/hello@nonexist should fail, still print origin info
+! go mod download -x -json vcs-test.golang.org/git/hello.git@nonexist
+cp stdout hellononexist.json
+stdout '"Version": "nonexist"'
+stdout '"Error":.*unknown revision nonexist'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+
+# go mod download vcstest/hello@1234567890123456789012345678901234567890 should fail, still print origin info
+# (40 hex digits is assumed to be a full hash and is a slightly different code path from @nonexist)
+! go mod download -x -json vcs-test.golang.org/git/hello.git@1234567890123456789012345678901234567890
+cp stdout hellononhash.json
+stdout '"Version": "1234567890123456789012345678901234567890"'
+stdout '"Error":.*unknown revision 1234567890123456789012345678901234567890'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+
+# go mod download vcstest/hello@v0.0.0-20220101120101-123456789abc should fail, still print origin info
+# (non-existent pseudoversion)
+! go mod download -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20220101120101-123456789abc
+cp stdout hellononpseudo.json
+stdout '"Version": "v0.0.0-20220101120101-123456789abc"'
+stdout '"Error":.*unknown revision 123456789abc'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
 
 # go mod download vcstest/tagtests should invoke git, print origin info
 go mod download -x -json vcs-test.golang.org/git/tagtests.git@latest
@@ -190,6 +214,36 @@ stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
 ! stdout '"(Ref|Hash)":'
 ! stdout '"(Dir|Info|GoMod|Zip)"'
 
+# reuse go mod download vcstest/hello@nonexist
+! go mod download -reuse=hellononexist.json -x -json vcs-test.golang.org/git/hello.git@nonexist
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "nonexist"'
+stdout '"Error":.*unknown revision nonexist'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello@1234567890123456789012345678901234567890
+! go mod download -reuse=hellononhash.json -x -json vcs-test.golang.org/git/hello.git@1234567890123456789012345678901234567890
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "1234567890123456789012345678901234567890"'
+stdout '"Error":.*unknown revision 1234567890123456789012345678901234567890'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello@v0.0.0-20220101120101-123456789abc
+! go mod download -reuse=hellononpseudo.json -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20220101120101-123456789abc
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.0.0-20220101120101-123456789abc"'
+stdout '"Error":.*unknown revision 123456789abc'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
 # reuse go mod download vcstest/tagtests result
 go mod download -reuse=tagtests.json -x -json vcs-test.golang.org/git/tagtests.git@latest
 ! stderr 'git fetch'