cmd/go: fetch history as needed to resolve recent tags

Fixes #26713.

Tested with Git 2.7.4. Older Gits may or may not work.

Change-Id: Ib72d751388dfbb50030191ae40f788d1402834b2
Reviewed-on: https://go-review.googlesource.com/126956
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 122 insertions, 45 deletions

diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
index 0f159bd519..06c452ff45 100644
--- a/src/cmd/go/internal/modfetch/codehost/git.go
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -355,19 +355,11 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 	// Last resort.
 	// Fetch all heads and tags and hope the hash we want is in the history.
 	if r.fetchLevel < fetchAll {
+		// TODO(bcmills): should we wait to upgrade fetchLevel until after we check
+		// err? If there is a temporary server error, we want subsequent fetches to
+		// try again instead of proceeding with an incomplete repo.
 		r.fetchLevel = fetchAll
-
-		// To work around a protocol version 2 bug that breaks --unshallow,
-		// add -c protocol.version=0.
-		// TODO(rsc): The bug is believed to be server-side, meaning only
-		// on Google's Git servers. Once the servers are fixed, drop the
-		// protocol.version=0. See Google-internal bug b/110495752.
-		var protoFlag []string
-		unshallowFlag := unshallow(r.dir)
-		if len(unshallowFlag) > 0 {
-			protoFlag = []string{"-c", "protocol.version=0"}
-		}
-		if _, err := Run(r.dir, "git", protoFlag, "fetch", unshallowFlag, "-f", r.remote, "refs/heads/*:refs/heads/*", "refs/tags/*:refs/tags/*"); err != nil {
+		if err := r.fetchUnshallow("refs/heads/*:refs/heads/*", "refs/tags/*:refs/tags/*"); err != nil {
 			return nil, err
 		}
 	}
@@ -375,6 +367,21 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 	return r.statLocal(rev, rev)
 }
 
+func (r *gitRepo) fetchUnshallow(refSpecs ...string) error {
+	// To work around a protocol version 2 bug that breaks --unshallow,
+	// add -c protocol.version=0.
+	// TODO(rsc): The bug is believed to be server-side, meaning only
+	// on Google's Git servers. Once the servers are fixed, drop the
+	// protocol.version=0. See Google-internal bug b/110495752.
+	var protoFlag []string
+	unshallowFlag := unshallow(r.dir)
+	if len(unshallowFlag) > 0 {
+		protoFlag = []string{"-c", "protocol.version=0"}
+	}
+	_, err := Run(r.dir, "git", protoFlag, "fetch", unshallowFlag, "-f", r.remote, refSpecs)
+	return err
+}
+
 // statLocal returns a RevInfo describing rev in the local git repository.
 // It uses version as info.Version.
 func (r *gitRepo) statLocal(version, rev string) (*RevInfo, error) {
@@ -512,6 +519,18 @@ func (r *gitRepo) ReadFileRevs(revs []string, file string, maxSize int64) (map[s
 		return nil, err
 	}
 
+	// TODO(bcmills): after the 1.11 freeze, replace the block above with:
+	//	if r.fetchLevel <= fetchSome {
+	//		r.fetchLevel = fetchSome
+	//		var refs []string
+	//		for _, tag := range redo {
+	//			refs = append(refs, "refs/tags/"+tag+":refs/tags/"+tag)
+	//		}
+	//		if _, err := Run(r.dir, "git", "fetch", "--update-shallow", "-f", r.remote, refs); err != nil {
+	//			return nil, err
+	//		}
+	//	}
+
 	if _, err := r.readFileRevs(redo, file, files); err != nil {
 		return nil, err
 	}
@@ -603,15 +622,65 @@ func (r *gitRepo) readFileRevs(tags []string, file string, fileMap map[string]*F
 }
 
 func (r *gitRepo) RecentTag(rev, prefix string) (tag string, err error) {
-	_, err = r.Stat(rev)
+	info, err := r.Stat(rev)
 	if err != nil {
 		return "", err
 	}
-	out, err := Run(r.dir, "git", "describe", "--first-parent", "--tags", "--always", "--abbrev=0", "--match", prefix+"v[0-9]*.[0-9]*.[0-9]*", "--tags", rev)
+	rev = info.Name // expand hash prefixes
+
+	// describe sets tag and err using 'git describe' and reports whether the
+	// result is definitive.
+	describe := func() (definitive bool) {
+		var out []byte
+		out, err = Run(r.dir, "git", "describe", "--first-parent", "--always", "--abbrev=0", "--match", prefix+"v[0-9]*.[0-9]*.[0-9]*", "--tags", rev)
+		if err != nil {
+			return true // Because we use "--always", describe should never fail.
+		}
+
+		tag = string(bytes.TrimSpace(out))
+		return tag != "" && !AllHex(tag)
+	}
+
+	if describe() {
+		return tag, err
+	}
+
+	// Git didn't find a version tag preceding the requested rev.
+	// See whether any plausible tag exists.
+	tags, err := r.Tags(prefix + "v")
 	if err != nil {
 		return "", err
 	}
-	return strings.TrimSpace(string(out)), nil
+	if len(tags) == 0 {
+		return "", nil
+	}
+
+	// There are plausible tags, but we don't know if rev is a descendent of any of them.
+	// Fetch the history to find out.
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if r.fetchLevel < fetchAll {
+		// Fetch all heads and tags and see if that gives us enough history.
+		if err := r.fetchUnshallow("refs/heads/*:refs/heads/*", "refs/tags/*:refs/tags/*"); err != nil {
+			return "", err
+		}
+		r.fetchLevel = fetchAll
+	}
+
+	// If we've reached this point, we have all of the commits that are reachable
+	// from all heads and tags.
+	//
+	// The only refs we should be missing are those that are no longer reachable
+	// (or never were reachable) from any branch or tag, including the master
+	// branch, and we don't want to resolve them anyway (they're probably
+	// unreachable for a reason).
+	//
+	// Try one last time in case some other goroutine fetched rev while we were
+	// waiting on r.mu.
+	describe()
+	return tag, err
 }
 
 func (r *gitRepo) ReadZip(rev, subdir string, maxSize int64) (zip io.ReadCloser, actualSubdir string, err error) {
diff --git a/src/cmd/go/testdata/script/mod_get_pseudo.txt b/src/cmd/go/testdata/script/mod_get_pseudo.txt
index 80bcd4718d..3945fdfa89 100644
--- a/src/cmd/go/testdata/script/mod_get_pseudo.txt
+++ b/src/cmd/go/testdata/script/mod_get_pseudo.txt
@@ -5,65 +5,73 @@ env GO111MODULE=on
 [!exec:git] skip
 env GOPROXY=
 
+# We can resolve the @master branch without unshallowing the local repository
+# (even with older gits), so try that before we do anything else.
+# (This replicates https://golang.org/issue/26713 with git 2.7.4.)
+go get -m github.com/rsc/legacytest@master
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.1-0\.\d{14}-7303f7796364\+incompatible$'
+
 # get should include incompatible tags in "latest" calculation.
+go get -m github.com/rsc/legacytest@latest
 go list
 go list -m all
 stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
 
-# v0.0.0-pseudo
-go get -m ...test@52853eb
+# v2.0.1-0.pseudo+incompatible
+go get -m ...test@7303f77
 go list -m all
-stdout '^github.com/rsc/legacytest v0\.0\.0-\d{14}-52853eb7b552$'
+stdout '^github.com/rsc/legacytest v2\.0\.1-0\.\d{14}-7303f7796364\+incompatible$'
 
-# v1.0.0
-go get -m ...test@7fff7f3
+# v2.0.0+incompatible by tag+incompatible
+go get -m ...test@v2.0.0+incompatible
 go list -m all
-stdout '^github.com/rsc/legacytest v1\.0\.0$'
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
 
-# v1.0.1-0.pseudo
-go get -m ...test@fa4f5d6
+# v2.0.0+incompatible by tag
+go get -m ...test@v2.0.0
 go list -m all
-stdout '^github.com/rsc/legacytest v1\.0\.1-0\.\d{14}-fa4f5d6a71c6$'
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
 
-# v1.1.0-pre (no longer on master)
-go get -m ...test@731e3b1
+# v2.0.0+incompatible by hash (back on master)
+go get -m ...test@d7ae1e4
 go list -m all
-stdout '^github.com/rsc/legacytest v1\.1\.0-pre$'
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
 
-# v1.1.0-pre.0.pseudo
-go get -m ...test@fb3c628
+# v1.2.1-0.pseudo
+go get -m ...test@d2d4c3e
 go list -m all
-stdout '^github.com/rsc/legacytest v1\.1\.0-pre\.0\.\d{14}-fb3c628075e3$'
+stdout '^github.com/rsc/legacytest v1\.2\.1-0\.\d{14}-d2d4c3ea6623$'
 
 # v1.2.0
 go get -m ...test@9f6f860
 go list -m all
 stdout '^github.com/rsc/legacytest v1\.2\.0$'
 
-# v1.2.1-0.pseudo
-go get -m ...test@d2d4c3e
+# v1.1.0-pre.0.pseudo
+go get -m ...test@fb3c628
 go list -m all
-stdout '^github.com/rsc/legacytest v1\.2\.1-0\.\d{14}-d2d4c3ea6623$'
+stdout '^github.com/rsc/legacytest v1\.1\.0-pre\.0\.\d{14}-fb3c628075e3$'
 
-# v2.0.0+incompatible by hash (back on master)
-go get -m ...test@d7ae1e4
+# v1.1.0-pre (no longer on master)
+go get -m ...test@731e3b1
 go list -m all
-stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+stdout '^github.com/rsc/legacytest v1\.1\.0-pre$'
 
-# v2.0.0+incompatible by tag
-go get -m ...test@v2.0.0
+# v1.0.1-0.pseudo
+go get -m ...test@fa4f5d6
 go list -m all
-stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+stdout '^github.com/rsc/legacytest v1\.0\.1-0\.\d{14}-fa4f5d6a71c6$'
 
-# v2.0.0+incompatible by tag+incompatible
-go get -m ...test@v2.0.0+incompatible
+# v1.0.0
+go get -m ...test@7fff7f3
 go list -m all
-stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+stdout '^github.com/rsc/legacytest v1\.0\.0$'
 
-# v2.0.1-0.pseudo+incompatible
-go get -m ...test@7303f77
+# v0.0.0-pseudo
+go get -m ...test@52853eb
 go list -m all
-stdout '^github.com/rsc/legacytest v2\.0\.1-0\.\d{14}-7303f7796364\+incompatible$'
+stdout '^github.com/rsc/legacytest v0\.0\.0-\d{14}-52853eb7b552$'
 
 -- go.mod --
 module x