cmd/go/internal/test: add opt-in file hashing instead of modtime for test caching (w/ git)

src/cmd/go/internal/githash/githash.go

@@ -0,0 +1,182 @@
+// Copyright 2026 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 githash
+
+import (
+	"bytes"
+	"io/fs"
+	"os"
+	"os/exec"
+	"path"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+// GitHash is a git hash in hex form.
+//
+// It's usually a SHA-1 hash, but could be SHA-256 depending on the git
+// configuration.
+type GitHash string
+
+// Enabled is whether git hash lookups are enabled via the CMD_GO_USE_GIT_HASH
+// environment variable.
+var Enabled bool
+
+func init() {
+	s := os.Getenv("CMD_GO_USE_GIT_HASH")
+	if s != "" {
+		Enabled, _ = strconv.ParseBool(s)
+	}
+}
+
+// gitHashKey is the key used to look up possible files in
+// a git repo that match the same base name & size.
+//
+// This is used to avoid statting all files in a git repo
+// when trying to find the git hash for a given file.
+// Instead, we only stat files that match on name & size.
+type gitHashKey struct {
+	baseName string // base name of file; as that's fs.FileInfo.Name gives us
+	size     int64
+}
+
+type gitHashMap struct {
+	gitRoot string // absolute path to git repo root
+
+	// cands is a list of files in the git repo, bucketed by their (base name,
+	// size) bucket key. This makes looking for a file faster later, without
+	// statting the whole world, yet still permitting lookup only from a
+	// fs.FileInfo that only has a base name & size & Sys info.
+	cands map[gitHashKey][]*gitHashCand
+}
+
+type gitHashCand struct {
+	rel  string // the relative git path from "git ls-files -r"
+	hash GitHash
+
+	statOnce sync.Once
+	stat     fs.FileInfo
+}
+
+func (c *gitHashCand) getStat(m *gitHashMap) fs.FileInfo {
+	c.statOnce.Do(func() {
+		fullPath := path.Join(m.gitRoot, c.rel)
+		info, err := os.Lstat(fullPath)
+		if err == nil {
+			c.stat = info
+		}
+	})
+	return c.stat
+}
+
+var getGitHashMap = sync.OnceValue(buildGitHashMap)
+
+func buildGitHashMap() *gitHashMap {
+	m := &gitHashMap{
+		cands: make(map[gitHashKey][]*gitHashCand),
+	}
+	gitRoot, err := exec.Command("git", "rev-parse", "--show-toplevel").Output()
+	if err != nil {
+		return nil
+	}
+	m.gitRoot = strings.TrimSpace(string(gitRoot))
+
+	cmd := exec.Command("git", "ls-tree",
+		"-r",     // recursive
+		"--long", // include file sizes
+		"-z",     // null-separated entries; don't have to deal with C quoting of some filenames
+		"HEAD",
+	)
+	cmd.Dir = m.gitRoot // effectively git -C <dir>; either way.
+	out, err := cmd.Output()
+	if err != nil {
+		return nil
+	}
+	// Parse lines of the form:
+	//
+	// 100644 blob cabbb1732c418125f9c773ce7a28ba34f2708554     639    .gitattributes
+	// 100644 blob 2b4a5fccdaf12f98cf8e255affa28cfd7e6a784d      95    .github/CODE_OF_CONDUCT.md
+	//
+	// .... but null-terminated instead of newline-terminated, so we don't have to deal
+	// with C quoting of filenames with certain characters.
+	//
+	// We don't care about the permissions.
+	remain := out
+	for len(remain) > 0 {
+		line, rest, ok := bytes.Cut(remain, []byte{0})
+		if !ok {
+			break
+		}
+		remain = rest
+		meta, nameB, ok := bytes.Cut(line, []byte("\t"))
+
+		_, hashAndSize, ok := bytes.Cut(meta, []byte(" blob "))
+		if !ok {
+			continue
+		}
+		hashB, sizeB, ok := bytes.Cut(hashAndSize, []byte(" "))
+		if !ok {
+			continue
+		}
+		size, err := strconv.ParseInt(strings.TrimSpace(string(sizeB)), 10, 64)
+		if err != nil {
+			continue
+		}
+		name := strings.TrimSpace(string(nameB))
+		hash := strings.TrimSpace(string(hashB))
+		k := gitHashKey{
+			baseName: path.Base(name),
+			size:     size,
+		}
+		m.cands[k] = append(m.cands[k], &gitHashCand{
+			rel:  name,
+			hash: GitHash(hash),
+		})
+	}
+	return m
+}
+
+// Hash returns the git hash for the given file info, if available.
+func Hash(info fs.FileInfo) (GitHash, bool) {
+	if !Enabled || info == nil || !info.Mode().IsRegular() {
+		return "", false
+	}
+	k := gitHashKey{
+		baseName: info.Name(),
+		size:     info.Size(),
+	}
+	m := getGitHashMap()
+	if m == nil {
+		return "", false
+	}
+	for _, cand := range m.cands[k] {
+		if os.SameFile(info, cand.getStat(m)) {
+			return cand.hash, true
+		}
+	}
+	return "", false
+}
+
+// ModTimeOrHash returns either the git hash (if enabled and available) or the
+// mod time of the given file info.
+//
+// For non-regular files (notably directories), it returns nil if git hash is
+// enabled.
+//
+// It always returns one of nil, time.Time, or GitHash (a string), all suitable
+// for use in Sprintf verb %v.
+func ModTimeOrHash(info fs.FileInfo) any {
+	if !Enabled {
+		return info.ModTime()
+	}
+	if h, ok := Hash(info); ok {
+		return h
+	}
+	if info.Mode().IsRegular() {
+		return info.ModTime()
+	}
+	return nil
+}

src/cmd/go/internal/modindex/read.go

@@ -28,6 +28,7 @@ import (
 	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
+	"cmd/go/internal/githash"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/str"
 	"cmd/internal/par"
@@ -109,11 +110,11 @@ func dirHash(modroot, pkgdir string) (cache.ActionID, error) {
 		if err != nil {
 			return cache.ActionID{}, ErrNotIndexed
 		}
-		if info.ModTime().After(cutoff) {
+		if !githash.Enabled && info.ModTime().After(cutoff) {
 			return cache.ActionID{}, ErrNotIndexed
 		}
 
-		fmt.Fprintf(h, "file %v %v %v\n", info.Name(), info.ModTime(), info.Size())
+		fmt.Fprintf(h, "file %v %v %v\n", info.Name(), githash.ModTimeOrHash(info), info.Size())
 	}
 	return h.Sum(), nil
 }

src/cmd/go/internal/test/test.go

@@ -27,6 +27,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/githash"
 	"cmd/go/internal/load"
 	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/modload"
@@ -2028,7 +2029,7 @@ func hashOpen(name string) (cache.ActionID, error) {
 				hashWriteStat(h, finfo)
 			}
 		}
-	} else if info.Mode().IsRegular() {
+	} else if info.Mode().IsRegular() && !githash.Enabled {
 		// Because files might be very large, do not attempt
 		// to hash the entirety of their content. Instead assume
 		// the mtime and size recorded in hashWriteStat above
@@ -2061,7 +2062,7 @@ func hashStat(name string) cache.ActionID {
 }
 
 func hashWriteStat(h io.Writer, info fs.FileInfo) {
-	fmt.Fprintf(h, "stat %d %x %v %v\n", info.Size(), uint64(info.Mode()), info.ModTime(), info.IsDir())
+	fmt.Fprintf(h, "stat %d %x %v %v\n", info.Size(), uint64(info.Mode()), githash.ModTimeOrHash(info), info.IsDir())
 }
 
 // testAndInputKey returns the actual cache key for the pair (testID, testInputsID).