Add better ModTime in gitfs and ArchGitTime helper (for SOURCE_DATE_EPOCH)

Author: tianon

cmd/bashbrew/git.go

@@ -11,6 +11,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
+	"time"
 
 	"github.com/urfave/cli"
 
@@ -110,6 +111,21 @@ func (r Repo) archGitFS(arch string, entry *manifest.Manifest2822Entry) (fs.FS,
 	return fs.Sub(gitFS, entry.ArchDirectory(arch))
 }
 
+// returns the timestamp of the ArchGitCommit -- useful for SOURCE_DATE_EPOCH
+func (r Repo) ArchGitTime(arch string, entry *manifest.Manifest2822Entry) (time.Time, error) {
+	f, err := r.archGitFS(arch, entry)
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	fi, err := fs.Stat(f, ".")
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	return fi.ModTime(), nil
+}
+
 func gitCommitFS(commit string) (fs.FS, error) {
 	if err := ensureGitInit(); err != nil {
 		return nil, err

pkg/gitfs/fs.go

@@ -17,6 +17,14 @@ import (
 
 // https://github.com/go-git/go-git/issues/296
 
+func CommitTime(commit *goGitPlumbingObject.Commit) time.Time {
+	if commit.Committer.When.After(commit.Author.When) {
+		return commit.Committer.When
+	} else {
+		return commit.Author.When
+	}
+}
+
 func CommitHash(repo *goGit.Repository, commit string) (fs.FS, error) {
 	gitCommit, err := repo.CommitObject(goGitPlumbing.NewHash(commit))
 	if err != nil {
@@ -31,6 +39,7 @@ func CommitHash(repo *goGit.Repository, commit string) (fs.FS, error) {
 			storer: repo.Storer,
 			tree:   tree,
 			name:   ".",
+			Mod:    CommitTime(gitCommit),
 		},
 	}, nil
 }
@@ -49,6 +58,8 @@ type gitFS struct {
 	tree   *goGitPlumbingObject.Tree
 	entry  *goGitPlumbingObject.TreeEntry // might be nil ("." at the top-level of the repo)
 
+	Mod time.Time
+
 	// cached values
 	name string // full path from the repository root
 	size int64  // Tree.Size value for non-directories (more efficient than opening/reading the blob)
@@ -342,7 +353,7 @@ func (f gitFS) Mode() fs.FileMode {
 
 // https://pkg.go.dev/io/fs#FileInfo: modification time
 func (f gitFS) ModTime() time.Time {
-	return time.Time{} // TODO maybe pass down whichever is more recent of commit.Author.When vs commit.Committer.When ?
+	return f.Mod
 }
 
 // https://pkg.go.dev/io/fs#FileInfo: abbreviation for Mode().IsDir()

pkg/gitfs/tarscrub_test.go

@@ -42,3 +42,35 @@ func ExampleGitVarnish() {
 	fmt.Printf("%x\n", h.Sum(nil))
 	// Output: 3aef5ac859b23d65dfe5e9f2a47750e9a32852222829cfba762a870c1473fad6
 }
+
+// this example is nice because it has a different committer vs author timestamp
+// https://github.com/tianon/docker-bash/commit/eb7e541caccc813d297e77cf4068f89553256673
+// https://github.com/docker-library/official-images/blob/8718b8afb62ff1a001d99bb4f77d95fe352ba187/library/bash
+func ExampleGitBash() {
+	repo, err := git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
+		URL:          "https://github.com/tianon/docker-bash.git",
+		SingleBranch: true,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	commit, err := gitfs.CommitHash(repo, "eb7e541caccc813d297e77cf4068f89553256673")
+	if err != nil {
+		panic(err)
+	}
+
+	f, err := fs.Sub(commit, "5.2")
+	if err != nil {
+		panic(err)
+	}
+
+	h := sha256.New()
+
+	if err := tarscrub.WriteTar(f, h); err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("%x\n", h.Sum(nil))
+	// Output: 011c8eda9906b94916a14e7969cfcff3974f1fbf2ff7b8e5c1867ff491dc01d3
+}