From cc8efb02ff49a95eff1568b2e7936d22eea5852a Mon Sep 17 00:00:00 2001
From: Toon Claes <toon@gitlab.com>
Date: Fri, 14 Nov 2025 12:41:38 +0100
Subject: [PATCH] WIP: implement git-replay

---
 internal/git/gitcmd/command_description.go |  3 +
 internal/git/localrepo/rebase.go           | 74 ++++++++++++++++++++--
 2 files changed, 72 insertions(+), 5 deletions(-)

diff --git a/internal/git/gitcmd/command_description.go b/internal/git/gitcmd/command_description.go
index 33c9a8e4d2..81b30c189f 100644
--- a/internal/git/gitcmd/command_description.go
+++ b/internal/git/gitcmd/command_description.go
@@ -259,6 +259,9 @@ var commandDescriptions = map[string]commandDescription{
 	"refs": {
 		flags: scNoRefUpdates,
 	},
+	"replay": {
+		flags: scNoRefUpdates,
+	},
 	"range-diff": {
 		flags: scNoRefUpdates | scNoEndOfOptions,
 	},
diff --git a/internal/git/localrepo/rebase.go b/internal/git/localrepo/rebase.go
index 62023922c6..1f8462e828 100644
--- a/internal/git/localrepo/rebase.go
+++ b/internal/git/localrepo/rebase.go
@@ -74,20 +74,84 @@ func (repo *Repo) Rebase(ctx context.Context, upstream, branch string, options .
 		return upstreamOID, nil
 	}
 
-	newOID, err := repo.rebaseUsingMergeTree(ctx, config, upstreamOID, branchOID)
+	objectHash, err := repo.ObjectHash(ctx)
 	if err != nil {
-		return "", structerr.NewInternal("rebase using merge-tree: %w", err)
+		return "", structerr.NewInternal("getting object hash %w", err)
+	}
+
+	var newOID git.ObjectID
+	if true {
+		baseOID, err := objectHash.FromHex(mergeBase)
+		if err != nil {
+			return "", structerr.NewInternal("parse merge-base: %w", err)
+		}
+
+		newOID, err = repo.rebaseUsingReplay(ctx, config, upstreamOID, branchOID, baseOID, objectHash)
+		if err != nil {
+			return "", structerr.NewInternal("rebase using replay: %w", err)
+		}
+	} else {
+		newOID, err = repo.rebaseUsingMergeTree(ctx, config, upstreamOID, branchOID, objectHash)
+		if err != nil {
+			return "", structerr.NewInternal("rebase using merge-tree: %w", err)
+		}
 	}
 
 	return newOID, nil
 }
 
-func (repo *Repo) rebaseUsingMergeTree(ctx context.Context, cfg rebaseConfig, upstreamOID, branchOID git.ObjectID) (git.ObjectID, error) {
-	objectHash, err := repo.ObjectHash(ctx)
+func (repo *Repo) rebaseUsingReplay(ctx context.Context, cfg rebaseConfig, upstreamOID, branchOID, baseOID git.ObjectID, objectHash git.ObjectHash) (git.ObjectID, error) {
+	flags := []gitcmd.Option{
+		gitcmd.ValueFlag{Name: "--onto", Value: upstreamOID.String()},
+	}
+
+	committer := cfg.committer
+	if committer == nil {
+		// TODO committer = getSignatureFromCommitAuthor(theirsCommit.GetCommitter())
+	}
+
+	env := []string{
+		fmt.Sprintf("GIT_COMMITTER_DATE=%s", git.FormatTime(committer.When)),
+		fmt.Sprintf("GIT_COMMITTER_NAME=%s", committer.Name),
+		fmt.Sprintf("GIT_COMMITTER_EMAIL=%s", committer.Email),
+	}
+
+	var stderr bytes.Buffer
+	cmd, err := repo.Exec(ctx, gitcmd.Command{
+		Name:  "replay",
+		Flags: flags,
+		Args:  []string{fmt.Sprintf("%s...%s", baseOID, branchOID)},
+	}, gitcmd.WithStderr(&stderr), gitcmd.WithSetupStdout(), gitcmd.WithEnv(env...))
 	if err != nil {
-		return "", structerr.NewInternal("getting object hash %w", err)
+		return "", structerr.NewInternal("start git rev-list: %w", err)
 	}
 
+	var newOID git.ObjectID
+	scanner := bufio.NewScanner(cmd)
+	for scanner.Scan() {
+		// if len(newHash) > 0 {
+		// 	return "", structerr.NewInternal("rebase replay only one ref expected")
+		// }
+		parts := strings.Split(strings.TrimSpace(scanner.Text()), " ")
+		if parts[0] != "update" {
+			return "", structerr.NewInternal("rebase replay expected 'update' instead of %q", parts[0])
+		}
+		newOID, err = objectHash.FromHex(parts[2])
+		if err != nil {
+			return "", structerr.NewInternal("rebase replay new OID %q not parsed: %w", parts[2], err)
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return "", fmt.Errorf("scanning rev-list output: %w", err)
+	}
+	if err := cmd.Wait(); err != nil {
+		return "", structerr.NewInternal("git replay: %w [%v]", err, stderr.String()).WithMetadata("stderr", stderr.String())
+	}
+
+	return newOID, nil
+}
+
+func (repo *Repo) rebaseUsingMergeTree(ctx context.Context, cfg rebaseConfig, upstreamOID, branchOID git.ObjectID, objectHash git.ObjectHash) (git.ObjectID, error) {
 	// Flags of git-rev-list to get the pick-only todo_list for a rebase.
 	// Currently we drop clean cherry-picks and merge commits, which is
 	// also what git2go does.
-- 
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