1 files changed, 115 insertions, 25 deletions

diff --git a/git-codereview/sync.go b/git-codereview/sync.go
index c38215c..b079c84 100644
--- a/git-codereview/sync.go
+++ b/git-codereview/sync.go
@@ -127,8 +127,9 @@ func writeSyncBranchStatus(status *syncBranchStatus) {
 func cmdSyncBranch(args []string) {
 	os.Setenv("GIT_EDITOR", ":") // do not bring up editor during merge, commit
 
-	var cont bool
+	var cont, mergeBackToParent bool
 	flags.BoolVar(&cont, "continue", false, "continue after merge conflicts")
+	flags.BoolVar(&mergeBackToParent, "merge-back-to-parent", false, "for shutting down the dev branch")
 	flags.Parse(args)
 	if len(flag.Args()) > 0 {
 		fmt.Fprintf(stderr(), "Usage: %s sync-branch %s [-continue]\n", progName, globalFlags)
@@ -155,10 +156,17 @@ func cmdSyncBranch(args []string) {
 	}
 
 	if cont {
+		// Note: There is no -merge-back-to-parent -continue
+		// because -merge-back-to-parent never has merge conflicts.
+		// (It requires that the parent be fully merged into the
+		// dev branch or it won't even attempt the reverse merge.)
+		if mergeBackToParent {
+			dief("cannot use -continue with -merge-back-to-parent")
+		}
 		if _, err := os.Stat(syncBranchStatusFile()); err != nil {
 			dief("cannot sync-branch -continue: no pending sync-branch status file found")
 		}
-		syncBranchContinue(" -continue", b, readSyncBranchStatus())
+		syncBranchContinue(syncBranchContinueFlag, b, readSyncBranchStatus())
 		return
 	}
 
@@ -192,11 +200,59 @@ func cmdSyncBranch(args []string) {
 	}
 	writeSyncBranchStatus(status)
 
+	parentHash, err := cmdOutputErr("git", "rev-parse", "origin/"+parent)
+	if err != nil {
+		dief("cannot sync-branch: cannot resolve origin/%s: %v\n%s", parent, err, parentHash)
+	}
+	branchHash, err := cmdOutputErr("git", "rev-parse", "origin/"+branch)
+	if err != nil {
+		dief("cannot sync-branch: cannot resolve origin/%s: %v\n%s", branch, err, branchHash)
+	}
+	parentHash = trim(parentHash)
+	branchHash = trim(branchHash)
+
+	// Only --merge-back-to-parent when there's nothing waiting
+	// to be merged in from parent. If a non-trivial merge needs
+	// to be done, it should be done first on the dev branch,
+	// not the parent branch.
+	if mergeBackToParent {
+		other := cmdOutput("git", "log", "--format=format:+ %cd %h %s", "--date=short", "origin/"+branch+"..origin/"+parent)
+		if other != "" {
+			dief("cannot sync-branch --merge-back-to-parent: parent has new commits.\n"+
+				"\trun 'git sync-branch' to bring them into this branch first:\n%s",
+				other)
+		}
+	}
+
 	// Start the merge.
-	_, err := cmdOutputErr("git", "merge", "origin/"+parent)
+	if mergeBackToParent {
+		// Change HEAD back to "parent" and merge "branch" into it,
+		// even though we could instead merge "parent" into "branch".
+		// This way the parent-branch lineage ends up the first parent
+		// of the merge, the same as it would when we are doing it by hand
+		// with a plain "git merge". This may help the display of the
+		// merge graph in some tools more closely reflect what we did.
+		run("git", "reset", "--hard", "origin/"+parent)
+		_, err = cmdOutputErr("git", "merge", "--no-ff", "origin/"+branch)
+	} else {
+		_, err = cmdOutputErr("git", "merge", "origin/"+parent)
+	}
 
-	// Resolve codereview.cfg the right way (never take it from the merge).
-	cmdOutputDir(repoRoot(), "git", "checkout", "HEAD", "--", "codereview.cfg")
+	// Resolve codereview.cfg the right way - never take it from the merge.
+	// For a regular sync-branch we keep the branch's.
+	// For a merge-back-to-parent we take the parent's.
+	// The codereview.cfg contains the branch config and we don't want
+	// it to change.
+	what := branchHash
+	if mergeBackToParent {
+		what = parentHash
+	}
+	cmdOutputDir(repoRoot(), "git", "checkout", what, "--", "codereview.cfg")
+
+	if mergeBackToParent {
+		syncBranchContinue(syncBranchMergeBackFlag, b, status)
+		return
+	}
 
 	if err != nil {
 		// Check whether the only listed file is codereview.cfg and try again if so.
@@ -224,8 +280,10 @@ func cmdSyncBranch(args []string) {
 
 	if err != nil {
 		if len(status.Conflicts) == 0 {
-			dief("cannot sync-branch: git merge failed but no conflicts found\n" +
-				"(unexpected error, please ask for help!)")
+			dief("cannot sync-branch: git merge failed but no conflicts found\n"+
+				"(unexpected error, please ask for help!)\n\ngit status:\n%s\ngit status -b --porcelain:\n%s",
+				cmdOutputDir(repoRoot(), "git", "status"),
+				cmdOutputDir(repoRoot(), "git", "status", "-b", "--porcelain"))
 		}
 		dief("sync-branch: merge conflicts in:\n\t- %s\n\n"+
 			"Please fix them (use 'git status' to see the list again),\n"+
@@ -246,6 +304,18 @@ func diePendingMerge(cmd string) {
 		cmd)
 }
 
+func prefixFor(branch string) string {
+	if strings.HasPrefix(branch, "dev.") || strings.HasPrefix(branch, "release-branch.") {
+		return "[" + branch + "] "
+	}
+	return ""
+}
+
+const (
+	syncBranchContinueFlag  = " -continue"
+	syncBranchMergeBackFlag = " -merge-back-to-parent"
+)
+
 func syncBranchContinue(flag string, b *Branch, status *syncBranchStatus) {
 	if h := gitHash("origin/" + status.Parent); h != status.ParentHash {
 		dief("cannot sync-branch%s: parent hash changed: %.7s -> %.7s", flag, status.ParentHash, h)
@@ -257,34 +327,43 @@ func syncBranchContinue(flag string, b *Branch, status *syncBranchStatus) {
 		dief("cannot sync-branch%s: branch changed underfoot: %s -> %s", flag, status.Local, b.Name)
 	}
 
-	branch := status.Branch
-	parent := status.Parent
-	branchHash := status.BranchHash
-	parentHash := status.ParentHash
+	var (
+		dst     = status.Branch
+		dstHash = status.BranchHash
+		src     = status.Parent
+		srcHash = status.ParentHash
+	)
+	if flag == syncBranchMergeBackFlag {
+		// This is a reverse merge: commits are flowing
+		// in the opposite direction from normal.
+		dst, src = src, dst
+		dstHash, srcHash = srcHash, dstHash
+	}
 
-	prefix := ""
-	if strings.HasPrefix(branch, "dev.") || strings.HasPrefix(branch, "release-branch.") {
-		prefix = "[" + branch + "] "
+	prefix := prefixFor(dst)
+	op := "merge"
+	if flag == syncBranchMergeBackFlag {
+		op = "REVERSE MERGE"
 	}
-	msg := fmt.Sprintf("%sall: merge %s (%.7s) into %s", prefix, parent, parentHash, branch)
+	msg := fmt.Sprintf("%sall: %s %s (%.7s) into %s", prefix, op, src, srcHash, dst)
 
-	if flag != "" {
+	if flag == syncBranchContinueFlag {
 		// Need to commit the merge.
 
 		// Check that the state of the client is the way we left it before any merge conflicts.
 		mergeHead, err := cmdOutputErr("git", "rev-parse", "MERGE_HEAD")
 		if err != nil {
 			dief("cannot sync-branch%s: no pending merge\n"+
-				"If you accidentally ran 'git merge --continue',\n"+
+				"If you accidentally ran 'git merge --continue' or 'git commit',\n"+
 				"then use 'git reset --hard HEAD^' to undo.\n", flag)
 		}
 		mergeHead = trim(mergeHead)
-		if mergeHead != parentHash {
-			dief("cannot sync-branch%s: MERGE_HEAD is %.7s, but origin/%s is %.7s", flag, mergeHead, parent, parentHash)
+		if mergeHead != srcHash {
+			dief("cannot sync-branch%s: MERGE_HEAD is %.7s, but origin/%s is %.7s", flag, mergeHead, src, srcHash)
 		}
 		head := gitHash("HEAD")
-		if head != branchHash {
-			dief("cannot sync-branch%s: HEAD is %.7s, but origin/%s is %.7s", flag, head, branch, branchHash)
+		if head != dstHash {
+			dief("cannot sync-branch%s: HEAD is %.7s, but origin/%s is %.7s", flag, head, dst, dstHash)
 		}
 
 		if HasUnstagedChanges() {
@@ -301,15 +380,24 @@ func syncBranchContinue(flag string, b *Branch, status *syncBranchStatus) {
 	// to use our standard format and list the incorporated CLs.
 
 	// Merge must never sync codereview.cfg,
-	// because it contains the parent and branch config.
-	// Force the on-branch copy back while amending the commit.
-	cmdOutputDir(repoRoot(), "git", "checkout", "origin/"+branch, "--", "codereview.cfg")
+	// because it contains the src and dst config.
+	// Force the on-dst copy back while amending the commit.
+	cmdOutputDir(repoRoot(), "git", "checkout", "origin/"+dst, "--", "codereview.cfg")
 
 	conflictMsg := ""
 	if len(status.Conflicts) > 0 {
 		conflictMsg = "Conflicts:\n\n- " + strings.Join(status.Conflicts, "\n- ") + "\n\n"
 	}
-	msg = fmt.Sprintf("%s\n\n%sMerge List:\n\n%s", msg, conflictMsg,
+
+	if flag == syncBranchMergeBackFlag {
+		msg += fmt.Sprintf("\n\n"+
+			"This commit is a REVERSE MERGE.\n"+
+			"It merges %s back into its parent branch, %s.\n"+
+			"This marks the end of development on %s.\n",
+			status.Branch, status.Parent, status.Branch)
+	}
+
+	msg += fmt.Sprintf("\n\n%sMerge List:\n\n%s", conflictMsg,
 		cmdOutput("git", "log", "--format=format:+ %cd %h %s", "--date=short", "HEAD^1..HEAD^2"))
 	run("git", "commit", "--amend", "-m", msg)
 
@@ -317,4 +405,6 @@ func syncBranchContinue(flag string, b *Branch, status *syncBranchStatus) {
 
 	cmdPending([]string{"-c", "-l"})
 	fmt.Fprintf(stderr(), "\n* Merge commit created.\nRun 'git codereview mail' to send for review.\n")
+
+	os.Remove(syncBranchStatusFile())
 }