replay: drop commits that become empty

If the changes in a commit being replayed are already in the branch
that the commits are being replayed onto, then "git replay" creates an
empty commit. This is confusing because the commit message no longer
matches the contents of the commit. Drop the commit instead. Commits
that start off empty are not dropped. This matches the behavior of
"git rebase --reapply-cherry-pick --empty=drop" and "git cherry-pick
--empty-drop".

If a branch points to a commit that is dropped it will be updated
to point to the last commit that was not dropped. This can be seen
in the new test where "topic1" is updated to point to the rebased
"C" as "F" is dropped because it is already upstream. While this is
a breaking change, "git replay" is marked as experimental to allow
improvements like this that change the behavior.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 31 insertions, 4 deletions

diff --git a/Documentation/git-replay.adoc b/Documentation/git-replay.adoc
index 4c61f3aa1f..dc966486ca 100644
--- a/Documentation/git-replay.adoc
+++ b/Documentation/git-replay.adoc
@@ -62,7 +62,9 @@ The default mode can be configured via the `replay.refAction` configuration vari
 	Range of commits to replay; see "Specifying Ranges" in
 	linkgit:git-rev-parse[1]. In `--advance <branch>` mode, the
 	range should have a single tip, so that it's clear to which tip the
-	advanced <branch> should point.
+	advanced <branch> should point. Any commits in the range whose
+	changes are already present in the branch the commits are being
+	replayed onto will be dropped.
 
 include::rev-list-options.adoc[]
 
diff --git a/replay.c b/replay.c
index 94fb76384b..f97d652f33 100644
--- a/replay.c
+++ b/replay.c
@@ -217,12 +217,12 @@ static struct commit *pick_regular_commit(struct repository *repo,
 					  struct merge_result *result)
 {
 	struct commit *base, *replayed_base;
-	struct tree *pickme_tree, *base_tree;
+	struct tree *pickme_tree, *base_tree, *replayed_base_tree;
 
 	base = pickme->parents->item;
 	replayed_base = mapped_commit(replayed_commits, base, onto);
 
-	result->tree = repo_get_commit_tree(repo, replayed_base);
+	replayed_base_tree = repo_get_commit_tree(repo, replayed_base);
 	pickme_tree = repo_get_commit_tree(repo, pickme);
 	base_tree = repo_get_commit_tree(repo, base);
 
@@ -232,7 +232,7 @@ static struct commit *pick_regular_commit(struct repository *repo,
 
 	merge_incore_nonrecursive(merge_opt,
 				  base_tree,
-				  result->tree,
+				  replayed_base_tree,
 				  pickme_tree,
 				  result);
 
@@ -240,6 +240,10 @@ static struct commit *pick_regular_commit(struct repository *repo,
 	merge_opt->ancestor = NULL;
 	if (!result->clean)
 		return NULL;
+	/* Drop commits that become empty */
+	if (oideq(&replayed_base_tree->object.oid, &result->tree->object.oid) &&
+	    !oideq(&pickme_tree->object.oid, &base_tree->object.oid))
+		return replayed_base;
 	return create_commit(repo, result->tree, pickme, replayed_base);
 }
 
diff --git a/t/t3650-replay-basics.sh b/t/t3650-replay-basics.sh
index c862aa39f3..a03f8f9293 100755
--- a/t/t3650-replay-basics.sh
+++ b/t/t3650-replay-basics.sh
@@ -25,6 +25,8 @@ test_expect_success 'setup' '
 	git switch -c topic3 &&
 	test_commit G &&
 	test_commit H &&
+	git switch -c empty &&
+	git commit --allow-empty -m empty &&
 	git switch -c topic4 main &&
 	test_commit I &&
 	test_commit J &&
@@ -160,6 +162,25 @@ test_expect_success 'using replay on bare repo to perform basic cherry-pick' '
 	test_cmp expect result-bare
 '
 
+test_expect_success 'commits that become empty are dropped' '
+	# Save original branches
+	git for-each-ref --format="update %(refname) %(objectname)" \
+		refs/heads/ >original-branches &&
+	test_when_finished "git update-ref --stdin <original-branches &&
+		rm original-branches" &&
+	# Cherry-pick tip of topic1 ("F"), from the middle of A..empty, to main
+	git replay --advance main topic1^! &&
+
+	# Replay all of A..empty onto main (which includes topic1 & thus F
+	# in the middle)
+	git replay --onto main --branches --ancestry-path=empty ^A \
+		>result &&
+	git log --format="%s%d" L..empty >actual &&
+	test_write_lines >expect \
+		"empty (empty)" "H (topic3)" G "C (topic1)" "F (main)" "M (tag: M)" &&
+	test_cmp expect actual
+'
+
 test_expect_success 'replay on bare repo fails with both --advance and --onto' '
 	test_must_fail git -C bare replay --advance main --onto main topic1..topic2 >result-bare
 '