Merge branch 'pw/replay-drop-empty' into seen

"git replay" is taught to drop commits that become empty (not the
ones that are empty in the original).

* pw/replay-drop-empty:
  replay: drop commits that become empty

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[]
 

replay.c

@@ -211,12 +211,12 @@ 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);
 
@@ -226,7 +226,7 @@ static struct commit *pick_regular_commit(struct repository *repo,
 
 	merge_incore_nonrecursive(merge_opt,
 				  base_tree,
-				  result->tree,
+				  replayed_base_tree,
 				  pickme_tree,
 				  result);
 
@@ -234,6 +234,10 @@ 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);
 }
 

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
 '