Re: [PATCH v2 08/10] commit-graph: handle mixed generation commit chains

From: Abhishek Kumar <redacted>

As corrected commit dates and topological levels cannot be compared
directly, we must handle commit graph chains with mixed generation
number definitions.

While reading a commit graph file, we disable generation numbers if the
chain contains mixed generation numbers.

While writing to commit graph chain, we write generation data chunk only
if the previous tip of chain had a generation data chunk. Using
`--split=replace` overwrites the existing chain and writes generation
data chunk regardless of previous tip.

In t5324-split-commit-graph, we set up a repo with twelve commits and
write a base commit graph file with no generation data chunk. When add
three commits and write to chain again, Git does not write generation
data chunk even without setting GIT_TEST_COMMIT_GRAPH_NO_GDAT=1. Then,
as we replace the existing chain, Git writes a commit graph file with
generation data chunk.

Signed-off-by: Abhishek Kumar <redacted>
---
 commit-graph.c                | 14 ++++++++
 t/t5324-split-commit-graph.sh | 66 +++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/commit-graph.c b/commit-graph.c
index d0f977852b..c6b6111adf 100644
--- a/commit-graph.c
+++ b/commit-graph.c

@@ -674,6 +674,14 @@ int generation_numbers_enabled(struct repository *r)
 	if (!g->num_commits)
 		return 0;
 
+	/* We cannot compare topological levels and corrected commit dates */
+	while (g->base_graph) {
+		warning(_("commit-graph-chain contains mixed generation versions"));

+			return 0;
+		g = g->base_graph;
+	}
+

 	first_generation = get_be32(g->chunk_commit_data +
 				    g->hash_len + 8) >> 2;
 

@@ -2186,6 +2194,9 @@ int write_commit_graph(struct object_directory *odb,
 
 		g = ctx->r->objects->commit_graph;
 
+		if (g && !g->chunk_generation_data)
+			ctx->write_generation_data = 0;
+
 		while (g) {
 			ctx->num_commit_graphs_before++;
 			g = g->base_graph;

@@ -2204,6 +2215,9 @@ int write_commit_graph(struct object_directory *odb,
 
 		if (ctx->split_opts)
 			replace = ctx->split_opts->flags & COMMIT_GRAPH_SPLIT_REPLACE;
+
+		if (replace)
+			ctx->write_generation_data = 1;
 	}

+test_expect_success 'setup repo for mixed generation commit-graph-chain' '
+	mkdir mixed &&
+	graphdir=".git/objects/info/commit-graphs" &&
+	cd "$TRASH_DIRECTORY/mixed" &&
+	git init &&
+	git config core.commitGraph true &&
+	git config gc.writeCommitGraph false &&
+	for i in $(test_seq 3)
+	do
+		test_commit $i &&
+		git branch commits/$i || return 1
+	done &&
+	git reset --hard commits/1 &&
+	for i in $(test_seq 4 5)
+	do
+		test_commit $i &&
+		git branch commits/$i || return 1
+	done &&
+	git reset --hard commits/2 &&
+	for i in $(test_seq 6 10)
+	do
+		test_commit $i &&
+		git branch commits/$i || return 1
+	done &&
+	git reset --hard commits/2 &&
+	git merge commits/4 &&
+	git branch merge/1 &&
+	git reset --hard commits/4 &&
+	git merge commits/6 &&
+	git branch merge/2 &&
+	GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --reachable --split &&
+	test-tool read-graph >output &&
+	cat >expect <<-EOF &&
+	header: 43475048 1 1 3 0
+	num_commits: 12
+	chunks: oid_fanout oid_lookup commit_metadata
+	EOF
+	test_cmp expect output
+'
+
+test_expect_success 'does not write generation data chunk if not present on existing tip' '
+	cd "$TRASH_DIRECTORY/mixed" &&
+	git reset --hard commits/3 &&
+	git merge merge/1 &&
+	git merge commits/5 &&
+	git merge merge/2 &&
+	git branch merge/3 &&
+	git commit-graph write --reachable --split &&
+	test-tool read-graph >output &&
+	cat >expect <<-EOF &&
+	header: 43475048 1 1 4 1
+	num_commits: 3
+	chunks: oid_fanout oid_lookup commit_metadata
+	EOF
+	test_cmp expect output
+'
+
+test_expect_success 'writes generation data chunk when commit-graph chain is replaced' '
+	cd "$TRASH_DIRECTORY/mixed" &&
+	git commit-graph write --reachable --split='replace' &&
+	test_path_is_file $graphdir/commit-graph-chain &&
+	test_line_count = 1 $graphdir/commit-graph-chain &&
+	verify_chain_files_exist $graphdir &&
+	graph_read_expect 15
+'