commit-graph: allow cross-alternate chains

In an environment like a fork network, it is helpful to have a
commit-graph chain that spans both the base repo and the fork repo. The
fork is usually a small set of data on top of the large repo, but
sometimes the fork is much larger. For example, git-for-windows/git has
almost double the number of commits as git/git because it rebases its
commits on every major version update.

To allow cross-alternate commit-graph chains, we need a few pieces:

1. When looking for a graph-{hash}.graph file, check all alternates.

2. When merging commit-graph chains, do not merge across alternates.

3. When writing a new commit-graph chain based on a commit-graph file
   in another object directory, do not allow success if the base file
   has of the name "commit-graph" instead of
   "commit-graphs/graph-{hash}.graph".

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 5cb5663a30..46f0832f68 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -90,6 +90,21 @@ test_expect_success 'add more commits, and write a new base graph' '
 	graph_read_expect 12
 '
 
+test_expect_success 'fork and fail to base a chain on a commit-graph file' '
+	test_when_finished rm -rf fork &&
+	git clone . fork &&
+	(
+		cd fork &&
+		rm .git/objects/info/commit-graph &&
+		echo "$(pwd)/../.git/objects" >.git/objects/info/alternates &&
+		test_commit new-commit &&
+		git commit-graph write --reachable --split &&
+		test_path_is_file $graphdir/commit-graph-chain &&
+		test_line_count = 1 $graphdir/commit-graph-chain &&
+		verify_chain_files_exist $graphdir
+	)
+'
+
 test_expect_success 'add three more commits, write a tip graph' '
 	git reset --hard commits/3 &&
 	git merge merge/1 &&
@@ -132,4 +147,26 @@ test_expect_success 'add one commit, write a merged graph' '
 
 graph_git_behavior 'merged commit-graph: commit 12 vs 6' commits/12 commits/6
 
+test_expect_success 'create fork and chain across alternate' '
+	git clone . fork &&
+	(
+		cd fork &&
+		git config core.commitGraph true &&
+		rm -rf $graphdir &&
+		echo "$(pwd)/../.git/objects" >.git/objects/info/alternates &&
+		test_commit 13 &&
+		git branch commits/13 &&
+		git commit-graph write --reachable --split &&
+		test_path_is_file $graphdir/commit-graph-chain &&
+		test_line_count = 3 $graphdir/commit-graph-chain &&
+		ls $graphdir/graph-*.graph >graph-files &&
+		test_line_count = 1 graph-files &&
+		git -c core.commitGraph=true  rev-list HEAD >expect &&
+		git -c core.commitGraph=false rev-list HEAD >actual &&
+		test_cmp expect actual
+	)
+'
+
+graph_git_behavior 'alternate: commit 13 vs 6' commits/13 commits/6
+
 test_done