[PATCH 2/2] commit-graph: propagate topo_levels slab to all chain layers

["Kristofer Karlsson via GitGitGadget" <gitgitgadget@gmail.com>]

From: Kristofer Karlsson <krka@spotify.com>

Fix a regression introduced in 199d452758 (commit-graph: fix
"filling in" topological levels, 2025-04-07) where the loop
propagating the topo_levels slab to each layer of the
commit-graph chain always assigned to `g->topo_levels`
(the topmost layer) instead of `chain->topo_levels` (the
current iteration variable).

This meant only the topmost layer had its topo_levels pointer
set.  When compute_reachable_generation_numbers() ran for an
incremental write, commits parsed from lower layers had their
topo levels left at zero in the slab, since
fill_commit_graph_info() could not store them without the
pointer.  The DFS then re-walked the entire commit ancestry
instead of stopping at commits with known levels.

On a repository with 2.78M commits and a multi-layer split
commit-graph, this caused a single incremental commit-graph
write to spend ~3.7 seconds in the generation DFS instead of
microseconds.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
---
 commit-graph.c                | 2 +-
 t/t5324-split-commit-graph.sh | 6 +-----
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 4e39a048c4..c2a711cceb 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -2610,7 +2610,7 @@ int write_commit_graph(struct odb_source *source,
 
 	g = prepare_commit_graph(ctx.r);
 	for (struct commit_graph *chain = g; chain; chain = chain->base_graph)
-		g->topo_levels = &topo_levels;
+		chain->topo_levels = &topo_levels;
 
 	if (flags & COMMIT_GRAPH_WRITE_BLOOM_FILTERS)
 		ctx.changed_paths = 1;
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index f9c57760f4..9e5ab7dbd0 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -738,11 +738,7 @@ test_expect_success 'incremental write reads topo levels from all layers' '
 		GIT_TRACE2_EVENT="$(pwd)/trace.txt" \
 			git commit-graph write --reachable --split=no-merge &&
 
-		# BUG: topo levels from lower graph layers are not
-		# propagated, so the DFS re-walks from base-3 down to
-		# the root (7 steps) instead of reading topo levels
-		# from the existing graph (1 step).
-		test_trace2_data commit-graph generation-dfs-steps 7 <trace.txt
+		test_trace2_data commit-graph generation-dfs-steps 1 <trace.txt
 	)
 '
 
-- 
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