commit-graph: add trace2 instrumentation for generation DFS

Count the number of steps taken in
compute_reachable_generation_numbers() and expose it via
trace2 to make it easier to detect performance regressions.

Add a failing test for such a regression, introduced in
199d452758 (commit-graph: return the prepared commit graph
from `prepare_commit_graph()`, 2025-09-04), where incremental
commit-graph writes do not see existing generation numbers
from lower graph layers and fall back to walking the full
ancestry.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/commit-graph.c b/commit-graph.c
index c6d9c5c740e94d3dd9d72100e5e2309fb9fb3476..702ba9731b0ed7c2e680dd424f6d5f5e71084c55 100644 (file)
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1653,6 +1653,7 @@ static void compute_reachable_generation_numbers(
 {
        int i;
        struct commit_list *list = NULL;
+       intmax_t steps = 0;
 
        for (i = 0; i < info->commits->nr; i++) {
                struct commit *c = info->commits->items[i];
@@ -1671,6 +1672,7 @@ static void compute_reachable_generation_numbers(
                        int all_parents_computed = 1;
                        timestamp_t max_gen = 0;
 
+                       steps++;
                        for (parent = current->parents; parent; parent = parent->next) {
                                repo_parse_commit(info->r, parent->item);
                                gen = info->get_generation(parent->item, info->data);
@@ -1694,6 +1696,9 @@ static void compute_reachable_generation_numbers(
                        }
                }
        }
+
+       trace2_data_intmax("commit-graph", info->r,
+                          "generation-dfs-steps", steps);
 }
 
 static timestamp_t get_topo_level(struct commit *c, void *data)

diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 49a057cc2eb65d60c661fe80668cf787a1213cd4..b41331e3dd930c152e099ed566591bde960d6865 100755 (executable)
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -718,6 +718,30 @@ test_expect_success 'write generation data chunk when commit-graph chain is repl
        )
 '
 
+test_expect_failure 'incremental write reads topo levels from all layers' '
+       git init topo-from-lower &&
+       (
+               cd topo-from-lower &&
+
+               for i in $(test_seq 5)
+               do
+                       test_commit base-$i || return 1
+               done &&
+               git commit-graph write --reachable &&
+
+               test_commit extra &&
+               git commit-graph write --reachable --split=no-merge &&
+
+               git checkout base-3 &&
+               test_commit new-branch &&
+
+               GIT_TRACE2_EVENT="$(pwd)/trace.txt" \
+                       git commit-graph write --reachable --split=no-merge &&
+
+               test_trace2_data commit-graph generation-dfs-steps 1 <trace.txt
+       )
+'
+
 test_expect_success 'temporary graph layer is discarded upon failure' '
        git init layer-discard &&
        (