commit-graph: introduce `commit_graph_generation_from_graph()`

In 2ee11f7261 (commit-graph: return generation from memory, 2023-03-20),
the `commit_graph_generation()` function stopped returning zeros when
asked to locate the generation number of a given commit.

This was done at the time to prepare for a later change which set
generation values in memory, meaning that we could no longer rely on
`graph_pos` alone to tell us whether or not to trust the generation
number returned by this function.

In 2ee11f7261, it was noted that this change only impacted very old
commit-graphs, which were written with all commits having generation
number 0. Indeed, zero is not a valid generation number, so we should
never expect to see that value outside of the aforementioned case.

The test fallout in 2ee11f7261 indicated that we were no longer able to
fsck a specific old case of commit-graph corruption, where we see a
non-zero generation number after having seen a generation number of 0
earlier.

Introduce a variant of `commit_graph_generation()` which behaves like
that function did prior to 2ee11f7261, known as
`commit_graph_generation_from_graph()`. Then use this function in the
context of `verify_one_commit_graph()`, where we only want to trust the
values from the graph.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/commit-graph.c b/commit-graph.c
index 0aa1640d15aca59b90617677afd30ef7c8d247c1..c68f5c6b3ac6ef0e3a924c6dc07f1e13efb1aee2 100644 (file)
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -128,6 +128,16 @@ timestamp_t commit_graph_generation(const struct commit *c)
        return GENERATION_NUMBER_INFINITY;
 }
 
+static timestamp_t commit_graph_generation_from_graph(const struct commit *c)
+{
+       struct commit_graph_data *data =
+               commit_graph_data_slab_peek(&commit_graph_data_slab, c);
+
+       if (!data || data->graph_pos == COMMIT_NOT_FROM_GRAPH)
+               return GENERATION_NUMBER_INFINITY;
+       return data->generation;
+}
+
 static struct commit_graph_data *commit_graph_data_at(const struct commit *c)
 {
        unsigned int i, nth_slab;
@@ -2659,7 +2669,7 @@ static int verify_one_commit_graph(struct repository *r,
                                             oid_to_hex(&graph_parents->item->object.oid),
                                             oid_to_hex(&odb_parents->item->object.oid));
 
-                       generation = commit_graph_generation(graph_parents->item);
+                       generation = commit_graph_generation_from_graph(graph_parents->item);
                        if (generation > max_generation)
                                max_generation = generation;
 
@@ -2671,7 +2681,7 @@ static int verify_one_commit_graph(struct repository *r,
                        graph_report(_("commit-graph parent list for commit %s terminates early"),
                                     oid_to_hex(&cur_oid));
 
-               if (!commit_graph_generation(graph_commit)) {
+               if (!commit_graph_generation_from_graph(graph_commit)) {
                        if (generation_zero == GENERATION_NUMBER_EXISTS)
                                graph_report(_("commit-graph has generation number zero for commit %s, but non-zero elsewhere"),
                                             oid_to_hex(&cur_oid));

diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 4df76173a8d774b56f60f12c7ee23372d11b7143..4e70820c74eba23ebecc9ba17a57fd909f4a71a1 100755 (executable)
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -598,7 +598,7 @@ test_expect_success 'detect incorrect generation number' '
 
 test_expect_success 'detect incorrect generation number' '
        corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\01" \
-               "commit-graph generation for commit"
+               "but zero elsewhere"
 '
 
 test_expect_success 'detect incorrect commit date' '