[PATCH v2 5/6] commit-graph: be extra careful about mixed generations

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Feb 2, 2021, 03:01 UTC
Message-ID: <9bccee8fb6381102d543d6b28e90959307ed08eb.1612234883.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.850.v2.git.1612234883.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

When upgrading to a commit-graph with corrected commit dates from one without, there are a few things that need to be considered.

When computing generation numbers for the new commit-graph file that expects to add the generation_data chunk with corrected commit dates, we need to ensure that the 'generation' member of the commit_graph_data struct is set to zero for these commits.

Unfortunately, the fallback to use topological level for generation number when corrected commit dates are not available are causing us harm here: parsing commits notices that read_generation_data is false and populates 'generation' with the topological level.

The solution is to iterate through the commits, parse the commits to populate initial values, then reset the generation values to zero to trigger recalculation. This loop only occurs when the existing commit-graph data has no corrected commit dates.

While this improves our situation somewhat, we have not completely solved the issue for correctly computing generation numbers for mixed layers. That follows in the next change.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 commit-graph.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 2790f70d113..ee8d5a0cdb4 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1036,7 +1036,8 @@ struct write_commit_graph_context {
 		 split:1,
 		 changed_paths:1,
 		 order_by_pack:1,
-		 write_generation_data:1;
+		 write_generation_data:1,
+		 trust_generation_numbers:1;
 
 	struct topo_level_slab *topo_levels;
 	const struct commit_graph_opts *opts;
@@ -1508,6 +1509,15 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 		ctx->progress = start_delayed_progress(
 					_("Computing commit graph generation numbers"),
 					ctx->commits.nr);
+
+	if (!ctx->trust_generation_numbers) {
+		for (i = 0; i < ctx->commits.nr; i++) {
+			struct commit *c = ctx->commits.list[i];
+			repo_parse_commit(ctx->r, c);
+			commit_graph_data_at(c)->generation = GENERATION_NUMBER_ZERO;
+		}
+	}
+
 	for (i = 0; i < ctx->commits.nr; i++) {
 		struct commit *c = ctx->commits.list[i];
 		timestamp_t corrected_commit_date;
@@ -2439,7 +2449,7 @@ int write_commit_graph(struct object_directory *odb,
 	} else
 		ctx->num_commit_graphs_after = 1;
 
-	validate_mixed_generation_chain(ctx->r->objects->commit_graph);
+	ctx->trust_generation_numbers = validate_mixed_generation_chain(ctx->r->objects->commit_graph);
 
 	compute_topological_levels(ctx);
 	if (ctx->write_generation_data)

-- 
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