Re: [PATCH 1/1] commit-graph.c: avoid unnecessary tag dereference when merging

From

Jeff King <peff@peff.net>

Date

Mar 22, 2020, 06:04 UTC

Message-ID

<20200322060434.GC578498@coredump.intra.peff.net>

In-Reply-To

<20200322054916.GB578498@coredump.intra.peff.net>

On Sun, Mar 22, 2020 at 01:49:16AM -0400, Jeff King wrote:

> [1] I'm actually not quite sure about correctness here. It should be
>     fine to generate a graph file without any given commit; readers will
>     just have to load that commit the old-fashioned way. But at this
>     phase of "commit-graph write", I think we'll already have done the
>     close_reachable() check. What does it mean to throw away a commit at
>     this stage? If we're the parent of another commit, then it will have
>     trouble referring to us by a uint32_t. Will the actual writing phase
>     barf, or will we generate an invalid graph file?

diff --git a/commit-graph.c b/commit-graph.c
index 3da52847e4..71419c2532 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1596,6 +1596,19 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 	}
 }
 
+static int pretend_commit_is_missing(const struct object_id *oid)
+{
+	static int initialized;
+	static struct object_id missing;
+	if (!initialized) {
+		const char *x = getenv("PRETEND_COMMIT_IS_MISSING");
+		if (x)
+			get_oid_hex(x, &missing);
+		initialized = 1;
+	}
+	return oideq(&missing, oid);
+}
+
 static void merge_commit_graph(struct write_commit_graph_context *ctx,
 			       struct commit_graph *g)
 {
@@ -1612,6 +1625,11 @@ static void merge_commit_graph(struct write_commit_graph_context *ctx,
 
 		load_oid_from_graph(g, i + offset, &oid);
 
+		if (pretend_commit_is_missing(&oid)) {
+			warning("pretending %s is missing", oid_to_hex(&oid));
+			continue;
+		}
+
 		/* only add commits if they still exist in the repo */
 		result = lookup_commit_reference_gently(ctx->r, &oid, 1);
 

and then I make a fully-graphed repo like this:

  git init repo
  cd repo
  for i in $(seq 10); do
    git commit --allow-empty -m $i
  done
  git commit-graph write --input=reachable --split=no-merge

if we pretend a parent is missing, I get a BUG():

  $ git rev-parse HEAD |
    PRETEND_COMMIT_IS_MISSING=$(git rev-parse HEAD^) \
    git commit-graph write --stdin-commits --split=merge-all
  warning: pretending 35e6e15c738cf2bfbe495957b2a941c2efe86dd9 is missing
  BUG: commit-graph.c:879: missing parent 35e6e15c738cf2bfbe495957b2a941c2efe86dd9 for commit d4141fb57a9bbe26b247f23c790d63d078977833
  Aborted

So it seems like just skipping here (either with the new patch or
without) isn't really a good strategy.

-Peff