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?
It doesn't seem great. If I instrument Git like this to simulate an
object temporarily "missing" (if it were really missing the whole repo
would be corrupt; we're trying to see what would happen if a race causes
us to momentarily not see it):
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
