On Thu, Sep 12, 2019 at 08:23:49AM -0400, Derrick Stolee wrote:
> > That creates an interesting problem for commits that have _already_ been
> > parsed using the commit graph. Their commit->object.parsed flag is set,
> > their commit->graph_pos is set, but their commit->maybe_tree may still
> > be NULL. When somebody later calls repo_get_commit_tree(), we see that
> > we haven't loaded the tree oid yet and try to get it from the commit
> > graph. But since it has been freed, we segfault!
>
> OOPS! That is certainly a bad thing. I'm glad you found it, but I
> am sorry for how you (probably) found it.
Heh. I'll admit it was quite a slog of debugging, but _most_ of that was
figuring out in which circumstance we'd have actually parsed the object.
Finding the problematic end state was pretty easy from a coredump. :)
> > diff --git a/commit-graph.c b/commit-graph.c
> > index 9b02d2c426..bc5dd5913f 100644
> > --- a/commit-graph.c
> > +++ b/commit-graph.c
> > @@ -41,6 +41,8 @@
> > #define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * GRAPH_CHUNKLOOKUP_WIDTH \
> > + GRAPH_FANOUT_SIZE + the_hash_algo->rawsz)
> >
> > +static int commit_graph_disabled;
>
> Should we be putting this inside the repository struct instead?
Probably. The only caller will just pass the_repository, but it doesn't
hurt to scope it down now.
It could potentially go into the commit_graph itself, but it looks like
with the incremental work we may have multiple such structs. It could
also go into raw_object_store, but I think conceptually it's a
repo-level thing.
So I put it straight into "struct repository".
> Your patch does not seem to actually cover the "I've already parsed some commits"
> case, as you are only preventing the commit-graph from being prepared. Instead,
> we need to have a short-circuit inside parse_commit() to avoid future parsing
> from the commit-graph file.
Maybe I was too clever, then. :)
I didn't want to have to sprinkle "are we disabled" in parse_commit(),
etc. But any such uses of the commit graph have to do:
if (!prepare_commit_graph(r))
return;
to lazy-load it. So the logic to prepare becomes (roughly):
if (disabled)
return 0;
if (already_loaded)
return 1;
return actually_load() ? 1 : 0;
and "disabled" takes precedence.
I've added this comment in prepare_commit_graph():
/*
* This must come before the "already attempted?" check below, because
* we want to disable even an already-loaded graph file.
*/
if (r->commit_graph_disabled)
return 0;
if (r->objects->commit_graph_attempted)
return !!r->objects->commit_graph;
r->objects->commit_graph_attempted = 1;
Does that make more sense?
Unrelated, but I also notice the top of prepare_commit_graph() has:
if (git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD, 0))
die("dying as requested by the '%s' variable on commit-graph load!",
GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD);
as the very first thing. Meaning we're calling getenv() as part of every
single parse_commit(), rather than just once per process. Seems like an
easy efficiency win.
-Peff
