From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-4.0 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_NONE shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 25FC41F463 for ; Thu, 12 Sep 2019 14:23:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732518AbfILOXI (ORCPT ); Thu, 12 Sep 2019 10:23:08 -0400 Received: from cloud.peff.net ([104.130.231.41]:47890 "HELO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1732444AbfILOXH (ORCPT ); Thu, 12 Sep 2019 10:23:07 -0400 Received: (qmail 13808 invoked by uid 109); 12 Sep 2019 14:23:07 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with SMTP; Thu, 12 Sep 2019 14:23:07 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 8103 invoked by uid 111); 12 Sep 2019 14:25:07 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.94) with (TLS_AES_256_GCM_SHA384 encrypted) ESMTPS; Thu, 12 Sep 2019 10:25:07 -0400 Authentication-Results: peff.net; auth=none Date: Thu, 12 Sep 2019 10:23:06 -0400 From: Jeff King To: Derrick Stolee Cc: git@vger.kernel.org, Taylor Blau , Derrick Stolee , =?utf-8?B?Tmd1eeG7hW4gVGjDoWkgTmfhu41j?= Duy Subject: Re: [PATCH] upload-pack: disable commit graph more gently for shallow traversal Message-ID: <20190912142306.GE23031@sigill.intra.peff.net> References: <20190912000414.GA31334@sigill.intra.peff.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org 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