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[104.178.186.189]) by smtp.gmail.com with ESMTPSA id j71-20020a25d24a000000b00c5fc63686f1sm417318ybg.16.2023.08.10.09.00.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Aug 2023 09:00:44 -0700 (PDT) Date: Thu, 10 Aug 2023 12:00:43 -0400 From: Taylor Blau To: Jeff King Cc: git@vger.kernel.org, Derrick Stolee Subject: Re: [RFC/PATCH] commit-graph: verify swapped zero/non-zero generation cases Message-ID: References: <20230808191536.GA4033224@coredump.intra.peff.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230808191536.GA4033224@coredump.intra.peff.net> Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Tue, Aug 08, 2023 at 03:15:36PM -0400, Jeff King wrote: > This is marked as RFC because I'm still confused about a lot of things. > For one, my explanation above about what the code is doing is mostly a > guess. It _looks_ to me like that's what the existing check is trying to > do. But if so, then why is the generation_zero flag defined outside the > loop over each object? I'd think it would be a per-object thing. I thought the same thing initially, but looking back at 1373e547f7 (commit-graph: verify generation number, 2018-06-27), I think the scope of generation_zero is correct. This is an artifact from when commit-graphs were written with all commit generation numbers equal to zero. So I think the logic is something like: - If the commit-graph has a generation number of 0 for some commit, but we saw a non-zero value from any another commit, report it. - Otherwise, if the commit-graph had a non-zero value for the commit's generation number, and we had previously seen a generation number of zero for some other commit, report it. IOW, I think we expect to see either all zeros, or all non-zero values in a single commit-graph's set of generation numbers. Earlier in your message, you wrote: > There's a matching GENERATION_NUMBER_EXISTS value, which in theory would > be used to find the case that we see the entries in the opposite order: > > 1. When we see an entry with a non-zero generation, we set the > generation_zero flag to GENERATION_NUMBER_EXISTS. > > 2. When we later see an entry with a zero generation, we complain if > the flag is GENERATION_NUMBER_EXISTS. > > But that doesn't work; step 2 is implemented, but there is no step 1. We > never use NUMBER_EXISTS at all, and Coverity rightly complains that step > 2 is dead code. So I think the missing part is setting GENERATION_NUMBER_EXISTS when we have a non-zero generation number from the commit-graph, but have generation_zero set to GENERATION_ZERO_EXISTS (IOW, we have seen at least one commit with generation number 0). --- 8< --- diff --git a/commit-graph.c b/commit-graph.c index 0aa1640d15..935bc15440 100644 --- a/commit-graph.c +++ b/commit-graph.c @@ -2676,9 +2676,11 @@ static int verify_one_commit_graph(struct repository *r, graph_report(_("commit-graph has generation number zero for commit %s, but non-zero elsewhere"), oid_to_hex(&cur_oid)); generation_zero = GENERATION_ZERO_EXISTS; - } else if (generation_zero == GENERATION_ZERO_EXISTS) + } else if (generation_zero == GENERATION_ZERO_EXISTS) { graph_report(_("commit-graph has non-zero generation number for commit %s, but zero elsewhere"), oid_to_hex(&cur_oid)); + generation_zero = GENERATION_NUMBER_EXISTS; + } if (generation_zero == GENERATION_ZERO_EXISTS) continue; --- >8 --- > So I kind of wonder if there's something I'm not getting here. Coverity > is definitely right that our "step 2" is dead code (because we never set > NUMBER_EXISTS). But I'm not sure if we should be deleting it, or trying > to fix an underlying bug. I think that above is correct in that we should be fixing an underlying bug. But the fact that this isn't caught by our existing tests indicates that there is a gap in coverage. Let me see if I can find a test case that highlights this bug... Thanks, Taylor