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From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Derrick Stolee <stolee@gmail.com>
Cc: git@vger.kernel.org, "Junio C Hamano" <gitster@pobox.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Eric Sunshine" <sunshine@sunshineco.com>,
	"Ramsay Jones" <ramsay@ramsayjones.plus.com>
Subject: Re: [PATCH v3 4/8] commit-graph: don't early exit(1) on e.g. "git status"
Date: Mon, 29 Apr 2019 16:30:38 +0200	[thread overview]
Message-ID: <87wojcrii9.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <3518ad3e-bc4a-c2c3-d4bd-c87f9e828b1c@gmail.com>


On Mon, Apr 29 2019, Derrick Stolee wrote:

> On 4/27/2019 9:06 AM, Ævar Arnfjörð Bjarmason wrote:
>>
>> There's still cases left where we'll exit early, e.g. if you do:
>>
>>     $ git diff -U1
>>     diff --git a/commit-graph.c b/commit-graph.c
>>     index 66865acbd7..63773764ce 100644
>>     --- a/commit-graph.c
>>     +++ b/commit-graph.c
>>     @@ -1074,3 +1074,3 @@ void write_commit_graph(const char *obj_dir,
>>             chunk_offsets[1] = chunk_offsets[0] + GRAPH_FANOUT_SIZE;
>>     -       chunk_offsets[2] = chunk_offsets[1] + hashsz * commits.nr;
>>     +       chunk_offsets[2] = chunk_offsets[0] + hashsz * commits.nr;
>>             chunk_offsets[3] = chunk_offsets[2] + (hashsz + 16) * commits.nr;
>>
>> Which is obviously bad, but something I encounterd while hacking up [1]
>> we'll still hard die as before this patch on:
>>
>>     $ git status
>>     fatal: invalid parent position 1734910766
>>     $
>
> I really appreciate you digging in deep into these kinds of issues. You
> seem to be hitting corrupted commit-graph files more often than we are
> (in VFS for Git world).

FWIW I've never encountered any of these in the wild. I just started
poking at this in 2ac138d568 ("commit-graph: fix segfault on e.g. "git
status"", 2019-03-25) because I was looking at the commit graph, running
its tests with -d, and we'd segfault previously on e.g. "git status" in
our own graph corruption tests.

> However, we should be _very careful_ when turning some of these errors
> to warnings. At the very least, we should do some high-level planning
> for how to handle this case.

Indeed. I should have been explicit, I don't think it's sane to do
anything except return NULL up the stack and say "the graph is screwed,
we can't use it" when initially parsing it/headers, but reading on...

> The biggest issue is that we have some logic that is run after a call to
> generation_numbers_enabled(), such as the `git rev-list --topo-order`
> logic, that relies on the commit-graph for correctness. If we output a
> warning and then stop using the commit-graph, then we will start having
> commits with finite generation pointing to commits with infinite generation.
>
> Perhaps, with some care, we can alert the algorithm to change the "minimum
> generation" that limits how far we dequeue the priority-queue. Changing it
> to zero will cause the algorithm to behave like the old algorithm.
>
> But, having an algorithm that usually takes 0.1 seconds suddenly take 10+
> seconds also violates some expectations.
>
> Q: How should we handle a detectably-invalid commit-graph?

...I don't think we need to do any paranoid algorithm fallback in
general. As you point out that's going to be a PITA as we read the
actual data in the graph in some cases.

> I think most of your patches have done a good job so far of detecting
> an invalid header, and responding by ignoring the commit-graph. This case
> of a detectable error in the chunk data itself is not something we can
> check on the first load without serious performance issues.
>
> I hope we can decide on a good solution.

...OK, so this is one of those PITA cases. I think it's fine to just
leave it.

Although maybe we'd still want to be more paranoid with O(n) cases like
"--contains" or "--topo-order" and cases that are surely just "look up
my immediate commit data", like "status" dying on this particular error.

But I think it's fine to just decide to nothing about this. I mainly
just wanted to make a note to myself & CC the list in case there was
interest...

  reply	other threads:[~2019-04-29 14:30 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-21 22:37 [PATCH 0/8] commit-graph: segfault & other fixes for broken graphs Ævar Arnfjörð Bjarmason
2019-02-21 22:37 ` [PATCH 1/8] commit-graph tests: split up corrupt_graph_and_verify() Ævar Arnfjörð Bjarmason
2019-02-21 22:37 ` [PATCH 2/8] commit-graph tests: test a graph that's too small Ævar Arnfjörð Bjarmason
2019-02-21 22:37 ` [PATCH 3/8] commit-graph: fix segfault on e.g. "git status" Ævar Arnfjörð Bjarmason
2019-02-21 22:37 ` [PATCH 4/8] commit-graph: don't early exit(1) " Ævar Arnfjörð Bjarmason
2019-02-21 22:37 ` [PATCH 5/8] commit-graph: don't pass filename to load_commit_graph_one_fd_st() Ævar Arnfjörð Bjarmason
2019-02-21 22:37 ` [PATCH 6/8] commit-graph verify: detect inability to read the graph Ævar Arnfjörð Bjarmason
2019-02-21 23:15   ` SZEDER Gábor
2019-02-21 22:37 ` [PATCH 7/8] commit-graph write: don't die if the existing graph is corrupt Ævar Arnfjörð Bjarmason
2019-02-22 23:13   ` Junio C Hamano
2019-02-23  9:29     ` Eric Sunshine
2019-02-21 22:37 ` [PATCH 8/8] commit-graph: improve & i18n error messages Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 0/8] commit-graph: segfault & other fixes for broken graphs Ævar Arnfjörð Bjarmason
2019-03-15  7:47   ` Eric Sunshine
2019-03-15 10:39     ` Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 " Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 1/8] commit-graph tests: split up corrupt_graph_and_verify() Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 2/8] commit-graph tests: test a graph that's too small Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 3/8] commit-graph: fix segfault on e.g. "git status" Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 4/8] commit-graph: don't early exit(1) " Ævar Arnfjörð Bjarmason
2019-04-27 13:06     ` Ævar Arnfjörð Bjarmason
2019-04-29 12:48       ` Derrick Stolee
2019-04-29 14:30         ` Ævar Arnfjörð Bjarmason [this message]
2019-05-01 18:31         ` Jeff King
2019-03-25 12:08   ` [PATCH v3 5/8] commit-graph: don't pass filename to load_commit_graph_one_fd_st() Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 6/8] commit-graph verify: detect inability to read the graph Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 7/8] commit-graph write: don't die if the existing graph is corrupt Ævar Arnfjörð Bjarmason
2019-03-25 12:08   ` [PATCH v3 8/8] commit-graph: improve & i18n error messages Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 1/8] commit-graph tests: split up corrupt_graph_and_verify() Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 2/8] commit-graph tests: test a graph that's too small Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 3/8] commit-graph: fix segfault on e.g. "git status" Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 4/8] commit-graph: don't early exit(1) " Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 5/8] commit-graph: don't pass filename to load_commit_graph_one_fd_st() Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 6/8] commit-graph verify: detect inability to read the graph Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 7/8] commit-graph write: don't die if the existing graph is corrupt Ævar Arnfjörð Bjarmason
2019-03-14 21:47 ` [PATCH v2 8/8] commit-graph: improve & i18n error messages Ævar Arnfjörð Bjarmason

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