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From: Jonathan Nieder <jrnieder@gmail.com>
To: Derrick Stolee <stolee@gmail.com>
Cc: Taylor Blau <me@ttaylorr.com>,
	git@vger.kernel.org, jonathantanmy@google.com, gitster@pobox.com,
	newren@gmail.com, Jay Conrod <jayconrod@google.com>
Subject: Re: [PATCH v2 2/2] shallow.c: use '{commit,rollback}_shallow_file'
Date: Wed, 3 Jun 2020 14:23:36 -0700	[thread overview]
Message-ID: <20200603212336.GD253041@google.com> (raw)
In-Reply-To: <1253efb6-f1bc-0a16-68e3-c1bc07e1bc18@gmail.com>

Hi,

Derrick Stolee wrote:
> On 6/3/2020 1:16 AM, Taylor Blau wrote:

>>   * Keep the shallow bit sticky, at least for fetch.writeCommitGraph
>>     (i.e., pretend as if fetch.writecommitgraph=0 in the case of
>>     '--unshallow').
>
> I'm in favor of this option, if possible. Anything that alters the
> commit history in-memory at any point in the Git process is unsafe to
> combine with a commit-graph read _or_ write. I'm sorry that the guards
> in commit_graph_compatible() are not enough here.

As described in [1], I agree --- this kind of "dirty bit" is a good
option and seems like the right thing to do here.

And I'm glad you said read _or_ write here.  I hadn't realized that
this check already applies for reads, and I'm very happy it does.

[...]
>>   * Dump the object cache upon un-shallowing, forcing us to re-discover
>>     the parents when they are no longer hidden behind a graft.
>>
>> The latter seems like the most complete feasible fix. The former should
>> work fine to address this case, but I wonder if there are other
>> call-sites that are affected by this behavior. My hunch is that this is
>> a unique case, since it requires going from shallow to unshallow in the
>> same process.
>
> The latter would solve issues that could arise outside of the commit-graph
> space. But it also presents an opportunity for another gap if someone edits
> the shallow logic without putting in the proper guards.

This, however, I don't agree with.

I'm a strong believer in having clear invariants --- without them,
code can only be understood empirically, and
https://ieeexplore.ieee.org/document/9068304 describes how painful of
a world that can be.

So because shallow is specifically about changing the set of parents
in objects used for traversal, I want to make sure that we as
reviewers will push back on any potential other new meaning of
shallow.  *If* we had a safe way to invalidate the object cache, it
would be sufficient and would be the right thing to do.  As described
in [1], we unfortunately don't have such a thing.

Okay, that's all an aside.  Now for the actual reason I'm replying:

I had been wondering why this wasn't caught at read time, but of
course --unshallow clears away the shallows so there was no way for
that to happen.  So what I wonder is, why isn't this caught by fsck?
Can "git fsck" run "git commit-graph verify" automatically and include
its result as part of what it reports?

Thanks,
Jonathan

[1] https://lore.kernel.org/git/20200603205151.GC253041@google.com/

  parent reply	other threads:[~2020-06-03 21:23 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-21 18:09 [PATCH] shallow.c: use 'reset_repository_shallow' when appropriate Taylor Blau
2020-04-21 20:41 ` Junio C Hamano
2020-04-21 20:45   ` Taylor Blau
2020-04-21 20:52     ` Junio C Hamano
2020-04-21 22:21       ` Taylor Blau
2020-04-21 23:06         ` Junio C Hamano
2020-04-22 18:05       ` Jonathan Tan
2020-04-22 18:02 ` Jonathan Tan
2020-04-22 18:15   ` Junio C Hamano
2020-04-23  0:14     ` Taylor Blau
2020-04-23  0:25       ` [PATCH v2 0/2] shallow.c: reset shallow-ness after updating Taylor Blau
2020-04-23  0:25         ` [PATCH v2 1/2] t5537: use test_write_lines, indented heredocs for readability Taylor Blau
2020-04-23  1:14           ` Jonathan Nieder
2020-04-24 17:11             ` Taylor Blau
2020-04-24 17:17               ` Jonathan Nieder
2020-04-24 20:45               ` Junio C Hamano
2020-04-23  0:25         ` [PATCH v2 2/2] shallow.c: use '{commit,rollback}_shallow_file' Taylor Blau
2020-04-23  1:23           ` Jonathan Nieder
2020-04-23 18:09           ` Jonathan Tan
2020-04-23 20:40             ` Junio C Hamano
2020-04-24 17:13               ` Taylor Blau
2020-06-03  3:42           ` Jonathan Nieder
2020-06-03  4:52             ` Taylor Blau
2020-06-03  5:16               ` Taylor Blau
2020-06-03 13:08                 ` Derrick Stolee
2020-06-03 19:26                   ` Taylor Blau
2020-06-03 21:23                   ` Jonathan Nieder [this message]
2020-06-03 20:51                 ` Jonathan Nieder
2020-06-03 22:14                   ` Taylor Blau
2020-06-03 23:06                     ` Jonathan Nieder
2020-06-04 17:45                       ` Taylor Blau
2020-04-23 19:05       ` [PATCH] shallow.c: use 'reset_repository_shallow' when appropriate Junio C Hamano

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