Re: [PATCH v3 2/2] apply: reload .gitattributes after patching it

From: Junio C Hamano <gitster@pobox.com>
To: "brian m. carlson" <sandals@crustytoothpaste.net>
Cc: git@vger.kernel.org, "SZEDER Gábor" <szeder.dev@gmail.com>,
	"Taylor Blau" <me@ttaylorr.com>, "Jeff King" <peff@peff.net>
Subject: Re: [PATCH v3 2/2] apply: reload .gitattributes after patching it
Date: Thu, 15 Aug 2019 15:10:29 -0700	[thread overview]
Message-ID: <xmqqftm22i7e.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <xmqq7e7h7xap.fsf@gitster-ct.c.googlers.com> (Junio C. Hamano's message of "Tue, 13 Aug 2019 11:08:46 -0700")

Junio C Hamano <gitster@pobox.com> writes:

> "brian m. carlson" <sandals@crustytoothpaste.net> writes:
>
>> When applying multiple patches with git am, or when rebasing using the
>> am backend, it's possible that one of our patches has updated a
>> gitattributes file. Currently, we cache this information, so if a
>> file in a subsequent patch has attributes applied, the file will be
>> written out with the attributes in place as of the time we started the
>> rebase or am operation, not with the attributes applied by the previous
>> patch. This problem does not occur when using the -m or -i flags to
>> rebase.
> ...
> "rebase -m" and "rebase -i" are not repeated run_command() calls
> that invoke "git cherry-pick" or "git merge" these days, either, so
> I am somewhat curious how they avoid fallilng into the same trap.
>
> Thanks for the fix.  Will queue.

Actually there still is one more thing I wasn't clear about the
change.

> To ensure we write the correct data into the working tree, expire the
> cache after each patch that touches a path ending in ".gitattributes".
> ...
> +			if (!flush_attributes && patch->new_name &&
> +			    ends_with_path_components(patch->new_name, GITATTRIBUTES_FILE))
> +				flush_attributes = 1;

When an attribute file is removed by a patch, we should forget what
we read earlier from the file before it got removed.  Would such a
case, where patch->new_name would be NULL, be handled correctly?

The call to ends_with_path_components() is almost no cost, and I
would suspect that this call is easier to reason about without the
"!flush_attributes &&" in the conditional part, by the way.

Thanks.