Re: [PATCH 1/1] diff: munmap() file contents before running external diff

From: Junio C Hamano <gitster@pobox.com>
Date: Jul 8, 2019, 21:54 UTC
Message-ID: <xmqq8st8xkdg.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <bef83fc20b9e2048872316e3f396da0cb6775086.1562231781.git.gitgitgadget@gmail.com>

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 6 quoted lines

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> When running an external diff from, say, a diff tool, it is safe to
> assume that we want to write the files in question. On Windows, that
> means that there cannot be any other process holding an open handle to
> said files.

Please add "It is not enough to close the file descriptor; having a region that is still mmapped keeps the file busy" or something like that at the end.

Show 22 quoted lines

> So let's make sure that `git diff` itself is not holding any open handle
> to the files in question.
>
> This fixes https://github.com/git-for-windows/git/issues/1315
>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  diff.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/diff.c b/diff.c
> index 4d3cf83a27..0afb76bbca 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -4206,6 +4206,10 @@ static void run_external_diff(const char *pgm,
>  	argv_array_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter);
>  	argv_array_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr);
>  
> +	if (one && one->should_munmap)
> +		diff_free_filespec_data(one);
> +	if (two && two->should_munmap)
> +		diff_free_filespec_data(two);

I wondered if a single diff_filespec instance can be used in two diff_filepair instances (e.g. file A is in-place modified and also used to create file C), and if so after showing the diff for file A, we have problems with showing file C. But I do not think it should pose a problem, as "free data after comparing a pair" is what we do for the in-core codepath in builtin_diff().

We can lose the NULL-ness test for one and two if these "free the resource once we no longer need it" is done inside "if (one && two)". After all, once add_external_diff_name()[*1*] does its thing, we do not need the data for these diff_filespec instances, right?

Also, just like builtin_diff() unconditionally frees the resources held by diff_filespec instances, shouldn't this function do so, even the ones that are not marked with should_munmap?

>  	if (run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env.argv))
>  		die(_("external diff died, stopping at %s"), name);