Re: [PATCH v5] diff: make diff_free_filespec_data accept NULL

> diff_free_filespec_data crashes when passed a NULL fillspec pointer.
> Commit 3aef54e8b8 ("diff: munmap() file contents before running external
> diff", 2019-07-11) introduced calls to diff_free_filespec_data in
> run_external_diff without also checking if the argument is NULL.
>
> Git uses NULL filespecs to indicate unmerged files when merge conflict
> resolution is in progress.  Fortunately, other code paths bail out early
> even before NULL can reach diff_free_filespec_data(); however, difftool
> is expected to do a full-blown diff anyway regardless of conflict
> status.
>
> Fix this and prevent any similar bugs in the future by making
> `diff_free_filespec_data(NULL)` a no-op.
>
> Add a test case that confirms that running difftool --cached with
> unmerged files does not result in a SIGSEGV.
>
> Signed-off-by: Jinoh Kang <luke1337@theori.io>

> ---
>  diff.c              |  3 +++
>  t/t7800-difftool.sh | 13 +++++++++++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/diff.c b/diff.c
> index d24f47df99..ace4a1d387 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -4115,6 +4115,9 @@ void diff_free_filespec_blob(struct diff_filespec *s)
>
>  void diff_free_filespec_data(struct diff_filespec *s)
>  {
> +	if (!s)
> +		return;
> +
>  	diff_free_filespec_blob(s);
>  	FREE_AND_NULL(s->cnt_data);
>  }
> diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
> index 524f30f7dc..a578b35761 100755
> --- a/t/t7800-difftool.sh
> +++ b/t/t7800-difftool.sh
> @@ -728,6 +728,19 @@ test_expect_success 'add -N and difftool -d' '
>  	git difftool --dir-diff --extcmd ls
>  '
>
> +test_expect_success 'difftool --cached with unmerged files' '
> +	test_when_finished git reset --hard &&
> +
> +	test_commit conflicting &&
> +	test_commit conflict-a conflict.t a &&
> +	git reset --hard conflicting &&
> +	test_commit conflict-b conflict.t b &&
> +	test_must_fail git merge conflict-a &&
> +
> +	git difftool --cached --no-prompt >output &&
> +	test_must_be_empty output
> +'
> +
>  test_expect_success 'outside worktree' '
>  	echo 1 >1 &&
>  	echo 2 >2 &&
> --
> 2.26.2
>
>