Re: [PATCH v4] diff: make diff_free_filespec_data accept NULL

> Today, diff_free_filespec_data crashes when passed a NULL pointer.
> Commit 3aef54e8b8 ("diff: munmap() file contents before running external
> diff") introduced calls to diff_free_filespec_data in run_external_diff,
> which may pass NULL pointers.
>
> 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.
>
> Also, add a test case that confirms that running difftool --cached with
> unmerged files does not SIGSEGV.
>
> Signed-off-by: Jinoh Kang <luke1337@theori.io>
> ---
>  diff.c              |  3 +++
>  t/t7800-difftool.sh | 23 +++++++++++++++++++++++
>  2 files changed, 26 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..e9391abb54 100755
> --- a/t/t7800-difftool.sh
> +++ b/t/t7800-difftool.sh
> @@ -728,6 +728,29 @@ 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 &&
> +	echo base >file &&
> +	git add file &&
> +	git commit -m base &&
> +	git checkout -B conflict-a &&
> +	git checkout -B conflict-b &&
> +	git checkout conflict-a &&
> +	echo conflict-a >>file &&
> +	git add file &&
> +	git commit -m conflict-a &&
> +	git checkout conflict-b &&
> +	echo conflict-b >>file &&
> +	git add file &&
> +	git commit -m conflict-b &&
> +	git checkout master &&
> +	git merge conflict-a &&
> +	test_must_fail git merge conflict-b &&
> +	: >expect &&
> +	git difftool --cached --no-prompt >actual &&
> +	test_cmp expect actual
> +'
> +
>  test_expect_success 'outside worktree' '
>  	echo 1 >1 &&
>  	echo 2 >2 &&
> --
> 2.26.2
>