[PATCH v4] diff: make diff_free_filespec_data accept NULL

[Jinoh Kang <luke1337@theori.io>]

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