[PATCH] diff-lib: ignore all outsider if --relative asked

["Đoàn Trần Công Danh" <congdanhqx@gmail.com>]

For diff family commands, we can tell them to exclude changes outside
of some directories if --relative is requested.

In diff_unmerge(), NULL will be returned if the requested path is
outside of the interesting directories, thus we'll run into NULL
pointer dereference in run_diff_files when trying to dereference
its return value.

We can simply check for NULL there before dereferencing said
return value.  However, we can do better by not running diff
on those unintesting entries.  Let's do that instead.

Reported-by: Thomas De Zeeuw <thomas@slight.dev>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
---

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

Notes:
    Check for return value of diff_unmerge is not enough.
    
    Yes, it works with --name-only, however, with only --relative,
    git-diff shows unmerged entries outside of subdir, too.
    
    Furthermore, the filename in "diff --cc" ignores the relative prefix.
    Fixing this requires touching all over places, at least from my study.
    Let's fix the crash, first.
    
    We have two choices here:
    
    * Check pair, aka return value of diff_unmerge, like my original
      suggestion, and the unmerged entries from outside will be shown, too.
      Some inconsistent will be observed, --name-only won't list files
      outside of subdir, while the patch shows them.  At least, it doesn't
      create false impression of no change outside of subdir.
    
    * Skip all outsiders, like this patch.
    
    While I prefer this approach, I don't know all ramifications of this change,
    let's say an entry moved to outside of subdir in one side, and modified in
    other side.
    
    Because, I pick the different approach, Junio's ack isn't included here.
    

 diff-lib.c               |  4 +++
 t/t4045-diff-relative.sh | 53 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/diff-lib.c b/diff-lib.c
index f9eadc4fc1..ca085a03ef 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -117,6 +117,10 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		if (!ce_path_match(istate, ce, &revs->prune_data, NULL))
 			continue;
 
+		if (revs->diffopt.prefix &&
+		    strncmp(ce->name, revs->diffopt.prefix, revs->diffopt.prefix_length))
+			continue;
+
 		if (ce_stage(ce)) {
 			struct combine_diff_path *dpath;
 			struct diff_filepair *pair;
diff --git a/t/t4045-diff-relative.sh b/t/t4045-diff-relative.sh
index 61ba5f707f..8cbbe53262 100755
--- a/t/t4045-diff-relative.sh
+++ b/t/t4045-diff-relative.sh
@@ -162,4 +162,57 @@ check_diff_relative_option subdir file2 true --no-relative --relative
 check_diff_relative_option . file2 false --no-relative --relative=subdir
 check_diff_relative_option . file2 true --no-relative --relative=subdir
 
+test_expect_success 'setup diff --relative unmerged' '
+	test_commit zero file0 &&
+	test_commit base subdir/file0 &&
+	git switch -c br1 &&
+	test_commit one file0 &&
+	test_commit sub1 subdir/file0 &&
+	git switch -c br2 base &&
+	test_commit two file0 &&
+	git switch -c br3 &&
+	test_commit sub3 subdir/file0
+'
+
+test_expect_success 'diff --relative without change in subdir' '
+	git switch br2 &&
+	test_when_finished "git merge --abort" &&
+	test_must_fail git merge one &&
+	git -C subdir diff --relative >out &&
+	test_must_be_empty out &&
+	git -C subdir diff --relative --name-only >out &&
+	test_must_be_empty out
+'
+
+test_expect_success 'diff --relative --name-only with change in subdir' '
+	git switch br3 &&
+	test_when_finished "git merge --abort" &&
+	test_must_fail git merge sub1 &&
+	test_write_lines file0 file0 >expected &&
+	git -C subdir diff --relative --name-only >out &&
+	test_cmp expected out
+'
+
+test_expect_failure 'diff --relative with change in subdir' '
+	git switch br3 &&
+	br1_blob=$(git rev-parse --short --verify br1:subdir/file0) &&
+	br3_blob=$(git rev-parse --short --verify br3:subdir/file0) &&
+	test_when_finished "git merge --abort" &&
+	test_must_fail git merge br1 &&
+	cat >expected <<-EOF &&
+	diff --cc file0
+	index $br3_blob,$br1_blob..0000000
+	--- a/file0
+	+++ b/file0
+	@@@ -1,1 -1,1 +1,5 @@@
+	++<<<<<<< HEAD
+	 +sub3
+	++=======
+	+ sub1
+	++>>>>>>> sub1
+	EOF
+	git -C subdir diff --relative >out &&
+	test_cmp expected out
+'
+
 test_done
-- 
2.33.0.254.g68ee769121