diffcore-break: prevent dangling pointer

After we have freed the file pair, we should set the queue reference to null.
This prevents us from encountering a dangling pointer later on.

The test uses git reset to trigger prefetching after break-rewrites have freed
the file pair.

Signed-off-by: Han Young <hanyang.tony@bytedance.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: hanyang-tony

diffcore-break.c

@@ -222,6 +222,7 @@ void diffcore_break(struct repository *r, int break_score)
 				free(p); /* not diff_free_filepair(), we are
 					  * reusing one and two here.
 					  */
+				q->queue[i] = NULL;
 				continue;
 			}
 		}

t/t4067-diff-partial-clone.sh

@@ -132,6 +132,36 @@ test_expect_success 'diff with rename detection batches blobs' '
 	test_line_count = 1 done_lines
 '
 
+test_expect_success 'diff succeeds even if prefetch triggered by break-rewrites' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo xyz >server/foo &&
+	mkdir server/bar &&
+	test_seq -f "line %d" 1 100 >server/bar/baz &&
+	git -C server add -A &&
+	git -C server commit -m x &&
+
+
+	echo xyzz >server/foo &&
+	rm server/bar/baz &&
+	test_seq -f "line %d" 90 190 >server/bar/baz &&
+	git -C server add -A &&
+	git -C server commit -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Fetch bar/baz without fetching foo.
+	git -C client checkout HEAD~1 bar &&
+	# Ensure baz has diff
+	git -C client reset --hard HEAD &&
+
+	# reset's break-rewrites detection will trigger prefetch
+	git -C client reset HEAD~1
+'
+
 test_expect_success 'diff succeeds even if entries are removed from queue' '
 	test_when_finished "rm -rf server client trace" &&