t4058: add more tests and documentation for duplicate tree entry handling

newren · gitster · commit 5c72261c664e · 2020-12-14T09:34:50.000-08:00
Commit 4d6be03 ("diffcore-rename: avoid processing duplicate destinations", 2015-02-26) added t4058 to demonstrate that a workaround it added to avoid double frees (namely to just turn off rename detection when trees had duplicate entries) would indeed avoid segfaults. The tests, though, give the impression that the expected diffs are "correct" when in reality they are just "don't segfault, and do something semi-reasonable under the circumstances". Add some notes to make this clearer. Also, commit 25d5ea4 ("[PATCH] Redo rename/copy detection logic.", 2005-05-24) added a similar workaround to avoid segfaults, but for rename_src rather than rename_dst. I do not see any tests in the testsuite to cover the collision detection of entries limited to the source side, so add a couple. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t4058-diff-duplicates.sh b/t/t4058-diff-duplicates.sh
@@ -1,5 +1,14 @@
 #!/bin/sh
 
+# NOTICE:
+#   This testsuite does a number of diffs and checks that the output match.
+#   However, it is a "garbage in, garbage out" situation; the trees have
+#   duplicate entries for individual paths, and it results in diffs that do
+#   not make much sense.  As such, it is not clear that the diffs are
+#   "correct".  The primary purpose of these tests was to verify that
+#   diff-tree does not segfault, but there is perhaps some value in ensuring
+#   that the diff output isn't wildly unreasonable.
+
 test_description='test tree diff when trees have duplicate entries'
 . ./test-lib.sh
 
@@ -57,7 +66,16 @@ test_expect_success 'create trees with duplicate entries' '
 	git tag two $outer_two
 '
 
-test_expect_success 'diff-tree between trees' '
+test_expect_success 'create tree without duplicate entries' '
+	blob_one=$(echo one | git hash-object -w --stdin) &&
+	outer_three=$(make_tree \
+		100644 renamed $blob_one
+	) &&
+	git tag three $outer_three
+'
+
+test_expect_success 'diff-tree between duplicate trees' '
+	# See NOTICE at top of file
 	{
 		printf ":000000 100644 $ZERO_OID $blob_two A\touter/inner\n" &&
 		printf ":000000 100644 $ZERO_OID $blob_two A\touter/inner\n" &&
@@ -71,9 +89,34 @@ test_expect_success 'diff-tree between trees' '
 '
 
 test_expect_success 'diff-tree with renames' '
-	# same expectation as above, since we disable rename detection
+	# See NOTICE at top of file.
 	git diff-tree -M -r --no-abbrev one two >actual &&
 	test_cmp expect actual
 '
 
+test_expect_success 'diff-tree FROM duplicate tree' '
+	# See NOTICE at top of file.
+	{
+		printf ":100644 000000 $blob_one $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":000000 100644 $ZERO_OID $blob_one A\trenamed\n"
+	} >expect &&
+	git diff-tree -r --no-abbrev one three >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'diff-tree FROM duplicate tree, with renames' '
+	# See NOTICE at top of file.
+	{
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 100644 $blob_one $blob_one R100\touter/inner\trenamed\n"
+	} >expect &&
+	git diff-tree -M -r --no-abbrev one three >actual &&
+	test_cmp expect actual
+'
+
 test_done
