merge-recursive: handle rename-to-self case

Directory rename detection can cause transitive renames, e.g. if the two
different sides of history each do one half of:
    A/file -> B/file
    B/     -> C/
then directory rename detection transitively renames to give us
    A/file -> C/file

However, when C/ == A/, note that this gives us
    A/file -> A/file.

merge-recursive assumed that any rename D -> E would have D != E.  While
that is almost always true, the above is a special case where it is not.
So we cannot do things like delete the rename source, we cannot assume
that a file existing at path E implies a rename/add conflict and we have
to be careful about what stages end up in the output.

This change feels a bit hackish.  It took me surprisingly many hours to
find, and given merge-recursive's design causing it to attempt to
enumerate all combinations of edge and corner cases with special code
for each combination, I'm worried there are other similar fixes needed
elsewhere if we can just come up with the right special testcase.
Perhaps an audit would rule it out, but I have not the energy.
merge-recursive deserves to die, and since it is on its way out anyway,
fixing this particular bug narrowly will have to be good enough.

Reported-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -2804,12 +2804,19 @@ static int process_renames(struct merge_options *opt,
 			int renamed_stage = a_renames == renames1 ? 2 : 3;
 			int other_stage =   a_renames == renames1 ? 3 : 2;
 
+			/*
+			 * Directory renames have a funny corner case...
+			 */
+			int renamed_to_self = !strcmp(ren1_src, ren1_dst);
+
 			/* BUG: We should only remove ren1_src in the base
 			 * stage and in other_stage (think of rename +
 			 * add-source case).
 			 */
-			remove_file(opt, 1, ren1_src,
-				    renamed_stage == 2 || !was_tracked(opt, ren1_src));
+			if (!renamed_to_self)
+				remove_file(opt, 1, ren1_src,
+					    renamed_stage == 2 ||
+					    !was_tracked(opt, ren1_src));
 
 			oidcpy(&src_other.oid,
 			       &ren1->src_entry->stages[other_stage].oid);
@@ -2823,6 +2830,9 @@ static int process_renames(struct merge_options *opt,
 			    ren1->dir_rename_original_type == 'A') {
 				setup_rename_conflict_info(RENAME_VIA_DIR,
 							   opt, ren1, NULL);
+			} else if (renamed_to_self) {
+				setup_rename_conflict_info(RENAME_NORMAL,
+							   opt, ren1, NULL);
 			} else if (oideq(&src_other.oid, null_oid())) {
 				setup_rename_conflict_info(RENAME_DELETE,
 							   opt, ren1, NULL);
@@ -3180,7 +3190,6 @@ static int handle_rename_normal(struct merge_options *opt,
 	struct rename *ren = ci->ren1;
 	struct merge_file_info mfi;
 	int clean;
-	int side = (ren->branch == opt->branch1 ? 2 : 3);
 
 	/* Merge the content and write it out */
 	clean = handle_content_merge(&mfi, opt, path, was_dirty(opt, path),
@@ -3190,9 +3199,7 @@ static int handle_rename_normal(struct merge_options *opt,
 	    opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_CONFLICT &&
 	    ren->dir_rename_original_dest) {
 		if (update_stages(opt, path,
-				  NULL,
-				  side == 2 ? &mfi.blob : NULL,
-				  side == 2 ? NULL : &mfi.blob))
+				  &mfi.blob, &mfi.blob, &mfi.blob))
 			return -1;
 		clean = 0; /* not clean, but conflicted */
 	}

t/t6423-merge-rename-directories.sh

@@ -5000,7 +5000,7 @@ test_setup_12i () {
 	)
 }
 
-test_expect_merge_algorithm failure success '12i: Directory rename causes rename-to-self' '
+test_expect_success '12i: Directory rename causes rename-to-self' '
 	test_setup_12i &&
 	(
 		cd 12i &&
@@ -5058,7 +5058,7 @@ test_setup_12j () {
 	)
 }
 
-test_expect_merge_algorithm failure success '12j: Directory rename to root causes rename-to-self' '
+test_expect_success '12j: Directory rename to root causes rename-to-self' '
 	test_setup_12j &&
 	(
 		cd 12j &&
@@ -5116,7 +5116,7 @@ test_setup_12k () {
 	)
 }
 
-test_expect_merge_algorithm failure success '12k: Directory rename with sibling causes rename-to-self' '
+test_expect_success '12k: Directory rename with sibling causes rename-to-self' '
 	test_setup_12k &&
 	(
 		cd 12k &&