merge: turn on rewrite detection

We currently don't do break-detection at all in
merge-recursive. But there are some cases where it would
provide a more useful merge result.

For example, consider this case (which is the basis for the
new tests in t6039):

  1. You rename a header file foo.h to bar.h. You install a
     new foo.h that includes bar.h (for compatibility).

  2. Another branch makes changes to foo.h.

When you merge, you want the changes the other branch made
to foo.h to migrate to the rename destination, bar.h, just
as you would if you hadn't installed that compatibility
header.

Similarly, you want the compatibility header left untouched.
The other side's changes all ended up in bar.h, so there is
no reason to conflict with the new content in foo.h.

This patch turns on break detection for merge-recursive. In
addition to new tests in t6039, it makes a similar test in
t3030 pass.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -435,6 +435,7 @@ static struct string_list *get_renames(struct merge_options *o,
 	opts.rename_score = o->rename_score;
 	opts.warn_on_too_large_rename = 1;
 	opts.output_format = DIFF_FORMAT_NO_OUTPUT;
+	opts.break_opt = 0;
 	if (diff_setup_done(&opts) < 0)
 		die("diff setup failed");
 	diff_tree_sha1(o_tree->object.sha1, tree->object.sha1, "", &opts);

t/t3030-merge-recursive.sh

@@ -631,7 +631,7 @@ test_expect_success 'merge-recursive copy vs. rename' '
 
 if test_have_prereq SYMLINKS
 then
-	test_expect_failure 'merge-recursive rename vs. rename/symlink' '
+	test_expect_success 'merge-recursive rename vs. rename/symlink' '
 
 		git checkout -f rename &&
 		git merge rename-ln &&

t/t6039-merge-break.sh

@@ -0,0 +1,174 @@
+#!/bin/sh
+
+test_description='merging with renames from broken pairs
+
+This is based on a real-world practice of moving a header file to a
+new location, but installing a "replacement" file that points to
+the old one. We need break detection in the merge to find the
+rename.
+'
+. ./test-lib.sh
+
+# A fake header file; it needs a fair bit of content
+# for break detection and inexact rename detection to work.
+mksample() {
+	echo '#ifndef SAMPLE_H'
+	echo '#define SAMPLE_H'
+	for i in 0 1 2 3 4; do
+		for j in 0 1 2 3 4 5 6 7 8 9; do
+			echo "extern fun$i$j();"
+		done
+	done
+	echo '#endif /* SAMPLE_H */'
+}
+
+mvsample() {
+	sed 's/SAMPLE_H/NEW_H/' "$1" >"$2" &&
+	rm "$1"
+}
+
+# A replacement sample header file that references a new one.
+mkreplacement() {
+	echo '#ifndef SAMPLE_H'
+	echo '#define SAMPLE_H'
+	echo "#include \"$1\""
+	echo '#endif /* SAMPLE_H */'
+}
+
+# Tweak the header file in a minor way.
+tweak() {
+	sed 's,42.*,& /* secret of something-or-other */,' "$1" >"$1.tmp" &&
+	mv "$1.tmp" "$1"
+}
+
+reset() {
+	git reset --hard &&
+	git checkout master &&
+	git reset --hard base &&
+	git clean -f &&
+	{ git branch -D topic || true; }
+}
+
+test_expect_success 'setup baseline' '
+	mksample >sample.h &&
+	git add sample.h &&
+	git commit -m "add sample.h" &&
+	git tag base
+'
+
+setup_rename_plus_tweak() {
+	reset &&
+	mvsample sample.h new.h &&
+	mkreplacement new.h >sample.h &&
+	git add sample.h new.h &&
+	git commit -m 'rename sample.h to new.h, with replacement' &&
+	git checkout -b topic base &&
+	tweak sample.h &&
+	git commit -a -m 'tweak sample.h'
+}
+
+check_tweak_result() {
+	mksample >expect.orig &&
+	mvsample expect.orig expect &&
+	tweak expect &&
+	test_cmp expect new.h &&
+	mkreplacement new.h >expect &&
+	test_cmp expect sample.h
+}
+
+test_expect_success 'merge rename to tweak finds rename' '
+	setup_rename_plus_tweak &&
+	git merge master &&
+	check_tweak_result
+'
+
+test_expect_success 'merge tweak to rename finds rename' '
+	setup_rename_plus_tweak &&
+	git checkout master &&
+	git merge topic &&
+	check_tweak_result
+'
+
+setup_double_rename_one_replacement() {
+	setup_rename_plus_tweak &&
+	mvsample sample.h new.h &&
+	git add new.h &&
+	git commit -a -m 'rename sample.h to new.h (no replacement)'
+}
+
+test_expect_success 'merge rename to rename/tweak (one replacement)' '
+	setup_double_rename_one_replacement &&
+	git merge master &&
+	check_tweak_result
+'
+
+test_expect_success 'merge rename/tweak to rename (one replacement)' '
+	setup_double_rename_one_replacement &&
+	git checkout master &&
+	git merge topic &&
+	check_tweak_result
+'
+
+setup_double_rename_two_replacements_same() {
+	setup_rename_plus_tweak &&
+	mvsample sample.h new.h &&
+	mkreplacement new.h >sample.h &&
+	git add sample.h new.h &&
+	git commit -m 'rename sample.h to new.h with replacement (same)'
+}
+
+test_expect_success 'merge rename to rename/tweak (two replacements, same)' '
+	setup_double_rename_two_replacements_same &&
+	git merge master &&
+	check_tweak_result
+'
+
+test_expect_success 'merge rename/tweak to rename (two replacements, same)' '
+	setup_double_rename_two_replacements_same &&
+	git checkout master &&
+	git merge topic &&
+	check_tweak_result
+'
+
+setup_double_rename_two_replacements_diff() {
+	setup_rename_plus_tweak &&
+	mvsample sample.h new.h &&
+	mkreplacement diff.h >sample.h &&
+	git add sample.h new.h &&
+	git commit -m 'rename sample.h to new.h with replacement (diff)'
+}
+
+test_expect_success 'merge rename to rename/tweak (two replacements, diff)' '
+	setup_double_rename_two_replacements_diff &&
+	test_must_fail git merge master &&
+	cat >expect <<-\EOF &&
+	#ifndef SAMPLE_H
+	#define SAMPLE_H
+	<<<<<<< HEAD
+	#include "diff.h"
+	=======
+	#include "new.h"
+	>>>>>>> master
+	#endif /* SAMPLE_H */
+	EOF
+	test_cmp expect sample.h
+'
+
+test_expect_success 'merge rename to rename/tweak (two replacements, diff)' '
+	setup_double_rename_two_replacements_diff &&
+	git checkout master &&
+	test_must_fail git merge topic &&
+	cat >expect <<-\EOF &&
+	#ifndef SAMPLE_H
+	#define SAMPLE_H
+	<<<<<<< HEAD
+	#include "new.h"
+	=======
+	#include "diff.h"
+	>>>>>>> topic
+	#endif /* SAMPLE_H */
+	EOF
+	test_cmp expect sample.h
+'
+
+test_done