[PATCH v2 09/10] rerere: teach rerere to handle nested conflicts

[Thomas Gummerer <t.gummerer@gmail.com>]

Currently rerere can't handle nested conflicts and will error out when
it encounters such conflicts.  Do that by recursively calling the
'handle_conflict' function to normalize the conflict.

The conflict ID calculation here deserves some explanation:

As we are using the same handle_conflict function, the nested conflict
is normalized the same way as for non-nested conflicts, which means
the ancestor in the diff3 case is stripped out, and the parts of the
conflict are ordered alphabetically.

The conflict ID is however is only calculated in the top level
handle_conflict call, so it will include the markers that 'rerere'
adds to the output.  e.g. say there's the following conflict:

    <<<<<<< HEAD
    1
    =======
    <<<<<<< HEAD
    3
    =======
    2
    >>>>>>> branch-2
    >>>>>>> branch-3~

it would be reordered as follows in the preimage:

    <<<<<<<
    1
    =======
    <<<<<<<
    2
    =======
    3
    >>>>>>>
    >>>>>>>

and the conflict ID would be calculated as
    sha1(1<NUL><<<<<<<
    2
    =======
    3
    >>>>>>><NUL>)

Stripping out vs. leaving the conflict markers in place should have no
practical impact, but it simplifies the implementation.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---

I couldn't actually get the conflict markers the right way just using
merge-recursive.  But I think that would be fixed either way by
d694a17986 ("ll-merge: use a longer conflict marker for internal
merge", 2016-04-14), if I read that correctly.

Either way I still think this can be an improvement for when the user
commits merge conflicts (even though they shouldn't do that in the
first place), and for possible other edge cases that I'm not able to
produce right now, but I may just not be creative enough for those.

 Documentation/technical/rerere.txt | 40 ++++++++++++++++++++++++++++++
 rerere.c                           | 14 ++++++++---
 t/t4200-rerere.sh                  | 38 ++++++++++++++++++++++++++++
 3 files changed, 88 insertions(+), 4 deletions(-)

diff --git a/Documentation/technical/rerere.txt b/Documentation/technical/rerere.txt
index 2c517fe0fc..7077ab4a08 100644
--- a/Documentation/technical/rerere.txt
+++ b/Documentation/technical/rerere.txt
@@ -140,3 +140,43 @@ SHA1('B<NUL>C<NUL>').
 If there are multiple conflicts in one file, the sha1 is calculated
 the same way with all hunks appended to each other, in the order in
 which they appear in the file, separated by a <NUL> character.
+
+Nested conflicts
+~~~~~~~~~~~~~~~~
+
+Nested conflicts are handled very similarly to "simple" conflicts.
+Same as before, labels on conflict markers and diff3 output is
+stripped, and the conflict hunks are sorted, for both the outer and
+the inner conflict.
+
+The only difference is in how the conflict ID is calculated.  For the
+inner conflict, the conflict markers themselves are not stripped out
+before calculating the sha1.
+
+Say we have the following conflict for example:
+
+    <<<<<<< HEAD
+    1
+    =======
+    <<<<<<< HEAD
+    3
+    =======
+    2
+    >>>>>>> branch-2
+    >>>>>>> branch-3~
+
+After stripping out the labels of the conflict markers, the conflict
+would look as follows:
+
+    <<<<<<<
+    1
+    =======
+    <<<<<<<
+    3
+    =======
+    2
+    >>>>>>>
+    >>>>>>>
+
+and finally the conflict ID would be calculated as:
+`sha1('1<NUL><<<<<<<\n3\n=======\n2\n>>>>>>><NUL>')`
diff --git a/rerere.c b/rerere.c
index fac90663b0..f611db7873 100644
--- a/rerere.c
+++ b/rerere.c
@@ -365,12 +365,18 @@ static int handle_conflict(struct strbuf *out, struct rerere_io *io,
 		RR_SIDE_1 = 0, RR_SIDE_2, RR_ORIGINAL
 	} hunk = RR_SIDE_1;
 	struct strbuf one = STRBUF_INIT, two = STRBUF_INIT;
-	struct strbuf buf = STRBUF_INIT;
+	struct strbuf buf = STRBUF_INIT, conflict = STRBUF_INIT;
 	int has_conflicts = 1;
 	while (!io->getline(&buf, io)) {
-		if (is_cmarker(buf.buf, '<', marker_size))
-			goto bad;
-		else if (is_cmarker(buf.buf, '|', marker_size)) {
+		if (is_cmarker(buf.buf, '<', marker_size)) {
+			if (handle_conflict(&conflict, io, marker_size, NULL) < 0)
+				goto bad;
+			if (hunk == RR_SIDE_1)
+				strbuf_addbuf(&one, &conflict);
+			else
+				strbuf_addbuf(&two, &conflict);
+			strbuf_release(&conflict);
+		} else if (is_cmarker(buf.buf, '|', marker_size)) {
 			if (hunk != RR_SIDE_1)
 				goto bad;
 			hunk = RR_ORIGINAL;
diff --git a/t/t4200-rerere.sh b/t/t4200-rerere.sh
index 5ce411b70d..f433848ccb 100755
--- a/t/t4200-rerere.sh
+++ b/t/t4200-rerere.sh
@@ -602,4 +602,42 @@ test_expect_success 'rerere with unexpected conflict markers does not crash' '
 	git rerere clear
 '
 
+test_expect_success 'rerere with inner conflict markers' '
+	git reset --hard &&
+
+	git checkout -b A master &&
+	echo "bar" >test &&
+	git add test &&
+	git commit -q -m two &&
+	echo "baz" >test &&
+	git add test &&
+	git commit -q -m three &&
+
+	git reset --hard &&
+	git checkout -b B master &&
+	echo "foo" >test &&
+	git add test &&
+	git commit -q -a -m one &&
+
+	test_must_fail git merge A~ &&
+	git add test &&
+	git commit -q -m "will solve conflicts later" &&
+	test_must_fail git merge A &&
+
+	echo "resolved" >test &&
+	git add test &&
+	git commit -q -m "solved conflict" &&
+
+	echo "resolved" >expect &&
+
+	git reset --hard HEAD~~ &&
+	test_must_fail git merge A~ &&
+	git add test &&
+	git commit -q -m "will solve conflicts later" &&
+	test_must_fail git merge A &&
+	cat test >actual &&
+	test_cmp expect actual
+'
+
+
 test_done
-- 
2.17.0.410.g65aef3a6c4