rerere: handle leftover rr-cache/$ID directory and postimage files

If by some accident there is only $GIT_DIR/rr-cache/$ID directory
existed, we wouldn't have recorded a preimage for a conflict that
is newly encountered, which would mean after a manual resolution,
we wouldn't have recorded it by storing the postimage, because the
logic used to be "if there is no rr-cache/$ID directory, then we are
the first so record the preimage".  Instead, record preimage if we
do not have one.

In addition, if there is only $GIT_DIR/rr-cache/$ID/postimage
without corresponding preimage, we would have tried to call into
merge() and punted.

These would have been a situation frustratingly hard to recover
from.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -117,7 +117,9 @@ static struct rerere_dir *find_rerere_dir(const char *hex)
 
 static int has_rerere_resolution(const struct rerere_id *id)
 {
-	return (id->collection->status & RR_HAS_POSTIMAGE);
+	const int both = RR_HAS_POSTIMAGE|RR_HAS_PREIMAGE;
+
+	return ((id->collection->status & both) == both);
 }
 
 static struct rerere_id *new_rerere_id_hex(char *hex)
@@ -806,24 +808,30 @@ static int do_plain_rerere(struct string_list *rr, int fd)
 		string_list_insert(rr, path)->util = id;
 
 		/*
-		 * If the directory does not exist, create
-		 * it.  mkdir_in_gitdir() will fail with
-		 * EEXIST if there already is one.
-		 *
-		 * NEEDSWORK: make sure "gc" does not remove
-		 * preimage without removing the directory.
+		 * Ensure that the directory exists.
+		 * mkdir_in_gitdir() will fail with EEXIST if there
+		 * already is one.
 		 */
-		if (mkdir_in_gitdir(rerere_path(id, NULL)))
-			continue;
+		if (mkdir_in_gitdir(rerere_path(id, NULL)) &&
+		    errno != EEXIST)
+			continue; /* NEEDSWORK: perhaps we should die? */
 
-		/*
-		 * We are the first to encounter this
-		 * conflict.  Ask handle_file() to write the
-		 * normalized contents to the "preimage" file.
-		 */
-		handle_file(path, NULL, rerere_path(id, "preimage"));
-		id->collection->status |= RR_HAS_PREIMAGE;
-		fprintf(stderr, "Recorded preimage for '%s'\n", path);
+		if (id->collection->status & RR_HAS_PREIMAGE) {
+			;
+		} else {
+			/*
+			 * We are the first to encounter this
+			 * conflict.  Ask handle_file() to write the
+			 * normalized contents to the "preimage" file.
+			 *
+			 * NEEDSWORK: what should happen if we had a
+			 * leftover postimage that is totally
+			 * unrelated?  Perhaps we should unlink it?
+			 */
+			handle_file(path, NULL, rerere_path(id, "preimage"));
+			id->collection->status |= RR_HAS_PREIMAGE;
+			fprintf(stderr, "Recorded preimage for '%s'\n", path);
+		}
 	}
 
 	for (i = 0; i < rr->nr; i++)

t/t4200-rerere.sh

@@ -184,12 +184,27 @@ test_expect_success 'rerere updates postimage timestamp' '
 '
 
 test_expect_success 'rerere clear' '
-	rm $rr/postimage &&
+	mv $rr/postimage .git/post-saved &&
 	echo "$sha1	a1" | perl -pe "y/\012/\000/" >.git/MERGE_RR &&
 	git rerere clear &&
 	! test -d $rr
 '
 
+test_expect_success 'leftover directory' '
+	git reset --hard &&
+	mkdir -p $rr &&
+	test_must_fail git merge first &&
+	test -f $rr/preimage
+'
+
+test_expect_success 'missing preimage' '
+	git reset --hard &&
+	mkdir -p $rr &&
+	cp .git/post-saved $rr/postimage &&
+	test_must_fail git merge first &&
+	test -f $rr/preimage
+'
+
 test_expect_success 'set up for garbage collection tests' '
 	mkdir -p $rr &&
 	echo Hello >$rr/preimage &&