rerere: split conflict ID further

The plan is to keep assigning the backward compatible conflict ID
based on the hash of the (normalized) text of conflicts, keep using
that conflict ID as the directory name under $GIT_DIR/rr-cache/, but
allow each conflicted path to use a separate "variant" to record
resolutions, i.e. having more than one <preimage,postimage> pairs
under $GIT_DIR/rr-cache/$ID/ directory.  As the first step in that
direction, separate the shared "conflict ID" out of the rerere_id
structure.

The plan is to keep information per $ID in rerere_dir, that can be
shared among rerere_id that is per conflicted path.

When we are done with rerere(), which can be directly called from
other programs like "git apply", "git commit" and "git merge", the
shared rerere_dir structures can be freed entirely, so they are not
reference-counted and they are not freed when we release rerere_id's
that reference them.

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

rerere.c

@@ -8,6 +8,7 @@
 #include "ll-merge.h"
 #include "attr.h"
 #include "pathspec.h"
+#include "sha1-lookup.h"
 
 #define RESOLVED 0
 #define PUNTED 1
@@ -22,6 +23,23 @@ static int rerere_autoupdate;
 
 static char *merge_rr_path;
 
+static int rerere_dir_nr;
+static int rerere_dir_alloc;
+
+static struct rerere_dir {
+	unsigned char sha1[20];
+} **rerere_dir;
+
+static void free_rerere_dirs(void)
+{
+	int i;
+	for (i = 0; i < rerere_dir_nr; i++)
+		free(rerere_dir[i]);
+	free(rerere_dir);
+	rerere_dir_nr = rerere_dir_alloc = 0;
+	rerere_dir = NULL;
+}
+
 static void free_rerere_id(struct string_list_item *item)
 {
 	free(item->util);
@@ -29,7 +47,7 @@ static void free_rerere_id(struct string_list_item *item)
 
 static const char *rerere_id_hex(const struct rerere_id *id)
 {
-	return id->hex;
+	return sha1_to_hex(id->collection->sha1);
 }
 
 const char *rerere_path(const struct rerere_id *id, const char *file)
@@ -40,6 +58,37 @@ const char *rerere_path(const struct rerere_id *id, const char *file)
 	return git_path("rr-cache/%s/%s", rerere_id_hex(id), file);
 }
 
+static const unsigned char *rerere_dir_sha1(size_t i, void *table)
+{
+	struct rerere_dir **rr_dir = table;
+	return rr_dir[i]->sha1;
+}
+
+static struct rerere_dir *find_rerere_dir(const char *hex)
+{
+	unsigned char sha1[20];
+	struct rerere_dir *rr_dir;
+	int pos;
+
+	if (get_sha1_hex(hex, sha1))
+		return NULL; /* BUG */
+	pos = sha1_pos(sha1, rerere_dir, rerere_dir_nr, rerere_dir_sha1);
+	if (pos < 0) {
+		rr_dir = xmalloc(sizeof(*rr_dir));
+		hashcpy(rr_dir->sha1, sha1);
+		pos = -1 - pos;
+
+		/* Make sure the array is big enough ... */
+		ALLOC_GROW(rerere_dir, rerere_dir_nr + 1, rerere_dir_alloc);
+		/* ... and add it in. */
+		rerere_dir_nr++;
+		memmove(rerere_dir + pos + 1, rerere_dir + pos,
+			(rerere_dir_nr - pos - 1) * sizeof(*rerere_dir));
+		rerere_dir[pos] = rr_dir;
+	}
+	return rerere_dir[pos];
+}
+
 static int has_rerere_resolution(const struct rerere_id *id)
 {
 	struct stat st;
@@ -50,7 +99,7 @@ static int has_rerere_resolution(const struct rerere_id *id)
 static struct rerere_id *new_rerere_id_hex(char *hex)
 {
 	struct rerere_id *id = xmalloc(sizeof(*id));
-	xsnprintf(id->hex, sizeof(id->hex), "%s", hex);
+	id->collection = find_rerere_dir(hex);
 	return id;
 }
 
@@ -810,12 +859,14 @@ int setup_rerere(struct string_list *merge_rr, int flags)
 int rerere(int flags)
 {
 	struct string_list merge_rr = STRING_LIST_INIT_DUP;
-	int fd;
+	int fd, status;
 
 	fd = setup_rerere(&merge_rr, flags);
 	if (fd < 0)
 		return 0;
-	return do_plain_rerere(&merge_rr, fd);
+	status = do_plain_rerere(&merge_rr, fd);
+	free_rerere_dirs();
+	return status;
 }
 
 static int rerere_forget_one_path(const char *path, struct string_list *rr)
@@ -900,7 +951,7 @@ int rerere_forget(struct pathspec *pathspec)
 static struct rerere_id *dirname_to_id(const char *name)
 {
 	static struct rerere_id id;
-	xsnprintf(id.hex, sizeof(id.hex), "%s", name);
+	id.collection = find_rerere_dir(name);
 	return &id;
 }
 

rerere.h

@@ -15,8 +15,9 @@ struct pathspec;
  */
 extern void *RERERE_RESOLVED;
 
+struct rerere_dir;
 struct rerere_id {
-	char hex[41];
+	struct rerere_dir *collection;
 };
 
 extern int setup_rerere(struct string_list *, int);