rerere: release lockfile in non-writing functions

There's a bug in builtin/am.c in which we take a lock on
MERGE_RR recursively. But rather than fix am.c, this patch
fixes the confusing interface from rerere.c that caused the
bug. Read on for the gory details.

The setup_rerere() function both reads the existing MERGE_RR
file, and takes MERGE_RR.lock. In the rerere() and
rerere_forget() functions, we end up in write_rr(), which
will then commit the lock file.

But for functions like rerere_clear() that do not write to
MERGE_RR, we expect the caller to have handled
setup_rerere(). That caller would then need to release the
lockfile, but it can't; the lock struct is local to
rerere.c.

For builtin/rerere.c, this is OK. We run a single rerere
operation and then exit immediately, which has the side
effect of rolling back the lockfile.

But in builtin/am.c, this is actively wrong. If we run "git
am -3 --skip", we call setup-rerere twice without releasing
the lock:

  1. The "--skip" causes us to call am_rerere_clear(), which
     calls setup_rerere(), but never drops the lock.

  2. We then proceed to the next patch.

  3. The "--3way" may cause us to call rerere() to handle
     conflicts in that patch, but we are already holding the
     lock. The lockfile code dies with:

     BUG: prepare_tempfile_object called for active object

We could fix this by having rerere_clear() call
rollback_lock_file(). But it feels a bit odd for it to roll
back a lockfile that it did not itself take. So let's
simplify the interface further, and handle setup_rerere in
the function itself, taking away the question from the
caller over whether they need to do so.

We can give rerere_gc() the same treatment, as well (even
though it doesn't have any callers besides builtin/rerere.c
at this point). Note that these functions don't take flags
from their callers to pass along to setup_rerere; that's OK,
because the flags would not be meaningful for what they are
doing.

Both of those functions need to hold the lock because even
though they do not write to MERGE_RR, they are still writing
and should be protected from a simultaneous "rerere" run.
But rerere_remaining(), "rerere diff", and "rerere status"
are all read-only operations. They want to setup_rerere(),
but do not care about taking the lock in the first place.
Since our update of MERGE_RR is the usual atomic rename done
by commit_lock_file, they can just do a lockless read. For
that, we teach setup_rerere a READONLY flag to avoid the
lock.

As a bonus, this pushes builtin/rerere.c's setup_rerere call
closer to the functions that use it. Which means that "git
rerere totally-bogus-command" will no longer silently
exit(0) in a repository without rerere enabled.

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

1 files changed, 9 insertions, 9 deletions

diff --git a/builtin/rerere.c b/builtin/rerere.c
index 7afadd2ead..12535c9b4f 100644
--- a/builtin/rerere.c
+++ b/builtin/rerere.c
@@ -50,7 +50,7 @@ static int diff_two(const char *file1, const char *label1,
 int cmd_rerere(int argc, const char **argv, const char *prefix)
 {
 	struct string_list merge_rr = STRING_LIST_INIT_DUP;
-	int i, fd, autoupdate = -1, flags = 0;
+	int i, autoupdate = -1, flags = 0;
 
 	struct option options[] = {
 		OPT_SET_INT(0, "rerere-autoupdate", &autoupdate,
@@ -79,18 +79,16 @@ int cmd_rerere(int argc, const char **argv, const char *prefix)
 		return rerere_forget(&pathspec);
 	}
 
-	fd = setup_rerere(&merge_rr, flags);
-	if (fd < 0)
-		return 0;
-
 	if (!strcmp(argv[0], "clear")) {
 		rerere_clear(&merge_rr);
 	} else if (!strcmp(argv[0], "gc"))
 		rerere_gc(&merge_rr);
-	else if (!strcmp(argv[0], "status"))
+	else if (!strcmp(argv[0], "status")) {
+		if (setup_rerere(&merge_rr, flags | RERERE_READONLY) < 0)
+			return 0;
 		for (i = 0; i < merge_rr.nr; i++)
 			printf("%s\n", merge_rr.items[i].string);
-	else if (!strcmp(argv[0], "remaining")) {
+	} else if (!strcmp(argv[0], "remaining")) {
 		rerere_remaining(&merge_rr);
 		for (i = 0; i < merge_rr.nr; i++) {
 			if (merge_rr.items[i].util != RERERE_RESOLVED)
@@ -100,13 +98,15 @@ int cmd_rerere(int argc, const char **argv, const char *prefix)
 				 * string_list_clear() */
 				merge_rr.items[i].util = NULL;
 		}
-	} else if (!strcmp(argv[0], "diff"))
+	} else if (!strcmp(argv[0], "diff")) {
+		if (setup_rerere(&merge_rr, flags | RERERE_READONLY) < 0)
+			return 0;
 		for (i = 0; i < merge_rr.nr; i++) {
 			const char *path = merge_rr.items[i].string;
 			const char *name = (const char *)merge_rr.items[i].util;
 			diff_two(rerere_path(name, "preimage"), path, path, path);
 		}
-	else
+	} else
 		usage_with_options(rerere_usage, options);
 
 	string_list_clear(&merge_rr, 1);