libalpm: Force update when database is marked as corrupted.

When a database and its signature is updated non-atomically on a server,
there is a window where a user may update the database but grab the old
signature.  The database is marked as invalid by libalpm, which can be
fixed by forcing a refresh (assuming the server has caught up and the
user realizes what has happened) or with a future update of the repo.
Work around this by forcing a repository refresh whenever a database is
invalid.

Signed-off-by: Sebastian Lackner <sebastian@fds-team.de>
Signed-off-by: Allan McRae <allan@archlinux.org>

lib/libalpm/be_sync.c

@@ -198,6 +198,11 @@ int SYMEXPORT alpm_db_update(int force, alpm_db_t *db)
 		return -1;
 	}
 
+	/* force update of invalid databases to fix potential mismatched database/signature */
+	if(db->status & DB_STATUS_INVALID) {
+		force = 1;
+	}
+
 	/* make sure we have a sane umask */
 	oldmask = umask(0022);