ntdb: reduce race between creating file and getting open lock.

In tdb, we grab the open lock immediately after we open the file.  In
ntdb, we usually did some work first.  tdbtorture managed to get in
before the creator grabbed the lock:

	testing with 3 processes, 5000 loops, seed=1338246020
	ntdb:torture.ntdb:IO Error:ntdb_open: torture.ntdb is not a ntdb file
	29023:torture.ntdb:db open failed

At cost of a little duplicated code, we can reduce the race.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 30 insertions, 23 deletions

diff --git a/lib/ntdb/open.c b/lib/ntdb/open.c
index 28aff0a2eaf..8dce4ce6f2a 100644
--- a/lib/ntdb/open.c
+++ b/lib/ntdb/open.c
@@ -508,48 +508,55 @@ _PUBLIC_ struct ntdb_context *ntdb_open(const char *name, int ntdb_flags,
 		ntdb->file = find_file(st.st_dev, st.st_ino);
 
 	if (!ntdb->file) {
-		int fd;
+		ecode = ntdb_new_file(ntdb);
+		if (ecode != NTDB_SUCCESS) {
+			goto fail;
+		}
 
-		if ((fd = open(name, open_flags, mode)) == -1) {
+		/* Set this now, as ntdb_nest_lock examines it. */
+		ntdb->file->map_size = 0;
+
+		if ((ntdb->file->fd = open(name, open_flags, mode)) == -1) {
 			/* errno set by open(2) */
 			saved_errno = errno;
 			ntdb_logerr(ntdb, NTDB_ERR_IO, NTDB_LOG_ERROR,
 				   "ntdb_open: could not open file %s: %s",
 				   name, strerror(errno));
+
+			goto fail_errno;
+		}
+
+		/* ensure there is only one process initialising at once:
+		 * do it immediately to reduce the create/openlock race. */
+		ecode = ntdb_lock_open(ntdb, openlock,
+				       NTDB_LOCK_WAIT|NTDB_LOCK_NOCHECK);
+		if (ecode != NTDB_SUCCESS) {
+			saved_errno = errno;
 			goto fail_errno;
 		}
 
 		/* on exec, don't inherit the fd */
-		v = fcntl(fd, F_GETFD, 0);
-		fcntl(fd, F_SETFD, v | FD_CLOEXEC);
+		v = fcntl(ntdb->file->fd, F_GETFD, 0);
+		fcntl(ntdb->file->fd, F_SETFD, v | FD_CLOEXEC);
 
-		if (fstat(fd, &st) == -1) {
+		if (fstat(ntdb->file->fd, &st) == -1) {
 			saved_errno = errno;
 			ntdb_logerr(ntdb, NTDB_ERR_IO, NTDB_LOG_ERROR,
 				   "ntdb_open: could not stat open %s: %s",
 				   name, strerror(errno));
-			close(fd);
 			goto fail_errno;
 		}
 
-		ecode = ntdb_new_file(ntdb);
-		if (ecode != NTDB_SUCCESS) {
-			close(fd);
-			goto fail;
-		}
-
-		ntdb->file->fd = fd;
 		ntdb->file->device = st.st_dev;
 		ntdb->file->inode = st.st_ino;
-		ntdb->file->map_ptr = NULL;
-		ntdb->file->map_size = 0;
-	}
-
-	/* ensure there is only one process initialising at once */
-	ecode = ntdb_lock_open(ntdb, openlock, NTDB_LOCK_WAIT|NTDB_LOCK_NOCHECK);
-	if (ecode != NTDB_SUCCESS) {
-		saved_errno = errno;
-		goto fail_errno;
+	} else {
+		/* ensure there is only one process initialising at once */
+		ecode = ntdb_lock_open(ntdb, openlock,
+				       NTDB_LOCK_WAIT|NTDB_LOCK_NOCHECK);
+		if (ecode != NTDB_SUCCESS) {
+			saved_errno = errno;
+			goto fail_errno;
+		}
 	}
 
 	/* call their open hook if they gave us one. */
@@ -696,7 +703,7 @@ fail_errno:
 				} else
 					ntdb_munmap(ntdb->file);
 			}
-			if (close(ntdb->file->fd) != 0)
+			if (ntdb->file->fd != -1 && close(ntdb->file->fd) != 0)
 				ntdb_logerr(ntdb, NTDB_ERR_IO, NTDB_LOG_ERROR,
 					   "ntdb_open: failed to close ntdb fd"
 					   " on error: %s", strerror(errno));