gossip: Switch gossip_store to use a single fd

As proposed by @rustyrussell this makes it a bit easier to truncate and sync on
read errors.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

gossipd/gossip_store.c

@@ -9,7 +9,8 @@
 #define GOSSIP_STORE_FILENAME "gossip_store"
 
 struct gossip_store {
-	int read_fd, write_fd;
+	int fd;
+	off_t read_pos, write_pos;
 
 	/* What was the size of the gossip_store when we started replaying
 	 * it? */
@@ -18,17 +19,16 @@ struct gossip_store {
 
 static void gossip_store_destroy(struct gossip_store *gs)
 {
-	if (gs->read_fd != -1)
-		close(gs->read_fd);
-	close(gs->write_fd);
+	close(gs->fd);
 }
 
 struct gossip_store *gossip_store_new(const tal_t *ctx)
 {
 	struct gossip_store *gs = tal(ctx, struct gossip_store);
-	gs->write_fd = open(GOSSIP_STORE_FILENAME, O_RDWR|O_APPEND|O_CREAT, 0600);
-	gs->read_fd = open(GOSSIP_STORE_FILENAME, O_RDONLY);
-	gs->replaysize = lseek(gs->write_fd, 0, SEEK_END);
+	gs->fd = open(GOSSIP_STORE_FILENAME, O_RDWR|O_APPEND|O_CREAT, 0600);
+	gs->read_pos = 0;
+	gs->write_pos = lseek(gs->fd, 0, SEEK_END);
+	gs->replaysize = gs->write_pos;
 
 	tal_add_destructor(gs, gossip_store_destroy);
 
@@ -40,17 +40,11 @@ void gossip_store_append(struct gossip_store *gs, const u8 *msg)
 	u16 msglen = tal_len(msg);
 	beint16_t belen = cpu_to_be16(msglen);
 
-	/* FIXME: this method of detecting replayed messages is best effort
-	 * only. It should avoid doubling the store file size on every start,
-	 * but it'll allow the last few messages to be duplicated since replay
-	 * and write are async, and we'll think we are done replaying a bit too
-	 * early. */
-        /* Check if we are replaying the store */
-	if (lseek(gs->read_fd, 0, SEEK_CUR) < gs->replaysize)
+	if (pwrite(gs->fd, &belen, sizeof(belen), gs->write_pos) != 2 ||
+	    pwrite(gs->fd, msg, msglen, gs->write_pos + 2) != msglen) {
 		return;
-
-	write_all(gs->write_fd, &belen, sizeof(belen));
-	write_all(gs->write_fd, msg, msglen);
+	} else
+		gs->write_pos += 2 + msglen;
 }
 
 const u8 *gossip_store_read_next(const tal_t *ctx, struct gossip_store *gs)
@@ -60,22 +54,29 @@ const u8 *gossip_store_read_next(const tal_t *ctx, struct gossip_store *gs)
 	u8 *msg;
 
 	/* Did we already reach the end of the gossip_store? */
-	if (gs->read_fd == -1)
+	if (gs->read_pos == -1)
 		return NULL;
 
 	/* Can we read one message? */
-	if (!read_all(gs->read_fd, &belen, sizeof(belen))) {
-		gs->read_fd = -1;
+	if (pread(gs->fd, &belen, sizeof(belen), gs->read_pos) != 2) {
+		gs->read_pos = -1;
 		return NULL;
 	}
 
 	msglen = be16_to_cpu(belen);
 	msg = tal_arr(ctx, u8, msglen);
 
-	if (!read_all(gs->read_fd, msg, msglen))
-		status_failed(
-		    STATUS_FAIL_INTERNAL_ERROR,
-		    "Short read from gossip-store, expected lenght %d", msglen);
+	if (!pread(gs->fd, msg, msglen, gs->read_pos + 2)) {
+		status_trace("Short read from gossip-store, expected lenght %d",
+			     msglen);
+
+		/* Reset write_pos to truncate this message and disable future
+		 * reads */
+		gs->write_pos = gs->read_pos;
+		gs->read_pos = -1;
+		ftruncate(gs->fd, gs->write_pos);
+	} else
+		gs->read_pos += 2 + msglen;
 
 	return msg;
 }