gossip: Avoid storing messages from gossip_store twice

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

gossipd/gossip_store.c

@@ -4,11 +4,16 @@
 #include <ccan/read_write_all/read_write_all.h>
 #include <common/status.h>
 #include <fcntl.h>
+#include <unistd.h>
 
 #define GOSSIP_STORE_FILENAME "gossip_store"
 
 struct gossip_store {
 	int read_fd, write_fd;
+
+	/* What was the size of the gossip_store when we started replaying
+	 * it? */
+	__off_t replaysize;
 };
 
 struct gossip_store *gossip_store_new(const tal_t *ctx)
@@ -16,6 +21,8 @@ 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);
+
 	return gs;
 }
 
@@ -24,6 +31,15 @@ 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)
+		return;
+
 	write_all(gs->write_fd, &belen, sizeof(belen));
 	write_all(gs->write_fd, msg, msglen);
 }