common/gossip_store: expose routine to read one header.

This is useful when you're writing routines to scan it.

Signed-off-by: Rusty Russell <[email protected]>

common/gossip_store.c

@@ -182,25 +182,48 @@ u8 *gossip_store_next(const tal_t *ctx,
 	return msg;
 }
 
-size_t find_gossip_store_end(int gossip_store_fd, size_t off)
+/* We cheat and read first two bytes of message too. */
+struct hdr_and_type {
+	struct gossip_hdr hdr;
+	be16 type;
+};
+/* Beware padding! */
+#define HDR_AND_TYPE_SIZE (sizeof(struct gossip_hdr) + sizeof(u16))
+
+bool gossip_store_readhdr(int gossip_store_fd, size_t off,
+			  size_t *len,
+			  u32 *timestamp,
+			  u16 *flags,
+			  u16 *type)
 {
-	/* We cheat and read first two bytes of message too. */
-	struct {
-		struct gossip_hdr hdr;
-		be16 type;
-	} buf;
+	struct hdr_and_type buf;
 	int r;
 
-	while ((r = pread(gossip_store_fd, &buf,
-			 sizeof(buf.hdr) + sizeof(buf.type), off))
-	       == sizeof(buf.hdr) + sizeof(buf.type)) {
-		u16 msglen = be16_to_cpu(buf.hdr.len);
+	r = pread(gossip_store_fd, &buf, HDR_AND_TYPE_SIZE, off);
+	if (r != HDR_AND_TYPE_SIZE)
+		return false;
+	*len = be16_to_cpu(buf.hdr.len);
+	if (flags)
+		*flags = be16_to_cpu(buf.hdr.flags);
+	if (timestamp)
+		*timestamp = be32_to_cpu(buf.hdr.timestamp);
+	if (type)
+		*type = be16_to_cpu(buf.type);
+	return true;
+}
+
+size_t find_gossip_store_end(int gossip_store_fd, size_t off)
+{
+	size_t msglen;
+	u16 type;
 
+	while (gossip_store_readhdr(gossip_store_fd, off,
+				    &msglen, NULL, NULL, &type)) {
 		/* Don't swallow end marker! */
-		if (buf.type == CPU_TO_BE16(WIRE_GOSSIP_STORE_ENDED))
+		if (type == WIRE_GOSSIP_STORE_ENDED)
 			break;
 
-		off += sizeof(buf.hdr) + msglen;
+		off += sizeof(struct gossip_hdr) + msglen;
 	}
 	return off;
 }

common/gossip_store.h

@@ -69,6 +69,27 @@ u8 *gossip_store_next(const tal_t *ctx,
 		      bool with_spam,
 		      size_t *off, size_t *end);
 
+/**
+ * Direct store accessor: read gossip msg hdr from store.
+ * @gossip_store_fd: the readable file descriptor
+ * @off: the offset to read
+ * @len (out): the length of the message (not including header)
+ * @timestamp (out): if non-NULL, set to the timestamp.
+ * @flags (out): if non-NULL, set to the flags.
+ * @type (out): if non-NULL, set to the msg type.
+ *
+ * Returns false if there are no more gossip msgs.  If you
+ * want to read the message, use gossip_store_next, if you
+ * want to skip, simply add sizeof(gossip_hdr) + *len to *off.
+ * Note: it's possible that entire record isn't there yet,
+ * so gossip_store_next can fail.
+ */
+bool gossip_store_readhdr(int gossip_store_fd, size_t off,
+			  size_t *len,
+			  u32 *timestamp,
+			  u16 *flags,
+			  u16 *type);
+
 /**
  * Gossipd will be writing to this, and it's not atomic!  Safest
  * way to find the "end" is to walk through.