common: add gossmap_chan_is_dying() helper to check flags.

And fix up gossip_store backwards comment!

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

common/gossip_store.h

@@ -44,8 +44,8 @@ struct gossip_rcvd_filter;
  * gossip_hdr -- On-disk format header.
  */
 struct gossip_hdr {
-	beint16_t flags; /* Length of message after header. */
-	beint16_t len; /* GOSSIP_STORE_xxx_BIT flags. */
+	beint16_t flags; /* GOSSIP_STORE_xxx_BIT flags. */
+	beint16_t len; /* Length of message after header. */
 	beint32_t crc; /* crc of message of timestamp, after header. */
 	beint32_t timestamp; /* timestamp of msg. */
 };

common/gossmap.c

@@ -1093,6 +1093,19 @@ bool gossmap_chan_is_localmod(const struct gossmap *map,
 	return c->cann_off >= map->map_size;
 }
 
+bool gossmap_chan_is_dying(const struct gossmap *map,
+			   const struct gossmap_chan *c)
+{
+	struct gossip_hdr ghdr;
+	size_t off;
+
+	/* FIXME: put this flag in plus_scid_off instead? */
+	off = c->cann_off - sizeof(ghdr);
+	map_copy(map, off, &ghdr, sizeof(ghdr));
+
+	return ghdr.flags & CPU_TO_BE16(GOSSIP_STORE_DYING_BIT);
+}
+
 bool gossmap_chan_get_capacity(const struct gossmap *map,
 			       const struct gossmap_chan *c,
 			       struct amount_sat *amount)

common/gossmap.h

@@ -122,6 +122,10 @@ void gossmap_remove_localmods(struct gossmap *map,
 bool gossmap_chan_is_localmod(const struct gossmap *map,
 			      const struct gossmap_chan *c);
 
+/* Is this channel dying? */
+bool gossmap_chan_is_dying(const struct gossmap *map,
+			   const struct gossmap_chan *c);
+
 /* Each channel has a unique (low) index. */
 u32 gossmap_node_idx(const struct gossmap *map, const struct gossmap_node *node);
 u32 gossmap_chan_idx(const struct gossmap *map, const struct gossmap_chan *chan);