gossipd: write uuid record on startup.

diff --git a/common/gossmap.c b/common/gossmap.c
index 09d9272c..69a3dbef 100644
--- a/common/gossmap.c
+++ b/common/gossmap.c
@@ -832,6 +832,9 @@ static bool map_catchup(struct gossmap *map, bool must_be_clean, bool *changed)
 		} else if (type == WIRE_GOSSIP_STORE_CHAN_DYING) {
 			/* We don't really care until it's deleted */
 			continue;
+		} else if (type == WIRE_GOSSIP_STORE_UUID) {
+			/* We handled this reopen, otherwise we don't care. */
+			continue;
 		} else {
 			map->logcb(map->cbarg, LOG_BROKEN,
 				   "Unknown record %u@%u (size %zu) in gossmap: ignoring",
diff --git a/gossipd/gossip_store.c b/gossipd/gossip_store.c
index 32c2ae7a..91c6c28f 100644
--- a/gossipd/gossip_store.c
+++ b/gossipd/gossip_store.c
@@ -179,6 +179,18 @@ static bool upgrade_field(u8 oldversion,
 	return true;
 }
 
+static u8 *new_uuid_record(const tal_t *ctx, int fd, u64 *off)
+{
+	u8 *uuid = tal_arr(ctx, u8, 32);
+
+	for (size_t i = 0; i < tal_bytelen(uuid); i++)
+		uuid[i] = pseudorand(256);
+	append_msg(fd, towire_gossip_store_uuid(tmpctx, uuid), 0, off, NULL);
+	/* append_msg does not change file offset, so do that now. */
+	lseek(fd, 0, SEEK_END);
+	return uuid;
+}
+
 /* Read gossip store entries, copy non-deleted ones.  Check basic
  * validity, but this code is written as simply and robustly as
  * possible!
@@ -198,6 +210,7 @@ static int gossip_store_compact(struct daemon *daemon,
 	struct stat st;
 	struct timemono start = time_mono();
 	const char *bad;
+	u8 *uuid = NULL;
 
 	*populated = false;
 	old_len = 1;
@@ -241,6 +254,10 @@ static int gossip_store_compact(struct daemon *daemon,
 
 	cur_off = old_len = sizeof(oldversion);
 
+	/* Make up uuid for old version */
+	if (oldversion < 16)
+		uuid = new_uuid_record(tmpctx, new_fd, total_len);
+
 	/* Read everything, write non-deleted ones to new_fd.  If something goes wrong,
 	 * we end up with truncated store. */
 	while (read_all(old_fd, &hdr, sizeof(hdr))) {
@@ -327,6 +344,13 @@ static int gossip_store_compact(struct daemon *daemon,
 		case WIRE_NODE_ANNOUNCEMENT:
 			nannounces++;
 			break;
+		case WIRE_GOSSIP_STORE_UUID:
+			uuid = tal_arr(tmpctx, u8, 32);
+			if (!fromwire_gossip_store_uuid(msg, uuid)) {
+				bad = "Corrupt uuid";
+				goto badmsg;
+			}
+			break;
 		}
 
 		if (!write_all(new_fd, &hdr, sizeof(hdr))
@@ -348,6 +372,16 @@ static int gossip_store_compact(struct daemon *daemon,
 	}
 
 rename_new:
+	/* If we didn't copy a uuid, do so now. */
+	if (!uuid) {
+		if (lseek(new_fd, 0, SEEK_END) != 1) {
+			bad = tal_fmt(tmpctx,
+				      "missing uuid in version %u", oldversion);
+			goto badmsg;
+		}
+		uuid = new_uuid_record(tmpctx, new_fd, total_len);
+	}
+
 	if (rename(GOSSIP_STORE_TEMP_FILENAME, GOSSIP_STORE_FILENAME) != 0) {
 		status_failed(STATUS_FAIL_INTERNAL_ERROR,
 			      "gossip_store_compact: rename failed: %s",
@@ -357,7 +391,6 @@ rename_new:
 	/* Create end marker now new file exists. */
 	if (old_fd != -1) {
 		/* FIXME: real uuid! */
-		u8 uuid[32] = {0};
 		append_msg(old_fd, towire_gossip_store_ended(tmpctx, *total_len, uuid),
 			   0, &old_len, NULL);
 		close(old_fd);
diff --git a/tests/test_gossip.py b/tests/test_gossip.py
index b29f3b97..e6a2bb07 100644
--- a/tests/test_gossip.py
+++ b/tests/test_gossip.py
@@ -1207,7 +1207,7 @@ def test_gossip_store_load(node_factory):
 
     l1.start()
     # May preceed the Started msg waited for in 'start'.
-    wait_for(lambda: l1.daemon.is_in_log('Read 1/1/1/0 cannounce/cupdate/nannounce/delete from store in 832 bytes, now 778 bytes'))
+    wait_for(lambda: l1.daemon.is_in_log('Read 1/1/1/0 cannounce/cupdate/nannounce/delete from store in 832 bytes, now 824 bytes'))
     assert not l1.daemon.is_in_log('gossip_store.*truncating')
 
 
@@ -1275,13 +1275,13 @@ def test_gossip_store_load_announce_before_update(node_factory):
 
     l1.start()
     # May preceed the Started msg waited for in 'start'.
-    wait_for(lambda: l1.daemon.is_in_log('Read 1/1/1/1 cannounce/cupdate/nannounce/delete from store in 950 bytes, now 778 bytes'))
+    wait_for(lambda: l1.daemon.is_in_log('Read 1/1/1/1 cannounce/cupdate/nannounce/delete from store in 982 bytes, now 824 bytes'))
     assert not l1.daemon.is_in_log('gossip_store.*truncating')
 
 
 def test_gossip_store_load_amount_truncated(node_factory):
     """Make sure we can read canned gossip store with truncated amount"""
-    l1 = node_factory.get_node(start=False, broken_log=r'gossip_store only processed 1 bytes of 445 \(expected 445\)|Moving to gossip_store.corrupt|plugin-cln-renepay:.*unable to fetch channel capacity')
+    l1 = node_factory.get_node(start=False, broken_log=r'gossip_store only processed 47 bytes of 491 \(expected 491\)|Moving to gossip_store.corrupt|plugin-cln-renepay:.*unable to fetch channel capacity')
     with open(os.path.join(l1.daemon.lightning_dir, TEST_NETWORK, 'gossip_store'), 'wb') as f:
         f.write(bytearray.fromhex("0c"        # GOSSIP_STORE_VERSION
                                   "000001b0"  # len
@@ -1292,9 +1292,9 @@ def test_gossip_store_load_amount_truncated(node_factory):
 
     l1.start()
     # May preceed the Started msg waited for in 'start'.
-    wait_for(lambda: l1.daemon.is_in_log(r'\*\*BROKEN\*\* gossipd: gossip_store only processed 1 bytes of 445 \(expected 445\)'))
+    wait_for(lambda: l1.daemon.is_in_log(r'\*\*BROKEN\*\* gossipd: gossip_store only processed 47 bytes of 491 \(expected 491\)'))
     wait_for(lambda: l1.daemon.is_in_log(r'\*\*BROKEN\*\* gossipd: gossip_store: Moving to gossip_store.corrupt'))
-    wait_for(lambda: l1.daemon.is_in_log(r'gossip_store: Read 0/0/0/0 cannounce/cupdate/nannounce/delete from store in 1 bytes, now 1 bytes \(populated=false\)'))
+    wait_for(lambda: l1.daemon.is_in_log(r'gossip_store: Read 0/0/0/0 cannounce/cupdate/nannounce/delete from store in 1 bytes, now 47 bytes \(populated=false\)'))
     assert os.path.exists(os.path.join(l1.daemon.lightning_dir, TEST_NETWORK, 'gossip_store.corrupt'))
 
 
@@ -1658,7 +1658,7 @@ def test_gossip_store_load_no_channel_update(node_factory):
     l1.start()
 
     # May preceed the Started msg waited for in 'start'.
-    wait_for(lambda: l1.daemon.is_in_log('Read 1/0/1/0 cannounce/cupdate/nannounce/delete from store in 650 bytes, now 628 bytes'))
+    wait_for(lambda: l1.daemon.is_in_log('Read 1/0/1/0 cannounce/cupdate/nannounce/delete from store in 682 bytes, now 674 bytes'))
     assert not os.path.exists(os.path.join(l1.daemon.lightning_dir, TEST_NETWORK, 'gossip_store.corrupt'))