gossipd: dev-compact-gossip-store to manually invoke compaction.

diff --git a/gossipd/gossipd.c b/gossipd/gossipd.c
index 5cb2541c..6c0d0739 100644
--- a/gossipd/gossipd.c
+++ b/gossipd/gossipd.c
@@ -542,6 +542,12 @@ static struct io_plan *recv_req(struct io_conn *conn,
 			goto done;
 		}
 		/* fall thru */
+	case WIRE_GOSSIPD_DEV_COMPACT_STORE:
+		if (daemon->developer) {
+			gossmap_manage_handle_dev_compact_store(daemon->gm, msg);
+			goto done;
+		}
+		/* fall thru */
 
 	/* We send these, we don't receive them */
 	case WIRE_GOSSIPD_INIT_CUPDATE:
@@ -549,6 +555,7 @@ static struct io_plan *recv_req(struct io_conn *conn,
 	case WIRE_GOSSIPD_INIT_REPLY:
 	case WIRE_GOSSIPD_GET_TXOUT:
 	case WIRE_GOSSIPD_DEV_MEMLEAK_REPLY:
+	case WIRE_GOSSIPD_DEV_COMPACT_STORE_REPLY:
 	case WIRE_GOSSIPD_ADDGOSSIP_REPLY:
 	case WIRE_GOSSIPD_NEW_BLOCKHEIGHT_REPLY:
 	case WIRE_GOSSIPD_REMOTE_CHANNEL_UPDATE:
diff --git a/gossipd/gossipd_wire.csv b/gossipd/gossipd_wire.csv
index 79de7512..8d407aa7 100644
--- a/gossipd/gossipd_wire.csv
+++ b/gossipd/gossipd_wire.csv
@@ -50,6 +50,13 @@ msgtype,gossipd_dev_memleak,3033
 msgtype,gossipd_dev_memleak_reply,3133
 msgdata,gossipd_dev_memleak_reply,leak,bool,
 
+# master -> gossipd: please rewrite the gossip_store
+msgtype,gossipd_dev_compact_store,3034
+
+# gossipd -> master: empty string means no problem.
+msgtype,gossipd_dev_compact_store_reply,3134
+msgdata,gossipd_dev_compact_store_reply,result,wirestring,
+
 # master -> gossipd: blockheight increased.
 msgtype,gossipd_new_blockheight,3026
 msgdata,gossipd_new_blockheight,blockheight,u32,
diff --git a/gossipd/gossmap_manage.c b/gossipd/gossmap_manage.c
index ed11baff..385cd66d 100644
--- a/gossipd/gossmap_manage.c
+++ b/gossipd/gossmap_manage.c
@@ -70,6 +70,7 @@ struct cannounce_map {
 struct compactd {
 	struct io_conn *in_conn;
 	u64 old_size;
+	bool dev_compact;
 	u8 ignored;
 	int outfd;
 	pid_t pid;
@@ -1582,6 +1583,24 @@ bool gossmap_manage_populated(const struct gossmap_manage *gm)
 	return gm->gossip_store_populated;
 }
 
+static void compactd_broken(const struct gossmap_manage *gm,
+			    const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	status_vfmt(LOG_BROKEN, NULL, fmt, ap);
+	va_end(ap);
+
+	if (gm->compactd->dev_compact) {
+		va_start(ap, fmt);
+		daemon_conn_send(gm->daemon->master,
+				 take(towire_gossipd_dev_compact_store_reply(NULL,
+									     tal_vfmt(tmpctx, fmt, ap))));
+		va_end(ap);
+	}
+}
+
 static void compactd_done(struct io_conn *unused, struct gossmap_manage *gm)
 {
 	int status;
@@ -1594,18 +1613,18 @@ static void compactd_done(struct io_conn *unused, struct gossmap_manage *gm)
 			      strerror(errno));
 
 	if (!WIFEXITED(status)) {
-		status_broken("compactd failed with signal %u",
-			      WTERMSIG(status));
+		compactd_broken(gm, "compactd failed with signal %u",
+				WTERMSIG(status));
 		goto failed;
 	}
 	if (WEXITSTATUS(status) != 0) {
-		status_broken("compactd exited with status %u",
+		compactd_broken(gm, "compactd exited with status %u",
 			      WEXITSTATUS(status));
 		goto failed;
 	}
 
 	if (stat(GOSSIP_STORE_COMPACT_FILENAME, &st) != 0) {
-		status_broken("compactd did not create file? %s",
+		compactd_broken(gm, "compactd did not create file? %s",
 			      strerror(errno));
 		goto failed;
 	}
@@ -1639,6 +1658,9 @@ static void compactd_done(struct io_conn *unused, struct gossmap_manage *gm)
 			 towire_gossip_store_ended(tmpctx, st.st_size, gm->compactd->uuid),
 			 0);
 	gossip_store_reopen(gm->gs);
+	if (gm->compactd->dev_compact)
+		daemon_conn_send(gm->daemon->master,
+				 take(towire_gossipd_dev_compact_store_reply(NULL, "")));
 
 failed:
 	gm->compactd = tal_free(gm->compactd);
@@ -1666,7 +1688,7 @@ static struct io_plan *init_compactd_conn_in(struct io_conn *conn,
 		       compactd_read_done, gm);
 }
 /* Returns false if already running */
-static bool gossmap_compact(struct gossmap_manage *gm)
+static bool gossmap_compact(struct gossmap_manage *gm, bool dev_compact)
 {
 	int childin[2], execfail[2], childout[2];
 	int saved_errno;
@@ -1753,6 +1775,7 @@ static bool gossmap_compact(struct gossmap_manage *gm)
 	}
 	close(execfail[0]);
 
+	gm->compactd->dev_compact = dev_compact;
 	gm->compactd->outfd = childout[1];
 	gm->compactd->in_conn = io_new_conn(gm->compactd, childin[0],
 					    init_compactd_conn_in, gm);
@@ -1776,7 +1799,7 @@ void gossmap_manage_maybe_compact(struct gossmap_manage *gm)
 	if (num_dead < 4 * num_live)
 		return;
 
-	compact_started = gossmap_compact(gm);
+	compact_started = gossmap_compact(gm, false);
 	status_debug("%s gossmap compaction:"
 		     " %"PRIu64" with"
 		     " %"PRIu64" live records and %"PRIu64" dead records",
@@ -1784,3 +1807,15 @@ void gossmap_manage_maybe_compact(struct gossmap_manage *gm)
 		     gossip_store_len_written(gm->gs),
 		     num_live, num_dead);
 }
+
+void gossmap_manage_handle_dev_compact_store(struct gossmap_manage *gm, const u8 *msg)
+{
+	if (!fromwire_gossipd_dev_compact_store(msg))
+		master_badmsg(WIRE_GOSSIPD_DEV_COMPACT_STORE, msg);
+
+	if (!gossmap_compact(gm, true))
+		daemon_conn_send(gm->daemon->master,
+				 take(towire_gossipd_dev_compact_store_reply(NULL,
+									     "Already compacting")));
+}
+
diff --git a/gossipd/gossmap_manage.h b/gossipd/gossmap_manage.h
index c1357227..464c61f4 100644
--- a/gossipd/gossmap_manage.h
+++ b/gossipd/gossmap_manage.h
@@ -118,6 +118,10 @@ bool gossmap_manage_populated(const struct gossmap_manage *gm);
  */
 void gossmap_manage_maybe_compact(struct gossmap_manage *gm);
 
+/* For testing */
+void gossmap_manage_handle_dev_compact_store(struct gossmap_manage *gm,
+					     const u8 *msg);
+
 /* For memleak to see inside of maps */
 void gossmap_manage_memleak(struct htable *memtable, const struct gossmap_manage *gm);
 #endif /* LIGHTNING_GOSSIPD_GOSSMAP_MANAGE_H */
diff --git a/lightningd/gossip_control.c b/lightningd/gossip_control.c
index 038eb1f7..9c698cd3 100644
--- a/lightningd/gossip_control.c
+++ b/lightningd/gossip_control.c
@@ -196,11 +196,13 @@ static unsigned gossip_msg(struct subd *gossip, const u8 *msg, const int *fds)
 	case WIRE_GOSSIPD_GET_TXOUT_REPLY:
 	case WIRE_GOSSIPD_OUTPOINTS_SPENT:
 	case WIRE_GOSSIPD_DEV_MEMLEAK:
+	case WIRE_GOSSIPD_DEV_COMPACT_STORE:
 	case WIRE_GOSSIPD_NEW_BLOCKHEIGHT:
 	case WIRE_GOSSIPD_ADDGOSSIP:
 	/* This is a reply, so never gets through to here. */
 	case WIRE_GOSSIPD_INIT_REPLY:
 	case WIRE_GOSSIPD_DEV_MEMLEAK_REPLY:
+	case WIRE_GOSSIPD_DEV_COMPACT_STORE_REPLY:
 	case WIRE_GOSSIPD_ADDGOSSIP_REPLY:
 	case WIRE_GOSSIPD_NEW_BLOCKHEIGHT_REPLY:
 		break;
@@ -488,3 +490,46 @@ static const struct json_command dev_set_max_scids_encode_size = {
 	.dev_only = true,
 };
 AUTODATA(json_command, &dev_set_max_scids_encode_size);
+
+static void dev_compact_gossip_store_reply(struct subd *gossip UNUSED,
+					   const u8 *reply,
+					   const int *fds UNUSED,
+					   struct command *cmd)
+{
+	char *result;
+
+	if (!fromwire_gossipd_dev_compact_store_reply(cmd, reply, &result)) {
+		was_pending(command_fail(cmd, LIGHTNINGD,
+					 "Gossip gave bad dev_gossip_compact_store_reply"));
+		return;
+	}
+
+	if (streq(result, ""))
+		was_pending(command_success(cmd, json_stream_success(cmd)));
+	else
+		was_pending(command_fail(cmd, LIGHTNINGD,
+					 "gossip_compact_store failed: %s", result));
+}
+
+static struct command_result *json_dev_compact_gossip_store(struct command *cmd,
+							    const char *buffer,
+							    const jsmntok_t *obj UNNEEDED,
+							    const jsmntok_t *params)
+{
+	u8 *msg;
+
+	if (!param(cmd, buffer, params, NULL))
+		return command_param_failed();
+
+	msg = towire_gossipd_dev_compact_store(NULL);
+	subd_req(cmd->ld->gossip, cmd->ld->gossip,
+		 take(msg), -1, 0, dev_compact_gossip_store_reply, cmd);
+	return command_still_pending(cmd);
+}
+
+static const struct json_command dev_compact_gossip_store = {
+	"dev-compact-gossip-store",
+	json_dev_compact_gossip_store,
+	.dev_only = true,
+};
+AUTODATA(json_command, &dev_compact_gossip_store);
diff --git a/tests/plugins/compacter-slow.sh b/tests/plugins/compacter-slow.sh
new file mode 100755
index 00000000..0812c62e
--- /dev/null
+++ b/tests/plugins/compacter-slow.sh
@@ -0,0 +1,11 @@
+#! /bin/sh -e
+# This pretends to be lightning_gossip_compactd, but waits until the file "compactd-continue"
+# exists.  This lets us test race conditions.
+
+if [ x"$1" != x"--version" ]; then
+    while [ ! -f "compactd-continue" ]; do
+	sleep 1
+    done
+fi
+
+exec "$(dirname "$0")"/../../lightningd/lightning_gossip_compactd "$@"
diff --git a/tests/test_gossip.py b/tests/test_gossip.py
index 440a9a7c..c17c7bf2 100644
--- a/tests/test_gossip.py
+++ b/tests/test_gossip.py
@@ -1281,6 +1281,9 @@ def test_gossip_store_load_announce_before_update(node_factory):
     l1.daemon.wait_for_log("gossipd: gossip_store: 3 live records, 0 deleted")
     assert not l1.daemon.is_in_log('gossip_store.*corrupt')
 
+    # Extra sanity check if we can.
+    l1.rpc.call('dev-compact-gossip-store')
+
 
 def test_gossip_store_load_amount_truncated(node_factory):
     """Make sure we can read canned gossip store with truncated amount"""
@@ -1300,6 +1303,11 @@ def test_gossip_store_load_amount_truncated(node_factory):
     wait_for(lambda: l1.daemon.is_in_log('gossip_store: 0 live records, 0 deleted'))
     assert os.path.exists(os.path.join(l1.daemon.lightning_dir, TEST_NETWORK, 'gossip_store.corrupt'))
 
+    # Extra sanity check if we can.
+    l1.rpc.call('dev-compact-gossip-store')
+    l1.restart()
+    l1.rpc.call('dev-compact-gossip-store')
+
 
 @pytest.mark.openchannel('v1')
 @pytest.mark.openchannel('v2')
@@ -1628,10 +1636,105 @@ def test_gossip_store_corrupt(node_factory, bitcoind):
 def test_gossip_store_load_complex(node_factory, bitcoind):
     l2 = setup_gossip_store_test(node_factory, bitcoind)
 
+    l2.rpc.call('dev-compact-gossip-store')
+    l2.restart()
+
+    wait_for(lambda: l2.daemon.is_in_log('gossip_store: 9 live records, 0 deleted'))
+
+
+def test_gossip_store_compact(node_factory, bitcoind):
+    l2 = setup_gossip_store_test(node_factory, bitcoind)
+
+    # Now compact store.
+    l2.rpc.call('dev-compact-gossip-store')
+    # Splicing changes features, making this size 2365 bytes -> 2065 bytes.
+    l2.daemon.wait_for_logs(['gossipd: compaction done: 236[25] -> 206[25] bytes',
+                             'connectd: Reopened gossip_store, reduced to offset 206[25]'])
+
+    # Should still be connected.
+    time.sleep(1)
+    assert len(l2.rpc.listpeers()['peers']) == 2
+
+    # Should restart ok.
     l2.restart()
+    wait_for(lambda: l2.daemon.is_in_log('gossipd: gossip_store: 9 live records, 0 deleted'))
+
+
+def test_gossip_store_compact_while_extending(node_factory, bitcoind, executor):
+    """We change gossip_store file (additions, deletion) during compaction"""
+    l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True,
+                                         opts=[{'subdaemon': 'gossip_compactd:../tests/plugins/compacter-slow.sh'}, {}, {}])
+    l2.rpc.setchannel(l3.info['id'], 20, 1000)
+    l3.rpc.setchannel(l2.info['id'], 21, 1001)
+
+    # Wait for it to hit l2's gossip store.
+    wait_for(lambda: sorted([c['fee_per_millionth'] for c in l1.rpc.listchannels()['channels']]) == [10, 10, 1000, 1001])
+
+    # We start compaction, but gossip store continues
+    fut = executor.submit(l1.rpc.call, 'dev-compact-gossip-store')
+    # Make sure it started!
+    l1.daemon.wait_for_log('Executing lightning_gossip_compactd')
+
+    # We make another channel, remove old one
+    l4 = node_factory.get_node()
+    node_factory.join_nodes([l3, l4], wait_for_announce=True)
+    scid34 = only_one(l4.rpc.listpeerchannels()['channels'])['short_channel_id']
+    wait_for(lambda: len(l1.rpc.listchannels(scid34)['channels']) == 2)
+
+    scid23 = only_one(l2.rpc.listpeerchannels(l3.info['id'])['channels'])['short_channel_id']
+    l2.rpc.close(l3.info['id'])
+    bitcoind.generate_block(13, wait_for_mempool=1)
+    wait_for(lambda: l1.rpc.listchannels(scid23) == {'channels': []})
 
+    l1.rpc.setchannel(l2.info['id'], 41, 1004)
+    scid12 = only_one(l1.rpc.listpeerchannels()['channels'])['short_channel_id']
+    wait_for(lambda: sorted([c['fee_per_millionth'] for c in l1.rpc.listchannels(scid12)['channels']]) == [10, 1004])
+
+    pre_channels = l1.rpc.listchannels()
+    pre_nodes = sorted(l1.rpc.listnodes()['nodes'], key=lambda n: n['nodeid'])
+
+    # Compaction "continues".
+    with open(os.path.join(l1.daemon.lightning_dir, TEST_NETWORK, 'compactd-continue'), 'wb') as f:
+        f.write(b'')
+
+    fut.result(TIMEOUT)
+    # Exact gossip size varies with EXPERIMENTAL_SPLICING.
+    l1.daemon.wait_for_logs(['gossipd: compaction done',
+                             'connectd: Reopened gossip_store, reduced to offset 224[59]'])
+
+    post_channels = l1.rpc.listchannels()
+    post_nodes = sorted(l1.rpc.listnodes()['nodes'], key=lambda n: n['nodeid'])
+    l1.daemon.wait_for_log('topology: Reopened gossip_store, reduced to offset 224[59]')
+
+    assert post_channels == pre_channels
+    assert post_nodes == pre_nodes
+
+
+def test_gossip_store_compact_miss_update(node_factory, bitcoind, executor):
+    """If we compact twice, you should notice the UUID difference."""
+    l2 = setup_gossip_store_test(node_factory, bitcoind)
+
+    pre_channels = l2.rpc.listchannels()
+
+    # Now compact store twice.
+    l2.rpc.call('dev-compact-gossip-store')
+    l2.rpc.call('dev-compact-gossip-store')
+
+    post_channels = l2.rpc.listchannels()
+    l2.daemon.wait_for_log('topology: Reopened gossip_store, but we missed some')
+    assert pre_channels == post_channels
+
+
+def test_gossip_store_compact_restart(node_factory, bitcoind):
+    l2 = setup_gossip_store_test(node_factory, bitcoind)
+
+    # Should restart ok.
+    l2.restart()
     wait_for(lambda: l2.daemon.is_in_log('gossip_store: 9 live records, 2 deleted'))
 
+    # Now compact store.
+    l2.rpc.call('dev-compact-gossip-store')
+
 
 def test_gossip_store_load_no_channel_update(node_factory):
     """Make sure we can read truncated gossip store with a channel_announcement and no channel_update"""
diff --git a/tools/bench-gossipd.sh b/tools/bench-gossipd.sh
index 4bea1552..b98295b5 100755
--- a/tools/bench-gossipd.sh
+++ b/tools/bench-gossipd.sh
@@ -5,7 +5,7 @@ set -e
 
 DIR=""
 TARGETS=""
-DEFAULT_TARGETS=" store_load_msec vsz_kb listnodes_sec listchannels_sec routing_sec peer_write_all_sec peer_read_all_sec "
+DEFAULT_TARGETS=" store_load_msec vsz_kb store_rewrite_sec listnodes_sec listchannels_sec routing_sec peer_write_all_sec peer_read_all_sec "
 MCP_DIR=../million-channels-project/data/1M/gossip/
 CSV=false
 
@@ -111,6 +111,12 @@ if [ -z "${TARGETS##* vsz_kb *}" ]; then
     ps -o vsz= -p "$(pidof lightning_gossipd)" | print_stat vsz_kb
 fi
 
+# How long does rewriting the store take?
+if [ -z "${TARGETS##* store_rewrite_sec *}" ]; then
+    # shellcheck disable=SC2086
+    /usr/bin/time --append -f %e $LCLI1 dev-compact-gossip-store 2>&1 > /dev/null | print_stat store_rewrite_sec
+fi
+
 # Now, how long does listnodes take?
 if [ -z "${TARGETS##* listnodes_sec *}" ]; then
     # shellcheck disable=SC2086