lightningd: only run watchman under --experimental-bwatch

diff --git a/lightningd/lightningd.c b/lightningd/lightningd.c
index 6810baee..682900ce 100644
--- a/lightningd/lightningd.c
+++ b/lightningd/lightningd.c
@@ -46,6 +46,7 @@
 
 /*~ This is common code: routines shared by one or more executables
  *  (separate daemons, or the lightning-cli program). */
+#include <common/configvar.h>
 #include <common/daemon.h>
 #include <common/deprecation.h>
 #include <common/ecdh_hsmd.h>
@@ -288,6 +289,11 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 	 * so set it to NULL explicitly now. */
 	ld->wallet = NULL;
 
+	/*~ Only created if the user opts into --experimental-bwatch, but
+	 * plugin startup (watchman_notify_plugin_ready) examines it, so set
+	 * it to NULL explicitly now. */
+	ld->watchman = NULL;
+
 	/*~ Behavioral options */
 	ld->accept_extra_tlv_types = tal_arr(ld, u64, 0);
 
@@ -1164,6 +1170,18 @@ static void setup_fd_limit(struct lightningd *ld, size_t num_channels)
 	}
 }
 
+/*~ Has the user opted into the experimental bwatch chain watcher?  The
+ * --experimental-bwatch flag is registered by the bwatch plugin, not by
+ * lightningd, so we look for it in the parsed configvars rather than
+ * keeping our own copy. */
+static bool bwatch_enabled(const struct lightningd *ld)
+{
+	const char **names = tal_arr(tmpctx, const char *, 1);
+
+	names[0] = "experimental-bwatch";
+	return configvar_first(ld->configvars, names) != NULL;
+}
+
 int main(int argc, char *argv[])
 {
 	struct lightningd *ld;
@@ -1349,8 +1367,16 @@ int main(int argc, char *argv[])
 	trace_span_end(ld->topology);
 
 	/*~ Stand up the watchman: it queues bwatch RPC requests until the
-	 * bwatch plugin reports ready, then replays them. */
-	ld->watchman = watchman_new(ld, ld);
+	 * bwatch plugin reports ready, then replays them.  Must come after
+	 * setup_topology so start_block reflects the last-processed height.
+	 *
+	 * bwatch is opt-in for now: the --experimental-bwatch flag is
+	 * registered by the bwatch plugin, so peek at the configvar to gate
+	 * the lightningd side too.  Without it, ld->watchman stays NULL and
+	 * the watchman_* entry points are no-ops, leaving chain_topology as
+	 * the only chain watcher. */
+	if (bwatch_enabled(ld))
+		ld->watchman = watchman_new(ld, ld);
 
 	db_begin_transaction(ld->wallet->db);
 	trace_span_start("delete_old_htlcs", ld->wallet);
diff --git a/lightningd/watchman.c b/lightningd/watchman.c
index 14d98541..15dd6b32 100644
--- a/lightningd/watchman.c
+++ b/lightningd/watchman.c
@@ -286,7 +286,13 @@ static void watchman_add(struct lightningd *ld, const char *method,
 			 const char *owner, const char *json_params)
 {
 	struct watchman *wm = ld->watchman;
-	char *op_id = tal_fmt(tmpctx, "%s:%s", method, owner);
+	char *op_id;
+
+	/* No-op unless --experimental-bwatch stood up the watchman. */
+	if (!wm)
+		return;
+
+	op_id = tal_fmt(tmpctx, "%s:%s", method, owner);
 
 	/* Remove any existing add for this owner */
 	watchman_ack(ld, op_id);
@@ -304,12 +310,18 @@ static void watchman_del(struct lightningd *ld, const char *method,
 			 const char *owner, const char *json_params)
 {
 	struct watchman *wm = ld->watchman;
-	char *op_id = tal_fmt(tmpctx, "%s:%s", method, owner);
+	char *op_id, *add_op_id;
+
+	/* No-op unless --experimental-bwatch stood up the watchman. */
+	if (!wm)
+		return;
+
+	op_id = tal_fmt(tmpctx, "%s:%s", method, owner);
 
 	/* Cancel any pending add for this owner.  All del-methods are named
 	 * "del<suffix>" and their paired add-method is "add<suffix>" */
 	assert(strstarts(method, "del"));
-	char *add_op_id = tal_fmt(tmpctx, "add%s:%s", method + strlen("del"), owner);
+	add_op_id = tal_fmt(tmpctx, "add%s:%s", method + strlen("del"), owner);
 	watchman_ack(ld, add_op_id);
 	enqueue_op(wm, method, op_id, json_params);
 }
@@ -326,6 +338,9 @@ void watchman_ack(struct lightningd *ld, const char *op_id)
 {
 	struct watchman *wm = ld->watchman;
 
+	if (!wm)
+		return;
+
 	for (size_t i = 0; i < tal_count(wm->pending_ops); i++) {
 		if (streq(wm->pending_ops[i]->op_id, op_id)) {
 			db_remove(wm, op_id);
@@ -346,6 +361,9 @@ void watchman_replay_pending(struct lightningd *ld)
 {
 	struct watchman *wm = ld->watchman;
 
+	if (!wm)
+		return;
+
 	for (size_t i = 0; i < tal_count(wm->pending_ops); i++) {
 		struct pending_op *op = wm->pending_ops[i];
 		send_to_bwatch(wm, method_from_op_id(tmpctx, op->op_id),
@@ -805,6 +823,9 @@ static struct command_result *json_chaininfo(struct command *cmd,
 		   NULL))
 		return command_param_failed();
 
+	if (!cmd->ld->watchman)
+		return command_fail(cmd, LIGHTNINGD, "Watchman not initialized");
+
 	if (!streq(chain, chainparams->bip70_name))
 		fatal("Wrong network! Our Bitcoin backend is running on '%s',"
 		      " but we expect '%s'.", chain, chainparams->bip70_name);