bcli: explicitly check at startup that bitcoind does relay transactions

We've had problems with blocksonly in the past and bitcoind still allows
to use outdated (thus potentially dangerous estimates) while running
bitcoin with -blocksonly.

ZmnSCPxj mentionned that we still don't document this, but i figured
that in this specific case an explicit check and error seems preferable.

Changelog-Added: We now explicitly check at startup that our default Bitcoin backend (bitcoind) does relay transactions.

Proposed-by: ZmnSCPxj <zmnscpxj@protonmail.com>
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

README.md

@@ -31,7 +31,7 @@ Don't hesitate to reach out to us on IRC at [#lightning-dev @ freenode.net][irc1
 
 ## Getting Started
 
-c-lightning only works on Linux and Mac OS, and requires a locally (or remotely) running `bitcoind` (version 0.16 or above) that is fully caught up with the network you're testing on.
+c-lightning only works on Linux and Mac OS, and requires a locally (or remotely) running `bitcoind` (version 0.16 or above) that is fully caught up with the network you're running on, and relays transactions (ie with `blocksonly=0`).
 Pruning (`prune=n` option in `bitcoin.conf`) is partially supported, see [here](#pruning) for more details.
 
 ### Installation

plugins/bcli.c

@@ -840,15 +840,16 @@ static void bitcoind_failure(struct plugin *p, const char *error_message)
 /* Do some sanity checks on bitcoind based on the output of `getnetworkinfo`. */
 static void parse_getnetworkinfo_result(struct plugin *p, const char *buf)
 {
-	const jsmntok_t *result, *versiontok;
-	bool valid;
+	const jsmntok_t *result, *versiontok, *relaytok;
+	bool valid, tx_relay;
+	u32 min_version = 160000;
 
 	result = json_parse_input(NULL,
 				  buf, strlen(buf),
 				  &valid);
 	if (!result || !valid)
-		plugin_err(p, "No or invalid response to '%s' ? Got '%s'. Can not"
-			      " continue without proceeding to sanity checks.",
+		plugin_err(p, "Invalid response to '%s': '%s'. Can not "
+			      "continue without proceeding to sanity checks.",
 			      gather_args(bitcoind, "getnetworkinfo", NULL), buf);
 
 	/* Check that we have a fully-featured `estimatesmartfee`. */
@@ -863,9 +864,21 @@ static void parse_getnetworkinfo_result(struct plugin *p, const char *buf)
 			      " continue without proceeding to sanity checks.",
 			      gather_args(bitcoind, "getnetworkinfo", NULL), buf);
 
-	if (bitcoind->version < 160000)
-		plugin_err(p, "Unsupported bitcoind version, you need to update"
-			      " Bitcoin Core.");
+	if (bitcoind->version < min_version)
+		plugin_err(p, "Unsupported bitcoind version %"PRIu32", at least"
+			      " %"PRIu32" required.", bitcoind->version, min_version);
+
+	/* We don't support 'blocksonly', as we rely on transaction relay for fee
+	 * estimates. */
+	relaytok = json_get_member(buf, result, "localrelay");
+	if (!relaytok || !json_to_bool(buf, relaytok, &tx_relay))
+		plugin_err(p, "No 'localrelay' in '%s' ? Got '%s'. Can not"
+			      " continue without proceeding to sanity checks.",
+			      gather_args(bitcoind, "getnetworkinfo", NULL), buf);
+
+	if (!tx_relay)
+		plugin_err(p, "The 'blocksonly' mode of bitcoind, or any option "
+			      "deactivating transaction relay is not supported.");
 
 	tal_free(result);
 }