common/json: make json_scan return an error string.

This makes for more useful errors.  It prints where it was up to in
the guide, but doesn't print the entire JSON it's scanning.

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

plugins/bcli.c

@@ -326,6 +326,7 @@ static struct command_result *process_getutxout(struct bitcoin_cli *bcli)
 	const jsmntok_t *tokens;
 	struct json_stream *response;
 	struct bitcoin_tx_output output;
+	const char *err;
 
 	/* As of at least v0.15.1.0, bitcoind returns "success" but an empty
 	   string on a spent txout. */
@@ -343,14 +344,14 @@ static struct command_result *process_getutxout(struct bitcoin_cli *bcli)
 		return command_err_bcli_badjson(bcli, "cannot parse");
 	}
 
-	if (!json_scan(bcli->output, tokens,
+	err = json_scan(tmpctx, bcli->output, tokens,
 		       "{value:%,scriptPubKey:{hex:%}}",
 		       JSON_SCAN(json_to_bitcoin_amount,
 				 &output.amount.satoshis), /* Raw: bitcoind */
 		       JSON_SCAN_TAL(bcli, json_tok_bin_from_hex,
-				     &output.script))) {
-		return command_err_bcli_badjson(bcli, "cannot scan");
-	}
+				     &output.script));
+	if (err)
+		return command_err_bcli_badjson(bcli, err);
 
 	response = jsonrpc_stream_success(bcli->cmd);
 	json_add_amount_sat_only(response, "amount", output.amount);
@@ -365,7 +366,7 @@ static struct command_result *process_getblockchaininfo(struct bitcoin_cli *bcli
 	struct json_stream *response;
 	bool ibd;
 	u32 headers, blocks;
-	const char *chain;
+	const char *chain, *err;
 
 	tokens = json_parse_simple(bcli->output,
 				   bcli->output, bcli->output_bytes);
@@ -373,14 +374,14 @@ static struct command_result *process_getblockchaininfo(struct bitcoin_cli *bcli
 		return command_err_bcli_badjson(bcli, "cannot parse");
 	}
 
-	if (!json_scan(bcli->output, tokens,
-		       "{chain:%,headers:%,blocks:%,initialblockdownload:%}",
-		       JSON_SCAN_TAL(tmpctx, json_strdup, &chain),
-		       JSON_SCAN(json_to_number, &headers),
-		       JSON_SCAN(json_to_number, &blocks),
-		       JSON_SCAN(json_to_bool, &ibd))) {
-		return command_err_bcli_badjson(bcli, "cannot scan");
-	}
+	err = json_scan(tmpctx, bcli->output, tokens,
+			"{chain:%,headers:%,blocks:%,initialblockdownload:%}",
+			JSON_SCAN_TAL(tmpctx, json_strdup, &chain),
+			JSON_SCAN(json_to_number, &headers),
+			JSON_SCAN(json_to_number, &blocks),
+			JSON_SCAN(json_to_bool, &ibd));
+	if (err)
+		return command_err_bcli_badjson(bcli, err);
 
 	response = jsonrpc_stream_success(bcli->cmd);
 	json_add_string(response, "chain", chain);
@@ -425,8 +426,8 @@ estimatefees_parse_feerate(struct bitcoin_cli *bcli, u64 *feerate)
 		return command_err_bcli_badjson(bcli, "cannot parse");
 	}
 
-	if (!json_scan(bcli->output, tokens, "{feerate:%}",
-		       JSON_SCAN(json_to_bitcoin_amount, feerate))) {
+	if (json_scan(tmpctx, bcli->output, tokens, "{feerate:%}",
+		      JSON_SCAN(json_to_bitcoin_amount, feerate)) != NULL) {
 		/* Paranoia: if it had a feerate, but was malformed: */
 		if (json_get_member(bcli->output, tokens, "feerate"))
 			return command_err_bcli_badjson(bcli, "cannot scan");
@@ -782,6 +783,7 @@ static void parse_getnetworkinfo_result(struct plugin *p, const char *buf)
 	const jsmntok_t *result;
 	bool tx_relay;
 	u32 min_version = 160000;
+	const char *err;
 
 	result = json_parse_simple(NULL, buf, strlen(buf));
 	if (!result)
@@ -790,12 +792,14 @@ static void parse_getnetworkinfo_result(struct plugin *p, const char *buf)
 			      gather_args(bitcoind, "getnetworkinfo", NULL), buf);
 
 	/* Check that we have a fully-featured `estimatesmartfee`. */
-	if (!json_scan(buf, result, "{version:%,localrelay:%}",
-		       JSON_SCAN(json_to_u32, &bitcoind->version),
-		       JSON_SCAN(json_to_bool, &tx_relay)))
-		plugin_err(p, "No 'version' or localrelay in '%s' ? Got '%s'. Can not"
-			      " continue without proceeding to sanity checks.",
-			      gather_args(bitcoind, "getnetworkinfo", NULL), buf);
+	err = json_scan(tmpctx, buf, result, "{version:%,localrelay:%}",
+			JSON_SCAN(json_to_u32, &bitcoind->version),
+			JSON_SCAN(json_to_bool, &tx_relay));
+	if (err)
+		plugin_err(p, "%s.  Got '%s'. Can not"
+			   " continue without proceeding to sanity checks.",
+			   err,
+			   gather_args(bitcoind, "getnetworkinfo", NULL), buf);
 
 	if (bitcoind->version < min_version)
 		plugin_err(p, "Unsupported bitcoind version %"PRIu32", at least"

plugins/keysend.c

@@ -251,10 +251,13 @@ static struct command_result *htlc_accepted_call(struct command *cmd,
 	struct keysend_in *ki;
 	struct out_req *req;
 	struct timeabs now = time_now();
+	const char *err;
 
-	if (!json_scan(buf, params, "{onion:{payload:%},htlc:{payment_hash:%}}",
-		       JSON_SCAN_TAL(cmd, json_tok_bin_from_hex, &rawpayload),
-		       JSON_SCAN(json_to_sha256, &payment_hash)))
+	err = json_scan(tmpctx, buf, params,
+			"{onion:{payload:%},htlc:{payment_hash:%}}",
+			JSON_SCAN_TAL(cmd, json_tok_bin_from_hex, &rawpayload),
+			JSON_SCAN(json_to_sha256, &payment_hash));
+	if (err)
 		return htlc_accepted_continue(cmd, NULL);
 
 	max = tal_bytelen(rawpayload);

plugins/libplugin.c

@@ -498,6 +498,7 @@ void rpc_scan(struct plugin *plugin,
 	      ...)
 {
 	bool error;
+	const char *err;
 	const jsmntok_t *contents;
 	int reqlen;
 	const char *p;
@@ -515,12 +516,13 @@ void rpc_scan(struct plugin *plugin,
 	p = membuf_consume(&plugin->rpc_conn->mb, reqlen);
 
 	va_start(ap, guide);
-	error = !json_scanv(p, contents, guide, ap);
+	err = json_scanv(tmpctx, p, contents, guide, ap);
 	va_end(ap);
 
-	if (error)
-		plugin_err(plugin, "Could not parse %s in reply to %s: '%.*s'",
-			   guide, method, reqlen, membuf_elems(&plugin->rpc_conn->mb));
+	if (err)
+		plugin_err(plugin, "Could not parse %s in reply to %s: %s: '%.*s'",
+			   guide, method, err,
+			   reqlen, membuf_elems(&plugin->rpc_conn->mb));
 }
 
 static void handle_rpc_reply(struct plugin *plugin, const jsmntok_t *toks)
@@ -810,19 +812,21 @@ static struct command_result *handle_init(struct command *cmd,
 	char *dir, *network;
 	struct plugin *p = cmd->plugin;
 	bool with_rpc = p->rpc_conn != NULL;
+	const char *err;
 
 	configtok = json_get_member(buf, params, "configuration");
-	if (!json_scan(buf, configtok,
-		       "{lightning-dir:%"
-		       ",network:%"
-		       ",feature_set:%"
-		       ",rpc-file:%}",
-		       JSON_SCAN_TAL(tmpctx, json_strdup, &dir),
-		       JSON_SCAN_TAL(tmpctx, json_strdup, &network),
-		       JSON_SCAN_TAL(p, json_to_feature_set, &p->our_features),
-		       JSON_SCAN_TAL(p, json_strdup, &p->rpc_location)))
-		plugin_err(p, "cannot scan init params: %.*s",
-			   json_tok_full_len(params),
+	err = json_scan(tmpctx, buf, configtok,
+			"{lightning-dir:%"
+			",network:%"
+			",feature_set:%"
+			",rpc-file:%}",
+			JSON_SCAN_TAL(tmpctx, json_strdup, &dir),
+			JSON_SCAN_TAL(tmpctx, json_strdup, &network),
+			JSON_SCAN_TAL(p, json_to_feature_set, &p->our_features),
+			JSON_SCAN_TAL(p, json_strdup, &p->rpc_location));
+	if (err)
+		plugin_err(p, "cannot scan init params: %s: %.*s",
+			   err, json_tok_full_len(params),
 			   json_tok_full(buf, params));
 
 	/* Move into lightning directory: other files are relative */

plugins/pay.c

@@ -399,9 +399,9 @@ get_remote_block_height(const char *buf, const jsmntok_t *error)
 	u16 type;
 
 	/* Is there even a raw_message?  */
-	if (!json_scan(buf, error, "{data:{raw_message:%}}",
-		       JSON_SCAN_TAL(tmpctx, json_tok_bin_from_hex,
-				     &raw_message)))
+	if (json_scan(tmpctx, buf, error, "{data:{raw_message:%}}",
+		      JSON_SCAN_TAL(tmpctx, json_tok_bin_from_hex,
+				    &raw_message)) != NULL)
 		return 0;
 
 	/* BOLT #4:
@@ -431,19 +431,24 @@ static struct command_result *waitsendpay_error(struct command *cmd,
 	struct pay_attempt *attempt = current_attempt(pc);
 	errcode_t code;
 	int failcode;
+	const char *err;
 
 	attempt_failed_tok(pc, "waitsendpay", buf, error);
 
-	if (!json_scan(buf, error, "{code:%}",
-		       JSON_SCAN(json_to_errcode, &code)))
-		plugin_err(cmd->plugin, "waitsendpay error gave no 'code'? '%.*s'",
-			   error->end - error->start, buf + error->start);
+	err = json_scan(tmpctx, buf, error, "{code:%}",
+			JSON_SCAN(json_to_errcode, &code));
+	if (err)
+		plugin_err(cmd->plugin, "waitsendpay error %s? '%.*s'",
+			   err,
+			   json_tok_full_len(error), json_tok_full(buf, error));
 
 	if (code != PAY_UNPARSEABLE_ONION) {
-		if (!json_scan(buf, error, "{data:{failcode:%}}",
-			       JSON_SCAN(json_to_int, &failcode)))
-			plugin_err(cmd->plugin, "waitsendpay error gave no 'failcode'? '%.*s'",
-				   error->end - error->start, buf + error->start);
+		err = json_scan(tmpctx, buf, error, "{data:{failcode:%}}",
+				JSON_SCAN(json_to_int, &failcode));
+		if (err)
+			plugin_err(cmd->plugin, "waitsendpay failcode error %s '%.*s'",
+				   err,
+				   json_tok_full_len(error), json_tok_full(buf, error));
 	}
 
 	/* Special case for WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS.
@@ -510,12 +515,15 @@ static struct command_result *waitsendpay_error(struct command *cmd,
 
 	if (failcode & NODE) {
 		struct node_id id;
-		const char *idstr;
+		const char *idstr, *err;
 
-		if (!json_scan(buf, error, "{data:{erring_node:%}}",
-			       JSON_SCAN(json_to_node_id, &id)))
-			plugin_err(cmd->plugin, "waitsendpay error no erring_node '%.*s'",
-				   error->end - error->start, buf + error->start);
+		err = json_scan(tmpctx, buf, error, "{data:{erring_node:%}}",
+				JSON_SCAN(json_to_node_id, &id));
+		if (err)
+			plugin_err(cmd->plugin, "waitsendpay error %s '%.*s'",
+				   err,
+				   json_tok_full_len(error),
+				   json_tok_full(buf, error));
 
 		/* FIXME: Keep as node_id, don't use strings. */
 		idstr = node_id_to_hexstr(tmpctx, &id);
@@ -530,13 +538,17 @@ static struct command_result *waitsendpay_error(struct command *cmd,
 	} else {
 		struct short_channel_id scid;
 		u32 dir;
-		const char *scidstr;
+		const char *scidstr, *err;
 
-		if (!json_scan(buf, error, "{data:{erring_channel:%,erring_direction:%}}",
-			       JSON_SCAN(json_to_short_channel_id, &scid),
-			       JSON_SCAN(json_to_number, &dir)))
-			plugin_err(cmd->plugin, "waitsendpay error no erring_channel/direction '%.*s'",
-				   error->end - error->start, buf + error->start);
+		err = json_scan(tmpctx, buf, error,
+				"{data:{erring_channel:%,erring_direction:%}}",
+				JSON_SCAN(json_to_short_channel_id, &scid),
+				JSON_SCAN(json_to_number, &dir));
+		if (err)
+			plugin_err(cmd->plugin, "waitsendpay error %s '%.*s'",
+				   err,
+				   json_tok_full_len(error),
+				   json_tok_full(buf, error));
 
 		scidstr = short_channel_id_to_str(tmpctx, &scid);
 		/* If failure is in routehint part, try next one */
@@ -736,6 +748,7 @@ static struct command_result *getroute_done(struct command *cmd,
 	struct amount_msat max_fee;
 	u32 delay;
 	struct out_req *req;
+	const char *err;
 
 	if (!t)
 		plugin_err(cmd->plugin, "getroute gave no 'route'? '%.*s'",
@@ -754,11 +767,13 @@ static struct command_result *getroute_done(struct command *cmd,
 	} else
 		attempt->route = json_strdup(pc->ps->attempts, buf, t);
 
-	if (!json_scan(buf, t, "[0:{msatoshi:%,delay:%}]",
-		       JSON_SCAN(json_to_msat, &fee),
-		       JSON_SCAN(json_to_number, &delay)))
-		plugin_err(cmd->plugin, "getroute with invalid msatoshi/delay? %.*s",
-			   result->end - result->start, buf);
+	err = json_scan(tmpctx, buf, t, "[0:{msatoshi:%,delay:%}]",
+			JSON_SCAN(json_to_msat, &fee),
+			JSON_SCAN(json_to_number, &delay));
+	if (err)
+		plugin_err(cmd->plugin, "getroute %s? %.*s",
+			   err,
+			   json_tok_full_len(result), json_tok_full(buf, result));
 
 	if (pc->maxfee_pct_millionths / 100 > UINT32_MAX)
 		plugin_err(cmd->plugin, "max fee percent too large: %lf",

plugins/spender/openchannel.c

@@ -547,15 +547,17 @@ static void json_peer_sigs(struct command *cmd,
 	struct channel_id cid;
 	const struct wally_psbt *psbt;
 	struct multifundchannel_destination *dest;
+	const char *err;
 
-	if (!json_scan(buf, params,
-		       "{openchannel_peer_sigs:"
-		       "{channel_id:%,signed_psbt:%}}",
-		       JSON_SCAN(json_to_channel_id, &cid),
-		       JSON_SCAN_TAL(cmd, json_to_psbt, &psbt)))
+	err = json_scan(tmpctx, buf, params,
+			"{openchannel_peer_sigs:"
+			"{channel_id:%,signed_psbt:%}}",
+			JSON_SCAN(json_to_channel_id, &cid),
+			JSON_SCAN_TAL(cmd, json_to_psbt, &psbt));
+	if (err)
 		plugin_err(cmd->plugin,
-			   "`openchannel_peer_sigs` did not scan",
-			   json_tok_full_len(params),
+			   "`openchannel_peer_sigs` did not scan: %s",
+			   err, json_tok_full_len(params),
 			   json_tok_full(buf, params));
 
 	/* Find the destination that's got this channel_id on it! */