lightningd: format JSON directly into json connection membuf.

My test case is a mainnet gossip store with 22107 channels, and
time to do `lightning-cli listchannels`:

Before: `lightning-cli listchannels` DEVELOPER=0
	real	0m1.303000-1.324000(1.3114+/-0.0091)s

After:
	real	0m0.629000-0.695000(0.64985+/-0.019)s

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

lightningd/json.c

@@ -491,31 +491,33 @@ struct json_result {
 	/* True if we haven't yet put an element in current wrapping */
 	bool empty;
 
-	/* tal_count() of this is strlen() + 1 */
-	char *s;
+	/* The command we're attached to */
+	struct command *cmd;
 };
 
 static void result_append(struct json_result *res, const char *str)
 {
-	size_t len = tal_count(res->s) - 1;
+	struct json_connection *jcon = res->cmd->jcon;
 
-	tal_resize(&res->s, len + strlen(str) + 1);
-	strcpy(res->s + len, str);
+	/* Don't do anything if they're disconnected. */
+	if (!jcon)
+		return;
+
+	jcon_append(jcon, str);
 }
 
 static void PRINTF_FMT(2,3)
 result_append_fmt(struct json_result *res, const char *fmt, ...)
 {
-	size_t len = tal_count(res->s) - 1, fmtlen;
+	struct json_connection *jcon = res->cmd->jcon;
 	va_list ap;
 
-	va_start(ap, fmt);
-	fmtlen = vsnprintf(NULL, 0, fmt, ap);
-	va_end(ap);
+	/* Don't do anything if they're disconnected. */
+	if (!jcon)
+		return;
 
-	tal_resize(&res->s, len + fmtlen + 1);
 	va_start(ap, fmt);
-	vsprintf(res->s + len, fmt, ap);
+	jcon_append_vfmt(jcon, fmt, ap);
 	va_end(ap);
 }
 
@@ -707,7 +709,7 @@ static struct json_result *new_json_stream(struct command *cmd)
 {
 	struct json_result *r = tal(cmd, struct json_result);
 
-	r->s = tal_strdup(r, "");
+	r->cmd = cmd;
 #if DEVELOPER
 	r->wrapping = tal_arr(r, jsmntype_t, 0);
 #endif
@@ -721,27 +723,33 @@ static struct json_result *new_json_stream(struct command *cmd)
 
 struct json_result *json_stream_success(struct command *cmd)
 {
-	cmd->failcode = 0;
-	return new_json_stream(cmd);
+	struct json_result *r;
+	r = new_json_stream(cmd);
+	result_append(r, "\"result\" : ");
+	return r;
 }
 
-struct json_result *json_stream_fail(struct command *cmd,
-				     int code,
-				     const char *errmsg)
+struct json_result *json_stream_fail_nodata(struct command *cmd,
+					    int code,
+					    const char *errmsg)
 {
+	struct json_result *r = new_json_stream(cmd);
+
 	assert(code);
 	assert(errmsg);
-	cmd->failcode = code;
-	cmd->errmsg = tal_strdup(cmd, errmsg);
-	return new_json_stream(cmd);
+
+	result_append_fmt(r, " \"error\" : "
+			  "{ \"code\" : %d,"
+			  " \"message\" : \"%s\"", code, errmsg);
+	return r;
 }
 
-const char *json_result_string(const struct json_result *result)
+struct json_result *json_stream_fail(struct command *cmd,
+				     int code,
+				     const char *errmsg)
 {
-#if DEVELOPER
-	assert(tal_count(result->wrapping) == 0);
-#endif
-	assert(result->indent == 0);
-	assert(tal_count(result->s) == strlen(result->s) + 1);
-	return result->s;
+	struct json_result *r = json_stream_fail_nodata(cmd, code, errmsg);
+
+	result_append(r, ", \"data\" : ");
+	return r;
 }

lightningd/json.h

@@ -159,7 +159,7 @@ bool json_tok_tok(struct command *cmd, const char *name,
 		  const jsmntok_t **out);
 
 
-/* Creating JSON strings */
+/* Creating JSON output */
 
 /* '"fieldname" : [ ' or '[ ' if fieldname is NULL */
 void json_array_start(struct json_result *ptr, const char *fieldname);
@@ -192,6 +192,18 @@ struct json_result *json_stream_fail(struct command *cmd,
 				     int code,
 				     const char *errmsg);
 
+/**
+ * json_stream_fail_nodata - start streaming a failed json result.
+ * @cmd: the command we're running.
+ * @code: the error code from lightningd/jsonrpc_errors.h
+ * @errmsg: the error string.
+ *
+ * This is used by command_fail(), which doesn't add any JSON data.
+ */
+struct json_result *json_stream_fail_nodata(struct command *cmd,
+					    int code,
+					    const char *errmsg);
+
 /* '"fieldname" : "value"' or '"value"' if fieldname is NULL.  Turns
  * any non-printable chars into JSON escapes, but leaves existing escapes alone.
  */
@@ -227,5 +239,4 @@ void json_add_hex_talarr(struct json_result *result,
 			 const tal_t *data);
 void json_add_object(struct json_result *result, ...);
 
-const char *json_result_string(const struct json_result *result);
 #endif /* LIGHTNING_LIGHTNINGD_JSON_H */