common: add brace hack for jsonrpc_async_parse.

This is a trick from bcli: we ask bitcoind for the block, and it hands
us a 2MB hex blob (which we read in multiple parts).  Our parser wades
through it all, but a quick search for '}' makes it much faster.

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

Author: rustyrussell

common/jsonrpc_io.c

@@ -64,6 +64,15 @@ const char *jsonrpc_io_parse(const tal_t *ctx,
 	*toks = NULL;
 	*buf = NULL;
 
+	/* Our JSON parser is pretty good at incremental parsing, but
+	 * `getrawblock` gives a giant 2MB token, which forces it to re-parse
+	 * every time until we have all of it. However, we can't complete a
+	 * JSON object without a '}', so we do a cheaper check here.
+	 */
+	if (!memchr(membuf_elems(&json_in->membuf), '}',
+		    membuf_num_elems(&json_in->membuf)))
+		return NULL;
+
 	if (!json_parse_input(&json_in->parser, &json_in->toks,
 			      membuf_elems(&json_in->membuf),
 			      membuf_num_elems(&json_in->membuf),

tests/test_misc.py

@@ -951,8 +951,8 @@ def test_malformed_rpc(node_factory):
     obj, _ = l1.rpc._readobj(sock, b'')
     assert obj['error']['code'] == -32600
 
-    # Complete crap (this makes it hang up!)
-    sock.sendall(b'[]')
+    # Complete crap: needs } to even try parsing, and also this makes it hang up!
+    sock.sendall(b'[]}')
     obj, _ = l1.rpc._readobj(sock, b'')
     assert obj['error']['code'] == -32600