lightningd: fix handling of truncated config options.

Do the same thing '--help' does with them; append `...`.

Valgrind noticed that we weren't NUL-terminarting if answer was over
78 characters.

Changelog-Fixed: JSONRPC: listconfigs appends '...' to truncated config options.

lightningd/options.c

@@ -1112,7 +1112,8 @@ static void add_config(struct lightningd *ld,
 		if (opt->desc == opt_hidden) {
 			/* Ignore hidden options (deprecated) */
 		} else if (opt->show) {
-			char *buf = tal_arr(name0, char, OPT_SHOW_LEN+1);
+			char buf[OPT_SHOW_LEN + sizeof("...")];
+			strcpy(buf + OPT_SHOW_LEN, "...");
 			opt->show(buf, opt->u.carg);
 
 			if (streq(buf, "true") || streq(buf, "false")
@@ -1126,7 +1127,8 @@ static void add_config(struct lightningd *ld,
 
 			/* opt_show_charp surrounds with "", strip them */
 			if (strstarts(buf, "\"")) {
-				buf[strlen(buf)-1] = '\0';
+				char *end = strrchr(buf, '"');
+				memmove(end, end + 1, strlen(end));
 				answer = buf + 1;
 			} else
 				answer = buf;

tests/test_misc.py

@@ -732,13 +732,16 @@ def test_address(node_factory):
 
 
 def test_listconfigs(node_factory, bitcoind, chainparams):
-    l1 = node_factory.get_node()
+    # Make extremely long entry, check it works
+    l1 = node_factory.get_node(options={'log-prefix': 'lightning1-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'})
 
     configs = l1.rpc.listconfigs()
     # See utils.py
     assert configs['allow-deprecated-apis'] is False
     assert configs['network'] == chainparams['name']
     assert configs['ignore-fee-limits'] is False
+    assert configs['ignore-fee-limits'] is False
+    assert configs['log-prefix'] == 'lightning1-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx...'
 
     # Test one at a time.
     for c in configs.keys():