common/json: restore json_delve() for simple plugin parsing.

This was removed (as unused) in 6269a4c55d592e8720b7f2a304c21f61f7931238;
now I've even added tests.

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

common/json.c

@@ -4,6 +4,7 @@
 #include <ccan/build_assert/build_assert.h>
 #include <ccan/str/hex/hex.h>
 #include <ccan/tal/str/str.h>
+#include <common/utils.h>
 #include <errno.h>
 #include <inttypes.h>
 #include <stdarg.h>
@@ -273,3 +274,39 @@ void json_tok_remove(jsmntok_t **tokens, jsmntok_t *tok, size_t num)
 	tal_resize(tokens, tal_count(*tokens) - remove_count);
 	(*tokens)->size -= num;
 }
+
+const jsmntok_t *json_delve(const char *buffer,
+			    const jsmntok_t *tok,
+			    const char *guide)
+{
+       while (*guide) {
+               const char *key;
+               size_t len = strcspn(guide+1, ".[]");
+
+               key = tal_strndup(tmpctx, guide+1, len);
+               switch (guide[0]) {
+               case '.':
+                       if (tok->type != JSMN_OBJECT)
+                               return NULL;
+                       tok = json_get_member(buffer, tok, key);
+                       if (!tok)
+                               return NULL;
+                       break;
+               case '[':
+                       if (tok->type != JSMN_ARRAY)
+                               return NULL;
+                       tok = json_get_arr(tok, atol(key));
+                       if (!tok)
+                               return NULL;
+                       /* Must be terminated */
+                       assert(guide[1+strlen(key)] == ']');
+                       len++;
+                       break;
+               default:
+                       abort();
+               }
+               guide += len + 1;
+       }
+
+       return tok;
+}

common/json.h

@@ -73,4 +73,9 @@ jsmntok_t *json_tok_copy(const tal_t *ctx, const jsmntok_t *tok);
  */
 void json_tok_remove(jsmntok_t **tokens, jsmntok_t *tok, size_t num);
 
+/* Guide is a string with . for members, [] around indexes. */
+const jsmntok_t *json_delve(const char *buffer,
+			    const jsmntok_t *tok,
+			    const char *guide);
+
 #endif /* LIGHTNING_COMMON_JSON_H */

common/test/run-json.c

@@ -1,4 +1,5 @@
 #include "../json.c"
+#include <ccan/mem/mem.h>
 #include <common/utils.h>
 #include <stdio.h>
 
@@ -38,11 +39,103 @@ static int test_json_tok_bitcoin_amount(void)
 	return 0;
 }
 
+static void test_json_delve(void)
+{
+	const jsmntok_t *toks, *t;
+	char *buf;
+	bool ok;
+
+	buf = "{\"1\":\"one\", \"2\":\"two\", \"3\":[\"three\", {\"deeper\": 17}]}";
+	toks = json_parse_input(tmpctx, buf, strlen(buf), &ok);
+	assert(ok);
+
+	t = json_delve(buf, toks, ".1");
+	assert(t);
+	assert(t->type == JSMN_STRING);
+	assert(json_tok_streq(buf, t, "one"));
+	assert(t == toks+2);
+
+	t = json_delve(buf, toks, ".2");
+	assert(t);
+	assert(t->type == JSMN_STRING);
+	assert(json_tok_streq(buf, t, "two"));
+	assert(t == toks+4);
+
+	t = json_delve(buf, toks, ".3");
+	assert(t);
+	assert(t->type == JSMN_ARRAY);
+	assert(t == toks+6);
+
+	t = json_delve(buf, toks, ".3[0]");
+	assert(t);
+	assert(t->type == JSMN_STRING);
+	assert(json_tok_streq(buf, t, "three"));
+	assert(t == toks+7);
+
+	t = json_delve(buf, toks, ".3[1]");
+	assert(t);
+	assert(t->type == JSMN_OBJECT);
+	assert(t == toks+8);
+
+	t = json_delve(buf, toks, ".3[1].deeper");
+	assert(t);
+	assert(t->type == JSMN_PRIMITIVE);
+	assert(memeq(buf + t->start, t->end - t->start, "17", strlen("17")));
+	assert(t == toks+10);
+
+	t = json_delve(buf, toks, ".4");
+	assert(!t);
+	t = json_delve(buf, toks, "[0]");
+	assert(!t);
+	t = json_delve(buf, toks, ".deeper");
+	assert(!t);
+	t = json_delve(buf, toks, ".3[2]");
+	assert(!t);
+	t = json_delve(buf, toks, ".3[2].deeper");
+	assert(!t);
+	t = json_delve(buf, toks, ".3[0].deeper");
+	assert(!t);
+	t = json_delve(buf, toks, ".3[1].deeper[0]");
+	assert(!t);
+	t = json_delve(buf, toks, ".3[1][0]");
+	assert(!t);
+
+	/* Now a real example. */
+	buf = "{\n"
+		"  \"jsonrpc\": \"2.0\", \n"
+		"  \"method\": \"init\", \n"
+		"  \"id\": 1, \n"
+		"  \"params\": {\n"
+		"    \"options\": {\n"
+		"    }, \n"
+		"    \"configuration\": {\n"
+		"      \"lightning-dir\": \"/tmp/ltests-n2hyd543/test_pay_plugin_1/lightning-2/\", \n"
+		"      \"rpc-file\": \"lightning-rpc\"\n"
+		"    }\n"
+		"  }\n"
+		"}\n"
+		"\n";
+	toks = json_parse_input(tmpctx, buf, strlen(buf), &ok);
+	assert(ok);
+
+	t = json_delve(buf, toks, ".rpcfile");
+	assert(!t);
+	t = json_delve(buf, toks, ".configuration.rpc-file");
+	assert(!t);
+	t = json_delve(buf, toks, ".params.configuration.rpc-file");
+	assert(t);
+	assert(t->type == JSMN_STRING);
+	assert(json_tok_streq(buf, t, "lightning-rpc"));
+}
+
 int main(void)
 {
 	setup_locale();
+	setup_tmpctx();
 
 	test_json_tok_bitcoin_amount();
+	test_json_delve();
 	assert(!taken_any());
 	take_cleanup();
+	tal_free(tmpctx);
 }