json_pointer: find null array elements

json_pointer_get_single_path() returned ENOENT when the array element
at an in-range index was NULL, which is how json-c stores a JSON null.
A null object member was already found, but on [0, null, 2] "/1" failed
with "not found", although RFC 6901 says it references the null value.

The array index is bounds-checked just before, so a NULL element now
resolves like a null member: json_pointer_get() returns 0 and sets
*res to NULL. This also fixes json_patch_apply(), which failed every
"test", "replace" and "remove" on a null array element, and every
"move" and "copy" from one, with "Did not find element referenced by
path field".

Added a json_pointer_get() test, and json_patch_tests.json cases that
mirror the existing null object member ones.

json_pointer.c

@@ -102,17 +102,11 @@ static int json_pointer_get_single_path(struct json_object *obj, char *path,
 			return -1;
 		}
 
-		obj = json_object_array_get_idx(obj, *idx);
-		if (obj)
-		{
+		/* A null element is stored as NULL, but it exists */
 		if (value)
-				*value = obj;
+			*value = json_object_array_get_idx(obj, *idx);
 		return 0;
 	}
-		/* Entry not found */
-		errno = ENOENT;
-		return -1;
-	}
 
 	/* RFC states that we first must eval all ~1 then all ~0 */
 	string_replace_all_occurrences_with_char(path, "~1", '/');

tests/json_patch_tests.json

@@ -291,6 +291,31 @@
       "expected": {"foo": null},
       "comment": "null value should still be valid obj property replace other value" },
 
+    { "doc": {"foo": [null]},
+      "patch": [{"op": "test", "path": "/foo/0", "value": null}],
+      "expected": {"foo": [null]},
+      "comment": "null value should be valid array element" },
+
+    { "doc": {"foo": [null]},
+      "patch": [{"op": "replace", "path": "/foo/0", "value": "truthy"}],
+      "expected": {"foo": ["truthy"]},
+      "comment": "null value should be valid array element to be replaced with something truthy" },
+
+    { "doc": {"foo": [null, 1]},
+      "patch": [{"op": "move", "from": "/foo/0", "path": "/bar"}],
+      "expected": {"foo": [1], "bar": null},
+      "comment": "null value should be valid array element to be moved" },
+
+    { "doc": {"foo": [null]},
+      "patch": [{"op": "copy", "from": "/foo/0", "path": "/foo/-"}],
+      "expected": {"foo": [null, null]},
+      "comment": "null value should be valid array element to be copied" },
+
+    { "doc": {"foo": [null, 1]},
+      "patch": [{"op": "remove", "path": "/foo/0"}],
+      "expected": {"foo": [1]},
+      "comment": "null value should be valid array element to be removed" },
+
     { "doc": {"foo": {"foo": 1, "bar": 2}},
       "patch": [{"op": "test", "path": "/foo", "value": {"bar": 2, "foo": 1}}],
       "expected": {"foo": {"foo": 1, "bar": 2}},

tests/test_json_patch.expected

@@ -89,6 +89,11 @@ Testing 'null value should be valid obj property to be moved', doc '{ "foo": nul
 Testing 'null value should be valid obj property to be copied', doc '{ "foo": null }' patch '[ { "op": "copy", "from": "\/foo", "path": "\/bar" } ]' : OK
 Testing 'null value should be valid obj property to be removed', doc '{ "foo": null }' patch '[ { "op": "remove", "path": "\/foo" } ]' : OK
 Testing 'null value should still be valid obj property replace other value', doc '{ "foo": "bar" }' patch '[ { "op": "replace", "path": "\/foo", "value": null } ]' : OK
+Testing 'null value should be valid array element', doc '{ "foo": [ null ] }' patch '[ { "op": "test", "path": "\/foo\/0", "value": null } ]' : OK
+Testing 'null value should be valid array element to be replaced with something truthy', doc '{ "foo": [ null ] }' patch '[ { "op": "replace", "path": "\/foo\/0", "value": "truthy" } ]' : OK
+Testing 'null value should be valid array element to be moved', doc '{ "foo": [ null, 1 ] }' patch '[ { "op": "move", "from": "\/foo\/0", "path": "\/bar" } ]' : OK
+Testing 'null value should be valid array element to be copied', doc '{ "foo": [ null ] }' patch '[ { "op": "copy", "from": "\/foo\/0", "path": "\/foo\/-" } ]' : OK
+Testing 'null value should be valid array element to be removed', doc '{ "foo": [ null, 1 ] }' patch '[ { "op": "remove", "path": "\/foo\/0" } ]' : OK
 Testing 'test should pass despite rearrangement', doc '{ "foo": { "foo": 1, "bar": 2 } }' patch '[ { "op": "test", "path": "\/foo", "value": { "bar": 2, "foo": 1 } } ]' : OK
 Testing 'test should pass despite (nested) rearrangement', doc '{ "foo": [ { "foo": 1, "bar": 2 } ] }' patch '[ { "op": "test", "path": "\/foo", "value": [ { "bar": 2, "foo": 1 } ] } ]' : OK
 Testing 'test should pass - no error', doc '{ "foo": { "bar": [ 1, 2, 5, 4 ] } }' patch '[ { "op": "test", "path": "\/foo", "value": { "bar": [ 1, 2, 5, 4 ] } } ]' : OK

tests/test_json_pointer.c

@@ -122,6 +122,18 @@ static void test_example_get(void)
 		test_example_int(jo1, json_pointers[i].s, json_pointers[i].i);
 
 	json_object_put(jo1);
+
+	/* Test /1 == null, a null array element is found and returned as NULL */
+	jo1 = json_tokener_parse("[0, null, 2]");
+	jo2 = jo1;
+	assert(0 == json_pointer_get(jo1, "/1", NULL));
+	assert(0 == json_pointer_get(jo1, "/1", &jo2));
+	assert(NULL == jo2);
+	errno = 0;
+	assert(0 != json_pointer_get(jo1, "/1/foo", NULL));
+	assert(errno == ENOENT);
+	printf("PASSED - GET - /1 == null\n");
+	json_object_put(jo1);
 }
 
 /* I'm not too happy with the RFC example to test the recursion of the json_pointer_get() function */

tests/test_json_pointer.expected

@@ -12,6 +12,7 @@ PASSED - GET -  /i\j == 5
 PASSED - GET -  /k"l == 6
 PASSED - GET -  /  == 7
 PASSED - GET -  /m~0n == 8
+PASSED - GET - /1 == null
 { "arr": [ { "obj": [ { }, { }, { "obj1": 0, "obj2": "1" } ] } ], "obj": { "obj": { "obj": [ { "obj1": 0, "obj2": "1" } ] } } }
 PASSED - GET - RECURSION TEST
 PASSED - GET - LOADED TEST JSON