json_patch: fix move/copy false rejection between sibling keys sharing a prefix

json_patch_apply_move_copy() checks that "from" is not a proper prefix
of "path" per RFC 6902 section 4.4 (a location cannot be moved into one
of its own children), but it did so with a plain strncmp() over the raw
pointer strings, without checking that the shared run of characters
actually ends on a JSON Pointer reference-token boundary in "path".

As a result, moving or copying between two sibling object keys where
one name is a textual prefix of the other, e.g. "/aa" to "/aab", was
rejected with "Invalid attempt to move parent under a child" even
though "/aa" is not an ancestor pointer of "/aab" at all, just an
unrelated key that happens to share a few leading characters.

The fix requires that the character in "path" right after the shared
"from" prefix be either the end of the string (equal paths, a no-op)
or a '/' (a genuine nested reference token), before treating it as an
attempt to move a location under its own descendant.

Added two cases to json_patch_tests.json covering the move and copy
operations between such sibling keys, and refreshed the corresponding
golden test_json_patch.expected output.

json_patch.c

@@ -215,7 +215,17 @@ static int json_patch_apply_move_copy(struct json_object **res,
 	}
 
 	from_s_len = strlen(from_s);
-	if (strncmp(from_s, path, from_s_len) == 0) {
+	/**
+	 * "from" is only a (possibly improper) prefix of "path" when the
+	 * shared run of characters ends on a reference-token boundary in
+	 * "path", i.e. the next character is '/' or the strings end there.
+	 * Without the boundary check, "from" being a plain string prefix of
+	 * "path" (e.g. "/aa" and "/aab") was mistaken for "from" being an
+	 * ancestor pointer of "path", rejecting moves/copies between mere
+	 * sibling keys that happen to share a name prefix.
+	 */
+	if (strncmp(from_s, path, from_s_len) == 0 &&
+	    (path[from_s_len] == '\0' || path[from_s_len] == '/')) {
 		/**
 		 * If lengths match, it's a noop, if they don't,
 		 * then we're trying to move a parent under a child
@@ -223,7 +233,7 @@ static int json_patch_apply_move_copy(struct json_object **res,
 		 *   The "from" location MUST NOT be a proper prefix of the "path"
 		 *   location; i.e., a location cannot be moved into one of its children.
 		 */
-		if (from_s_len == strlen(path))
+		if (path[from_s_len] == '\0')
 			return 0;
 		_set_err(EINVAL, "Invalid attempt to move parent under a child");
 		return -1;

tests/json_patch_tests.json

@@ -370,6 +370,16 @@
       "patch": [{"op": "move", "from": "/foo", "path": "/foo"}],
       "expected": {"foo": 1} },
 
+    { "comment": "Move between sibling keys sharing a name prefix is not a parent/child move",
+      "doc": {"aa": 1, "aab": 2},
+      "patch": [{"op": "move", "from": "/aa", "path": "/aab"}],
+      "expected": {"aab": 1} },
+
+    { "comment": "Copy between sibling keys sharing a name prefix is not a parent/child move",
+      "doc": {"aa": 1, "aab": 2},
+      "patch": [{"op": "copy", "from": "/aa", "path": "/aab"}],
+      "expected": {"aa": 1, "aab": 1} },
+
     { "doc": {"foo": 1, "baz": [{"qux": "hello"}]},
       "patch": [{"op": "move", "from": "/foo", "path": "/bar"}],
       "expected": {"baz": [{"qux": "hello"}], "bar": 1} },

tests/test_json_patch.expected

@@ -100,6 +100,8 @@ Testing 'Test the whole document, no match', doc '{ "foo": 1 }' patch '[ { "op":
 Testing 'Empty-string element', doc '{ "": 1 }' patch '[ { "op": "test", "path": "\/", "value": 1 } ]' : OK
 Testing '(null)', doc '{ "foo": [ "bar", "baz" ], "": 0, "a\/b": 1, "c%d": 2, "e^f": 3, "g|h": 4, "i\\j": 5, "k\"l": 6, " ": 7, "m~n": 8 }' patch '[ { "op": "test", "path": "\/foo", "value": [ "bar", "baz" ] }, { "op": "test", "path": "\/foo\/0", "value": "bar" }, { "op": "test", "path": "\/", "value": 0 }, { "op": "test", "path": "\/a~1b", "value": 1 }, { "op": "test", "path": "\/c%d", "value": 2 }, { "op": "test", "path": "\/e^f", "value": 3 }, { "op": "test", "path": "\/g|h", "value": 4 }, { "op": "test", "path": "\/i\\j", "value": 5 }, { "op": "test", "path": "\/k\"l", "value": 6 }, { "op": "test", "path": "\/ ", "value": 7 }, { "op": "test", "path": "\/m~0n", "value": 8 } ]' : OK
 Testing 'Move to same location has no effect', doc '{ "foo": 1 }' patch '[ { "op": "move", "from": "\/foo", "path": "\/foo" } ]' : OK
+Testing 'Move between sibling keys sharing a name prefix is not a parent/child move', doc '{ "aa": 1, "aab": 2 }' patch '[ { "op": "move", "from": "\/aa", "path": "\/aab" } ]' : OK
+Testing 'Copy between sibling keys sharing a name prefix is not a parent/child move', doc '{ "aa": 1, "aab": 2 }' patch '[ { "op": "copy", "from": "\/aa", "path": "\/aab" } ]' : OK
 Testing '(null)', doc '{ "foo": 1, "baz": [ { "qux": "hello" } ] }' patch '[ { "op": "move", "from": "\/foo", "path": "\/bar" } ]' : OK
 Testing '(null)', doc '{ "baz": [ { "qux": "hello" } ], "bar": 1 }' patch '[ { "op": "move", "from": "\/baz\/0\/qux", "path": "\/baz\/1" } ]' : OK
 Testing '(null)', doc '{ "baz": [ { "qux": "hello" } ], "bar": 1 }' patch '[ { "op": "copy", "from": "\/baz\/0", "path": "\/boo" } ]' : OK