util/jsoncomm: add pair constructor method

Add a new constructor method, `Socket.new_pair`, to create a pair
of connected sockets (via `socketpair`) and wrap both sides via
`jsoncomm.Socket`.
Add a simple test to check it.

osbuild/util/jsoncomm.py

@@ -254,6 +254,24 @@ class Socket(contextlib.AbstractContextManager):
 
         return cls(sock, (unlink, path[1]))
 
+    @classmethod
+    def new_pair(cls, *, blocking=True):
+        """Create a connected socket pair
+
+        Create a pair of connected sockets and return both as a tuple.
+
+        Parameters
+        ----------
+        blocking
+            The blocking mode for the socket pair.
+        """
+        a, b = socket.socketpair(socket.AF_UNIX, socket.SOCK_SEQPACKET)
+
+        a.setblocking(blocking)
+        b.setblocking(blocking)
+
+        return cls(a, None), cls(b, None)
+
     def fileno(self) -> int:
         assert self._socket is not None
         return self._socket.fileno()

test/mod/test_util_jsoncomm.py

@@ -176,3 +176,13 @@ class TestUtilJsonComm(unittest.TestCase):
 
         conn = listener.accept()
         self.assertIsNone(conn)
+
+    def test_socket_pair(self):
+        #
+        # Test creating a socket pair and sending, receiving of a simple message
+        a, b = jsoncomm.Socket.new_pair()
+
+        ping = {"osbuild": "yes"}
+        a.send(ping)
+        pong, _, _ = b.recv()
+        self.assertEqual(ping, pong)