Probe IPv6 first

Author: andystaples

tests/durabletask/_port_utils.py

@@ -21,7 +21,20 @@ def find_free_port() -> int:
     releases it and returns the chosen number. There is an inherent race
     between releasing the socket and the caller binding the port, but in
     practice it is reliable for tests and far safer than fixed ports.
+
+    The in-memory backend binds the IPv6 wildcard (``[::]``), so the probe
+    uses an IPv6 socket to mirror that bind and avoid returning a port that
+    is free on IPv4 but already in use on IPv6. If IPv6 is unavailable, it
+    falls back to IPv4.
     """
+    if socket.has_ipv6:
+        try:
+            with socket.socket(socket.AF_INET6, socket.SOCK_STREAM) as s:
+                s.bind(("::", 0))
+                return s.getsockname()[1]
+        except OSError:
+            pass  # IPv6 not usable on this host; fall back to IPv4.
+
     with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-        s.bind(("localhost", 0))
+        s.bind(("127.0.0.1", 0))
         return s.getsockname()[1]