Fix port availability check for dual-stack localhost binding

lightly_studio/src/lightly_studio/api/server.py

@@ -73,10 +73,12 @@ def _is_port_available(host: str, port: int) -> bool:
             # Fallback for hostnames like 'localhost'
             families = [socket.AF_INET, socket.AF_INET6]
 
+    # The port is available if we can bind to at least one address family.
     for family in families:
         with socket.socket(family, socket.SOCK_STREAM) as s:
             try:
                 s.bind((host, port))
+                return True
             except OSError:
-                return False
-    return True
+                continue
+    return False

lightly_studio/tests/api/test_server.py

@@ -81,3 +81,19 @@ def test__get_available_port__preferred_port_in_use() -> None:
     server = Server(host, port)
     s.close()
     assert server.port != port
+
+
+def test__get_available_port__localhost_with_ipv6_failure() -> None:
+    """Test that port is available even if IPv6 binding fails for localhost.
+
+    This tests the fix for the bug where _is_port_available returned False
+    when checking 'localhost' because IPv6 binding failed, even though IPv4
+    binding succeeded. The port should be considered available if at least
+    one address family can bind successfully.
+    """
+    host = "localhost"
+    port = 8002
+
+    # Verify that the port is available.
+    server = Server(host=host, port=port)
+    assert server.port == port