test(sync): probe broadcast by sending, not by listening

The broadcast check added in the previous commit bound INADDR_ANY to
receive its own probe datagram, and the free-port probe did the same to
pick a port. CodeQL flagged both, correctly: a test suite has no reason
to open a socket the whole network can reach.

Sending is enough for what the probe is actually for. An environment
that refuses broadcast raises on sendto, which is the case that occurs
in sandboxes and is the one worth skipping over; confirming delivery
would have required the listening socket. A network that accepts the
send and silently drops it still reaches the assertion, exactly as it
did before either commit. The port probe binds loopback -- it only needs
a number, and the manager's own bind is the one that has to succeed, with
the retry loop already covering a port taken elsewhere.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

test/test_sync_manager.py

@@ -853,29 +853,30 @@ class TestStop:
         make_manager(role=SyncRole.STANDALONE).stop()  # all sockets None
 
 
-def _broadcast_works(port):
-    """True when this environment can actually deliver a UDP broadcast.
+def _broadcast_available(port):
+    """True when a UDP broadcast can be sent at all in this environment.
 
-    The handshake below depends on it: the follower announces itself to
-    ("<broadcast>", port). Sandboxes and some CI networks drop or refuse
-    broadcast, and sync_manager swallows the sendto error, so without
-    this probe the test would just wait out its deadline and fail for a
-    reason that has nothing to do with the code.
+    The handshake below depends on broadcast: the follower announces
+    itself to ("<broadcast>", port), and sync_manager swallows any sendto
+    error. Without this probe, a sandbox or CI network that refuses
+    broadcast would make the test wait out its whole deadline and then
+    fail for a reason that has nothing to do with the code.
+
+    Sending is enough to detect the case that actually occurs — a
+    refusing environment raises here. Confirming *delivery* would mean
+    binding INADDR_ANY to receive, which is a listening socket this suite
+    has no reason to open; a network that accepts the send and silently
+    drops it still reaches the assertion, exactly as before.
     """
-    recv = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-    send = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
     try:
-        recv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-        recv.bind(("", port))
-        recv.settimeout(0.5)
-        send.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
-        send.sendto(b"probe", ("<broadcast>", port))
-        return recv.recvfrom(64)[0] == b"probe"
+        sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
+        sock.sendto(b"probe", ("<broadcast>", port))
+        return True
     except OSError:
         return False
     finally:
-        recv.close()
-        send.close()
+        sock.close()
 
 
 class TestRealSocketHandshake:
@@ -895,13 +896,17 @@ class TestRealSocketHandshake:
         # The free-port probe is inherently racy — the port can be taken
         # between release and rebind — so retry rather than fail on it.
         for attempt in range(5):
+            # Probed on loopback: this only needs a port number, and the
+            # manager's own bind is what has to succeed. If the port turns
+            # out to be taken on another interface, the retry below covers
+            # it — same as for the race.
             probe = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-            probe.bind(("", 0))
+            probe.bind(("127.0.0.1", 0))
             port = probe.getsockname()[1]
             probe.close()
 
-            if not _broadcast_works(port):
-                pytest.skip("environment cannot deliver UDP broadcast")
+            if not _broadcast_available(port):
+                pytest.skip("environment refuses UDP broadcast")
 
             try:
                 leader = DisplaySyncManager("leader", {"port": port}, hw, MagicMock())