python-zeroconf · bluetoothbot · May 18, 2026
diff --git a/src/zeroconf/_listener.pxd b/src/zeroconf/_listener.pxd
@@ -29,6 +29,7 @@ cdef class AsyncListener:
     cdef public object sock_description
     cdef public cython.dict _deferred
     cdef public cython.dict _timers
+    cdef public cython.dict _deferred_deadlines
 
     @cython.locals(now=double, debug=cython.bint)
     cpdef datagram_received(self, cython.bytes bytes, cython.tuple addrs)
@@ -38,7 +39,7 @@ cdef class AsyncListener:
 
     cdef _cancel_any_timers_for_addr(self, object addr)
 
-    @cython.locals(incoming=DNSIncoming, deferred=list)
+    @cython.locals(incoming=DNSIncoming, deferred=list, now=double, delay=double, deadline=object, fire_at=double)
     cpdef handle_query_or_defer(
         self,
         DNSIncoming msg,

diff --git a/src/zeroconf/_listener.py b/src/zeroconf/_listener.py
@@ -58,6 +58,7 @@ class AsyncListener:
 
     __slots__ = (
         "_deferred",
+        "_deferred_deadlines",
         "_query_handler",
         "_record_manager",
         "_registry",
@@ -82,6 +83,7 @@ def __init__(self, zc: Zeroconf) -> None:
         self.sock_description: str | None = None
         self._deferred: dict[str, list[DNSIncoming]] = {}
         self._timers: dict[str, asyncio.TimerHandle] = {}
+        self._deferred_deadlines: dict[str, float] = {}
         super().__init__()
 
     def datagram_received(self, data: _bytes, addrs: tuple[str, int] | tuple[str, int, int, int]) -> None:
@@ -203,12 +205,25 @@ def handle_query_or_defer(
             if incoming.data == msg.data:
                 return
         deferred.append(msg)
-        delay = millis_to_seconds(random.randint(*_TC_DELAY_RANDOM_INTERVAL))  # noqa: S311
         loop = self.zc.loop
         assert loop is not None
+        now = loop.time()
+        delay = millis_to_seconds(random.randint(*_TC_DELAY_RANDOM_INTERVAL))  # noqa: S311
+        # Bound the assembly window to first_arrival + max delay so a peer
+        # streaming TC packets cannot keep deferring the flush indefinitely.
+        deadline = self._deferred_deadlines.get(addr)
+        if deadline is None:
+            deadline = now + millis_to_seconds(_TC_DELAY_RANDOM_INTERVAL[1])
+            self._deferred_deadlines[addr] = deadline
+        fire_at = now + delay
+        if fire_at >= deadline:
+            # Existing timer (if any) already fires at or before the deadline.
+            if addr in self._timers:
+                return
+            fire_at = deadline
         self._cancel_any_timers_for_addr(addr)
         self._timers[addr] = loop.call_at(
-            loop.time() + delay,
+            fire_at,
             self._respond_query,
             None,
             addr,
@@ -232,6 +247,7 @@ def _respond_query(
     ) -> None:
         """Respond to a query and reassemble any truncated deferred packets."""
         self._cancel_any_timers_for_addr(addr)
+        self._deferred_deadlines.pop(addr, None)
         packets = self._deferred.pop(addr, [])
         if msg:
             packets.append(msg)

diff --git a/tests/test_core.py b/tests/test_core.py
@@ -743,6 +743,51 @@ def test_tc_bit_defers_last_response_missing():
     zc.close()
 
 
+def test_tc_bit_defer_window_is_bounded():
+    """TC-deferral assembly window must not slide past first_arrival + max delay."""
+    zc = Zeroconf(interfaces=["127.0.0.1"])
+    _wait_for_start(zc)
+    type_ = "_boundeddefer._tcp.local."
+    registration_name = f"knownname.{type_}"
+
+    info = r.ServiceInfo(
+        type_,
+        registration_name,
+        80,
+        0,
+        0,
+        {"path": "/~paulsm/"},
+        "ash-2.local.",
+        addresses=[socket.inet_aton("10.0.1.2")],
+    )
+    zc.registry.async_add(info)
+
+    protocol = zc.engine.protocols[0]
+    now_ms = r.current_time_millis()
+    _clear_cache(zc)
+    source_ip = "203.0.113.99"
+
+    generated = r.DNSOutgoing(const._FLAGS_QR_QUERY)
+    generated.add_question(r.DNSQuestion(type_, const._TYPE_PTR, const._CLASS_IN))
+    for _ in range(300):
+        generated.add_answer_at_time(info.dns_pointer(), now_ms)
+    packets = generated.packets()
+    assert len(packets) >= 3
+
+    # Pin the per-packet delay at its maximum so any subsequent reset would
+    # land past the deadline established by the first packet.
+    with patch("zeroconf._listener.random.randint", return_value=500):
+        threadsafe_query(zc, protocol, r.DNSIncoming(packets[0]), source_ip, const._MDNS_PORT, Mock(), ())
+        first_when = protocol._timers[source_ip].when()
+
+        for raw in packets[1:-1]:
+            threadsafe_query(zc, protocol, r.DNSIncoming(raw), source_ip, const._MDNS_PORT, Mock(), ())
+            assert protocol._timers[source_ip].when() <= first_when
+
+    zc.registry.async_remove(info)
+    zc.close()
+
+
 @pytest.mark.asyncio
 async def test_open_close_twice_from_async() -> None:
     """Test we can close twice from a coroutine when using Zeroconf.