python-zeroconf · bdraco · May 20, 2026 · May 18, 2026 · May 19, 2026 · May 19, 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 +39,10 @@ cdef class AsyncListener:
 
     cdef _cancel_any_timers_for_addr(self, object addr)
 
-    @cython.locals(incoming=DNSIncoming, deferred=list)
+    @cython.locals(deadline=object, fire_at=double)
+    cdef double _compute_deferred_fire_at(self, object addr, double now, double delay)
+
+    @cython.locals(incoming=DNSIncoming, deferred=list, now=double, delay=double, 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 +205,19 @@ 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
+        fire_at = self._compute_deferred_fire_at(addr, now, delay)
+        if fire_at < 0.0:
+            # Sentinel: a new reset would push the flush past the
+            # per-addr reassembly deadline, so leave the existing
+            # TimerHandle in place rather than re-arming it.
+            return
         self._cancel_any_timers_for_addr(addr)
         self._timers[addr] = loop.call_at(
-            loop.time() + delay,
+            fire_at,
             self._respond_query,
             None,
             addr,
@@ -217,6 +226,27 @@ def handle_query_or_defer(
             v6_flow_scope,
         )
 
+    def _compute_deferred_fire_at(self, addr: _str, now: _float, delay: _float) -> _float:
+        """Return the bounded call_at time for a TC-deferred flush, or -1.0 to keep the existing timer."""
+        # RFC 6762 §18.5 frames the random delay as a fixed reassembly budget
+        # starting at first arrival, not a sliding heartbeat.
+        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:
+            if addr in self._timers:
+                # Existing timer already fires at or before the deadline;
+                # signal the caller to leave it alone rather than reset it.
+                return -1.0
+            # First packet for this addr already proposes a fire-time at
+            # or past the deadline — clamp to the deadline so the flush
+            # still happens within the reassembly budget.
+            return deadline
+        # Within budget: schedule at the proposed fire-time.
+        return fire_at
+
     def _cancel_any_timers_for_addr(self, addr: _str) -> None:
         """Cancel any future truncated packet timers for the address."""
         if addr in self._timers:
@@ -232,6 +262,7 @@ 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
@@ -20,7 +20,7 @@
 
 import zeroconf as r
 from zeroconf import NotRunningException, Zeroconf, const, current_time_millis
-from zeroconf._listener import AsyncListener, _WrappedTransport
+from zeroconf._listener import _TC_DELAY_RANDOM_INTERVAL, AsyncListener, _WrappedTransport
 from zeroconf._protocol.incoming import DNSIncoming
 from zeroconf.asyncio import AsyncZeroconf
 
@@ -699,36 +699,41 @@ def test_tc_bit_defers_last_response_missing():
     assert len(packets) == 4
     expected_deferred = []
 
-    next_packet = r.DNSIncoming(packets.pop(0))
-    expected_deferred.append(next_packet)
-    threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
-    assert protocol._deferred[source_ip] == expected_deferred
-    timer1 = protocol._timers[source_ip]
-
-    next_packet = r.DNSIncoming(packets.pop(0))
-    expected_deferred.append(next_packet)
-    threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
-    assert protocol._deferred[source_ip] == expected_deferred
-    timer2 = protocol._timers[source_ip]
-    assert timer1.cancelled()
-    assert timer2 != timer1
-
-    # Send the same packet again to similar multi interfaces
-    threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
-    assert protocol._deferred[source_ip] == expected_deferred
-    assert source_ip in protocol._timers
-    timer3 = protocol._timers[source_ip]
-    assert not timer3.cancelled()
-    assert timer3 == timer2
-
-    next_packet = r.DNSIncoming(packets.pop(0))
-    expected_deferred.append(next_packet)
-    threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
-    assert protocol._deferred[source_ip] == expected_deferred
-    assert source_ip in protocol._timers
-    timer4 = protocol._timers[source_ip]
-    assert timer3.cancelled()
-    assert timer4 != timer3
+    # Pin per-packet delay to the minimum so each successive fire_at lands
+    # before the deadline established by the first packet — keeps the
+    # timer-replacement assertions deterministic under bounded TC-deferral.
+    min_delay_ms = _TC_DELAY_RANDOM_INTERVAL[0]
+    with patch("zeroconf._listener.random.randint", return_value=min_delay_ms):
+        next_packet = r.DNSIncoming(packets.pop(0))
+        expected_deferred.append(next_packet)
+        threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
+        assert protocol._deferred[source_ip] == expected_deferred
+        timer1 = protocol._timers[source_ip]
+
+        next_packet = r.DNSIncoming(packets.pop(0))
+        expected_deferred.append(next_packet)
+        threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
+        assert protocol._deferred[source_ip] == expected_deferred
+        timer2 = protocol._timers[source_ip]
+        assert timer1.cancelled()
+        assert timer2 != timer1
+
+        # Send the same packet again to similar multi interfaces
+        threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
+        assert protocol._deferred[source_ip] == expected_deferred
+        assert source_ip in protocol._timers
+        timer3 = protocol._timers[source_ip]
+        assert not timer3.cancelled()
+        assert timer3 == timer2
+
+        next_packet = r.DNSIncoming(packets.pop(0))
+        expected_deferred.append(next_packet)
+        threadsafe_query(zc, protocol, next_packet, source_ip, const._MDNS_PORT, Mock(), ())
+        assert protocol._deferred[source_ip] == expected_deferred
+        assert source_ip in protocol._timers
+        timer4 = protocol._timers[source_ip]
+        assert timer3.cancelled()
+        assert timer4 != timer3
 
     for _ in range(8):
         time.sleep(0.1)
@@ -743,6 +748,52 @@ 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.
+    max_delay_ms = _TC_DELAY_RANDOM_INTERVAL[1]
+    with patch("zeroconf._listener.random.randint", return_value=max_delay_ms):
+        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.