refactor: switch _seen_logs to dict[str, None] for FIFO eviction

bdraco · bdraco · commit 4cb3ecfc4734 · 2026-05-17T19:17:23.000-07:00
`set.pop()` removes an arbitrary element (hash-bucket order), not the oldest one — the "recency window" framing in ba5de3a was misleading. Switch the dedup containers to insertion-ordered `dict[str, None]` and evict via `del seen[next(iter(seen))]` so eviction is truly FIFO: the oldest entry is dropped per overflow, and `_MAX_SEEN_LOGS` becomes a genuine recency window. The eviction-path delta vs set+pop (~165 vs ~77 ns/call in a microbench) is dwarfed by the `str(sys.exc_info()[1])` interpolation on the calling path; this only runs when a packet has already failed to parse.
diff --git a/src/zeroconf/_logger.py b/src/zeroconf/_logger.py
@@ -40,22 +40,22 @@ def set_logger_level_if_unset() -> None:
 
 
 _MAX_SEEN_LOGS = 512
-_seen_logs: set[str] = set()
+_seen_logs: dict[str, None] = {}
 
 
-def _mark_seen(seen: set[str], key: str) -> bool:
+def _mark_seen(seen: dict[str, None], key: str) -> bool:
     """Record ``key`` in ``seen`` and return True if it was newly added.
 
-    Bounds the set so callers passing attacker-influenced keys (peer
+    Bounds the dict so callers passing attacker-influenced keys (peer
     addresses, packet offsets) cannot grow it without bound. Evicts
-    one arbitrary entry per overflow so warning-level re-emissions
-    stay smooth rather than arriving in bursts.
+    the oldest entry per overflow (dict preserves insertion order on
+    Python 3.7+), so ``_MAX_SEEN_LOGS`` is a recency window.
     """
     if key in seen:
         return False
     if len(seen) >= _MAX_SEEN_LOGS:
-        seen.pop()
-    seen.add(key)
+        del seen[next(iter(seen))]
+    seen[key] = None
     return True
 
 
diff --git a/src/zeroconf/_protocol/incoming.py b/src/zeroconf/_protocol/incoming.py
@@ -63,7 +63,7 @@
 DECODE_EXCEPTIONS = (IndexError, struct.error, IncomingDecodeError)
 
 
-_seen_logs: set[str] = set()
+_seen_logs: dict[str, None] = {}
 _str = str
 _int = int
 
