fix(snmp): close SnmpEngine after each poll to stop fd leak

A fresh SnmpEngine() was created on every poll_device() call and never
closed. pysnmp opens a UDP transport socket per engine and doesn't release
it on GC, so each scheduler tick (default 60s, per device) leaked a file
descriptor. Over hours of polling the process hit its fd ceiling and the
listening socket could no longer accept connections — OSError: [Errno 24]
Too many open files on socket.accept(), locking up the app.

Wrap the engine in try/finally and release its transport socket via a new
_close_engine() helper that probes both pysnmp API shapes (6.2.x lextudio
camelCase, canonical 7.x snake_case); all close paths are best-effort so a
failed close never breaks the poll loop. Regression tests cover both shapes
and the never-raises contract.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/snmp/poller.py

@@ -28,6 +28,44 @@ def _mp_model(version: str) -> int:
     return 0 if version == "1" else 1
 
 
+def _close_engine(engine) -> None:
+    """Release the engine's UDP transport socket.
+
+    pysnmp opens a UDP socket per ``SnmpEngine`` and never closes it on its own.
+    Since we build a fresh engine for every poll, an unclosed engine leaks one
+    file descriptor each scheduler tick; over hours of polling that exhausts the
+    process fd limit (``OSError: [Errno 24] Too many open files``), which also
+    takes down the app's listening socket. So close it explicitly here.
+
+    The close method/attribute names differ across pysnmp majors (6.2.x lextudio
+    is camelCase, canonical 7.x is snake_case), so probe for whatever exists.
+    All paths are best-effort — a failed close must never break the poll loop.
+    """
+    # 7.x exposes a convenience close directly on the engine.
+    for meth in ("close_dispatcher", "closeDispatcher"):
+        fn = getattr(engine, meth, None)
+        if callable(fn):
+            try:
+                fn()
+            except Exception:
+                pass
+            return
+    # 6.2.x: go through the transport dispatcher.
+    for attr in ("transport_dispatcher", "transportDispatcher"):
+        dispatcher = getattr(engine, attr, None)
+        if dispatcher is None:
+            continue
+        for meth in ("close_dispatcher", "closeDispatcher"):
+            fn = getattr(dispatcher, meth, None)
+            if callable(fn):
+                try:
+                    fn()
+                except Exception:
+                    pass
+                return
+        return
+
+
 async def poll_device(
     host: str,
     port: int,
@@ -81,6 +119,8 @@ async def poll_device(
 
     engine = SnmpEngine()
 
+    # Always release the engine's UDP socket — see _close_engine for why.
+    try:
         # Same host/port for every OID on this device, so build the transport once.
         try:
             if _transport_is_async:
@@ -127,3 +167,5 @@ async def poll_device(
                     logger.debug("SNMP non-numeric value for %s label=%s: %r", oid, label, val)
 
         return results
+    finally:
+        _close_engine(engine)

tests/plugins/snmp/test_poller.py

@@ -30,3 +30,47 @@ def test_poll_device_without_pysnmp_returns_empty(monkeypatch):
         poller.poll_device("192.0.2.1", 161, "public", "2c", [{"oid": "1.3.6.1.2.1.1.3.0"}])
     )
     assert out == {}
+
+
+# --- _close_engine: the fd-leak guard ---------------------------------------
+# A fresh SnmpEngine per poll leaks its UDP socket unless closed; _close_engine
+# must release it across both pysnmp API shapes. These fakes stand in for the
+# engine since the unit lane has no pysnmp.
+
+def test_close_engine_uses_engine_level_close_7x():
+    # canonical pysnmp 7.x: close method lives directly on the engine.
+    calls = []
+
+    class Engine:
+        def close_dispatcher(self):
+            calls.append("engine")
+
+    poller._close_engine(Engine())
+    assert calls == ["engine"]
+
+
+def test_close_engine_falls_back_to_dispatcher_6x():
+    # pysnmp-lextudio 6.2.x: no engine-level close; go through the dispatcher.
+    calls = []
+
+    class Dispatcher:
+        def closeDispatcher(self):
+            calls.append("dispatcher")
+
+    class Engine:
+        transportDispatcher = Dispatcher()
+
+    poller._close_engine(Engine())
+    assert calls == ["dispatcher"]
+
+
+def test_close_engine_never_raises():
+    # A failing close must not break the poll loop.
+    class Engine:
+        def close_dispatcher(self):
+            raise OSError("boom")
+
+    poller._close_engine(Engine())  # no exception
+
+    # No close path at all (defensive) is also fine.
+    poller._close_engine(object())