fix: use Twisted networking instead of asyncio in ClamAV module

Synapse runs on Twisted's reactor, not asyncio's event loop - the
original asyncio.open_connection/wait_for calls failed immediately
with "RuntimeError: no running event loop", silently fail-opening
every scan (confirmed live: EICAR test file passed through unscanned).
Rewritten using twisted.internet.endpoints.HostnameEndpoint/
connectProtocol and a custom Protocol for the INSTREAM conversation.

apps/production/clamav_spam_checker.py

@@ -1,9 +1,13 @@
 # Synapse spam-checker module (Issue #19): scans locally-stored and remote/federated media
 # through ClamAV's clamd daemon via its native INSTREAM protocol, before Synapse serves it.
 #
-# Talks to clamd using only the standard library (asyncio/socket) - no third-party
-# dependency, since Synapse's container runs with a read-only root filesystem and this
-# module is mounted in via a ConfigMap, not baked into a custom image.
+# Talks to clamd using Twisted's networking primitives - NOT asyncio's open_connection/
+# wait_for. Synapse runs on Twisted's reactor, which does not have a running asyncio event
+# loop underneath it, so raw asyncio socket calls fail immediately with
+# "RuntimeError: no running event loop" (confirmed live, 2026-07-29 - see git history for
+# the asyncio-based version that failed this way). Twisted Deferreds are natively awaitable
+# from an `async def` when Synapse wraps the callback via Deferred.fromCoroutine(), so this
+# stays plain async/await from the caller's perspective.
 #
 # clamd INSTREAM protocol (docs.clamav.net/manual/Usage/ClamdProtocol.html):
 #   1. send b"zINSTREAM\0"
@@ -16,10 +20,14 @@
 # so a scanner outage can't take down media uploads for the whole homeserver - logged
 # loudly so an outage is still visible in the logs.
 
-import asyncio
 import logging
 from typing import Any, Union
 
+from twisted.internet import reactor
+from twisted.internet.defer import Deferred, TimeoutError as TwistedTimeoutError
+from twisted.internet.endpoints import HostnameEndpoint, connectProtocol
+from twisted.internet.protocol import Protocol
+
 from synapse.module_api import ModuleApi, NOT_SPAM
 from synapse.module_api.errors import Codes
 
@@ -29,6 +37,31 @@ CHUNK_SIZE = 2**14  # matches ReadableFileWrapper.CHUNK_SIZE
 CLAMD_TIMEOUT_SECONDS = 30
 
 
+class _ClamdInstreamProtocol(Protocol):
+    """Speaks clamd's INSTREAM protocol for a single scan, then closes."""
+
+    def __init__(self, data: bytes, result: "Deferred[bytes]"):
+        self._data = data
+        self._result = result
+        self._buffer = bytearray()
+
+    def connectionMade(self) -> None:
+        self.transport.write(b"zINSTREAM\0")
+        for offset in range(0, len(self._data), CHUNK_SIZE):
+            chunk = self._data[offset : offset + CHUNK_SIZE]
+            self.transport.write(len(chunk).to_bytes(4, "big") + chunk)
+        self.transport.write((0).to_bytes(4, "big"))
+
+    def dataReceived(self, data: bytes) -> None:
+        self._buffer.extend(data)
+        if self._buffer.endswith(b"\0") or self._buffer.endswith(b"\n"):
+            self.transport.loseConnection()
+
+    def connectionLost(self, reason: Any = None) -> None:
+        if not self._result.called:
+            self._result.callback(bytes(self._buffer))
+
+
 class ClamAVSpamChecker:
     def __init__(self, config: dict, api: ModuleApi):
         self.api = api
@@ -53,9 +86,7 @@ class ClamAVSpamChecker:
         await file_wrapper.write_chunks_to(buffer.extend)
 
         try:
-            verdict = await asyncio.wait_for(
-                self._scan(bytes(buffer)), timeout=CLAMD_TIMEOUT_SECONDS
-            )
+            verdict = await self._scan(bytes(buffer))
         except Exception:
             logger.exception(
                 "ClamAV scan failed (clamd at %s:%s unreachable?) - "
@@ -73,19 +104,17 @@ class ClamAVSpamChecker:
 
     async def _scan(self, data: bytes) -> "str | None":
         """Returns the detected signature name, or None if the file is clean."""
-        reader, writer = await asyncio.open_connection(self.clamd_host, self.clamd_port)
-        try:
-            writer.write(b"zINSTREAM\0")
-            for offset in range(0, len(data), CHUNK_SIZE):
-                chunk = data[offset : offset + CHUNK_SIZE]
-                writer.write(len(chunk).to_bytes(4, "big") + chunk)
-            writer.write((0).to_bytes(4, "big"))
-            await writer.drain()
+        result: "Deferred[bytes]" = Deferred()
+        endpoint = HostnameEndpoint(reactor, self.clamd_host.encode(), self.clamd_port)
+        await connectProtocol(endpoint, _ClamdInstreamProtocol(data, result))
 
-            response = await reader.read()
-        finally:
-            writer.close()
-            await writer.wait_closed()
+        result.addTimeout(CLAMD_TIMEOUT_SECONDS, reactor)
+        try:
+            response = await result
+        except TwistedTimeoutError:
+            raise TimeoutError(
+                f"clamd at {self.clamd_host}:{self.clamd_port} did not respond in time"
+            )
 
         text = response.decode("utf-8", errors="replace").strip("\x00 \n")
         # "stream: OK" or "stream: <signature name> FOUND"