1 files changed, 88 insertions, 0 deletions
diff --git a/synapse/logging/handlers.py b/synapse/logging/handlers.py
new file mode 100644
index 0000000000..a6c212f300
--- /dev/null
+++ b/synapse/logging/handlers.py
@@ -0,0 +1,88 @@
+import logging
+import time
+from logging import Handler, LogRecord
+from logging.handlers import MemoryHandler
+from threading import Thread
+from typing import Optional
+
+from twisted.internet.interfaces import IReactorCore
+
+
+class PeriodicallyFlushingMemoryHandler(MemoryHandler):
+    """
+    This is a subclass of MemoryHandler that additionally spawns a background
+    thread to periodically flush the buffer.
+
+    This prevents messages from being buffered for too long.
+
+    Additionally, all messages will be immediately flushed if the reactor has
+    not yet been started.
+    """
+
+    def __init__(
+        self,
+        capacity: int,
+        flushLevel: int = logging.ERROR,
+        target: Optional[Handler] = None,
+        flushOnClose: bool = True,
+        period: float = 5.0,
+        reactor: Optional[IReactorCore] = None,
+    ) -> None:
+        """
+        period: the period between automatic flushes
+
+        reactor: if specified, a custom reactor to use. If not specifies,
+            defaults to the globally-installed reactor.
+            Log entries will be flushed immediately until this reactor has
+            started.
+        """
+        super().__init__(capacity, flushLevel, target, flushOnClose)
+
+        self._flush_period: float = period
+        self._active: bool = True
+        self._reactor_started = False
+
+        self._flushing_thread: Thread = Thread(
+            name="PeriodicallyFlushingMemoryHandler flushing thread",
+            target=self._flush_periodically,
+        )
+        self._flushing_thread.start()
+
+        def on_reactor_running():
+            self._reactor_started = True
+
+        reactor_to_use: IReactorCore
+        if reactor is None:
+            from twisted.internet import reactor as global_reactor
+
+            reactor_to_use = global_reactor  # type: ignore[assignment]
+        else:
+            reactor_to_use = reactor
+
+        # call our hook when the reactor start up
+        reactor_to_use.callWhenRunning(on_reactor_running)
+
+    def shouldFlush(self, record: LogRecord) -> bool:
+        """
+        Before reactor start-up, log everything immediately.
+        Otherwise, fall back to original behaviour of waiting for the buffer to fill.
+        """
+
+        if self._reactor_started:
+            return super().shouldFlush(record)
+        else:
+            return True
+
+    def _flush_periodically(self):
+        """
+        Whilst this handler is active, flush the handler periodically.
+        """
+
+        while self._active:
+            # flush is thread-safe; it acquires and releases the lock internally
+            self.flush()
+            time.sleep(self._flush_period)
+
+    def close(self) -> None:
+        self._active = False
+        super().close()