Reduce concurrent thread usage in media (#17567)

Follow on from #17558

Basically, we want to reduce the number of threads we want to use at a
time, i.e. reduce the number of threads that are paused/blocked. We do
this by returning from the thread when the consumer pauses the producer,
rather than pausing in the thread.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/17567.misc

@@ -0,0 +1 @@
+Speed up responding to media requests.

synapse/media/_base.py

@@ -22,7 +22,6 @@
 
 import logging
 import os
-import threading
 import urllib
 from abc import ABC, abstractmethod
 from types import TracebackType
@@ -56,6 +55,7 @@ from synapse.logging.context import (
     run_in_background,
 )
 from synapse.util import Clock
+from synapse.util.async_helpers import DeferredEvent
 from synapse.util.stringutils import is_ascii
 
 if TYPE_CHECKING:
@@ -620,10 +620,13 @@ class ThreadedFileSender:
     A producer that sends the contents of a file to a consumer, reading from the
     file on a thread.
 
-    This works by spawning a loop in a threadpool that repeatedly reads from the
-    file and sends it to the consumer. The main thread communicates with the
-    loop via two `threading.Event`, which controls when to start/pause reading
-    and when to terminate.
+    This works by having a loop in a threadpool repeatedly reading from the
+    file, until the consumer pauses the producer. There is then a loop in the
+    main thread that waits until the consumer resumes the producer and then
+    starts reading in the threadpool again.
+
+    This is done to ensure that we're never waiting in the threadpool, as
+    otherwise its easy to starve it of threads.
     """
 
     # How much data to read in one go.
@@ -643,12 +646,11 @@ class ThreadedFileSender:
 
         # Signals if the thread should keep reading/sending data. Set means
         # continue, clear means pause.
-        self.wakeup_event = threading.Event()
+        self.wakeup_event = DeferredEvent(self.reactor)
 
         # Signals if the thread should terminate, e.g. because the consumer has
-        # gone away. Both this and `wakeup_event` should be set to terminate the
-        # loop (otherwise the thread will block on `wakeup_event`).
-        self.stop_event = threading.Event()
+        # gone away.
+        self.stop_writing = False
 
     def beginFileTransfer(
         self, file: BinaryIO, consumer: interfaces.IConsumer
@@ -663,12 +665,7 @@ class ThreadedFileSender:
 
         # We set the wakeup signal as we should start producing immediately.
         self.wakeup_event.set()
-        run_in_background(
-            defer_to_threadpool,
-            self.reactor,
-            self.thread_pool,
-            self._on_thread_read_loop,
-        )
+        run_in_background(self.start_read_loop)
 
         return make_deferred_yieldable(self.deferred)
 
@@ -686,42 +683,52 @@ class ThreadedFileSender:
         # Unregister the consumer so we don't try and interact with it again.
         self.consumer = None
 
-        # Terminate the thread loop.
+        # Terminate the loop.
+        self.stop_writing = True
         self.wakeup_event.set()
-        self.stop_event.set()
 
         if not self.deferred.called:
             self.deferred.errback(Exception("Consumer asked us to stop producing"))
 
-    def _on_thread_read_loop(self) -> None:
-        """This is the loop that happens on a thread."""
-
+    async def start_read_loop(self) -> None:
+        """This is the loop that drives reading/writing"""
         try:
-            while not self.stop_event.is_set():
-                # We wait for the producer to signal that the consumer wants
-                # more data (or we should abort)
+            while not self.stop_writing:
+                # Start the loop in the threadpool to read data.
+                more_data = await defer_to_threadpool(
+                    self.reactor, self.thread_pool, self._on_thread_read_loop
+                )
+                if not more_data:
+                    # Reached EOF, we can just return.
+                    return
+
                 if not self.wakeup_event.is_set():
-                    ret = self.wakeup_event.wait(self.TIMEOUT_SECONDS)
+                    ret = await self.wakeup_event.wait(self.TIMEOUT_SECONDS)
                     if not ret:
                         raise Exception("Timed out waiting to resume")
-
-                # Check if we were woken up so that we abort the download
-                if self.stop_event.is_set():
-                    return
-
-                # The file should always have been set before we get here.
-                assert self.file is not None
-
-                chunk = self.file.read(self.CHUNK_SIZE)
-                if not chunk:
-                    return
-
-                self.reactor.callFromThread(self._write, chunk)
-
         except Exception:
-            self.reactor.callFromThread(self._error, Failure())
+            self._error(Failure())
         finally:
-            self.reactor.callFromThread(self._finish)
+            self._finish()
+
+    def _on_thread_read_loop(self) -> bool:
+        """This is the loop that happens on a thread.
+
+        Returns:
+            Whether there is more data to send.
+        """
+
+        while not self.stop_writing and self.wakeup_event.is_set():
+            # The file should always have been set before we get here.
+            assert self.file is not None
+
+            chunk = self.file.read(self.CHUNK_SIZE)
+            if not chunk:
+                return False
+
+            self.reactor.callFromThread(self._write, chunk)
+
+        return True
 
     def _write(self, chunk: bytes) -> None:
         """Called from the thread to write a chunk of data"""
@@ -729,7 +736,7 @@ class ThreadedFileSender:
             self.consumer.write(chunk)
 
     def _error(self, failure: Failure) -> None:
-        """Called from the thread when there was a fatal error"""
+        """Called when there was a fatal error"""
         if self.consumer:
             self.consumer.unregisterProducer()
             self.consumer = None
@@ -738,7 +745,7 @@ class ThreadedFileSender:
             self.deferred.errback(failure)
 
     def _finish(self) -> None:
-        """Called from the thread when it finishes (either on success or
+        """Called when we have finished writing (either on success or
         failure)."""
         if self.file:
             self.file.close()

synapse/util/async_helpers.py

@@ -885,3 +885,46 @@ class AwakenableSleeper:
             # Cancel the sleep if we were woken up
             if call.active():
                 call.cancel()
+
+
+class DeferredEvent:
+    """Like threading.Event but for async code"""
+
+    def __init__(self, reactor: IReactorTime) -> None:
+        self._reactor = reactor
+        self._deferred: "defer.Deferred[None]" = defer.Deferred()
+
+    def set(self) -> None:
+        if not self._deferred.called:
+            self._deferred.callback(None)
+
+    def clear(self) -> None:
+        if self._deferred.called:
+            self._deferred = defer.Deferred()
+
+    def is_set(self) -> bool:
+        return self._deferred.called
+
+    async def wait(self, timeout_seconds: float) -> bool:
+        if self.is_set():
+            return True
+
+        # Create a deferred that gets called in N seconds
+        sleep_deferred: "defer.Deferred[None]" = defer.Deferred()
+        call = self._reactor.callLater(timeout_seconds, sleep_deferred.callback, None)
+
+        try:
+            await make_deferred_yieldable(
+                defer.DeferredList(
+                    [sleep_deferred, self._deferred],
+                    fireOnOneCallback=True,
+                    fireOnOneErrback=True,
+                    consumeErrors=True,
+                )
+            )
+        finally:
+            # Cancel the sleep if we were woken up
+            if call.active():
+                call.cancel()
+
+        return self.is_set()