Immediately retry any requests that have backed off when a server comes back online. (#12500)

Otherwise it can take up to a minute for any in-flight `/send` requests to be retried.

2 files changed, 80 insertions, 1 deletions

diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index b91020117f..7f1d41eb3c 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -778,3 +778,60 @@ def delay_cancellation(awaitable: Awaitable[T]) -> Awaitable[T]:
     new_deferred: "defer.Deferred[T]" = defer.Deferred(handle_cancel)
     deferred.chainDeferred(new_deferred)
     return new_deferred
+
+
+class AwakenableSleeper:
+    """Allows explicitly waking up deferreds related to an entity that are
+    currently sleeping.
+    """
+
+    def __init__(self, reactor: IReactorTime) -> None:
+        self._streams: Dict[str, Set[defer.Deferred[None]]] = {}
+        self._reactor = reactor
+
+    def wake(self, name: str) -> None:
+        """Wake everything related to `name` that is currently sleeping."""
+        stream_set = self._streams.pop(name, set())
+        for deferred in stream_set:
+            try:
+                with PreserveLoggingContext():
+                    deferred.callback(None)
+            except Exception:
+                pass
+
+    async def sleep(self, name: str, delay_ms: int) -> None:
+        """Sleep for the given number of milliseconds, or return if the given
+        `name` is explicitly woken up.
+        """
+
+        # Create a deferred that gets called in N seconds
+        sleep_deferred: "defer.Deferred[None]" = defer.Deferred()
+        call = self._reactor.callLater(delay_ms / 1000, sleep_deferred.callback, None)
+
+        # Create a deferred that will get called if `wake` is called with
+        # the same `name`.
+        stream_set = self._streams.setdefault(name, set())
+        notify_deferred: "defer.Deferred[None]" = defer.Deferred()
+        stream_set.add(notify_deferred)
+
+        try:
+            # Wait for either the delay or for `wake` to be called.
+            await make_deferred_yieldable(
+                defer.DeferredList(
+                    [sleep_deferred, notify_deferred],
+                    fireOnOneCallback=True,
+                    fireOnOneErrback=True,
+                    consumeErrors=True,
+                )
+            )
+        finally:
+            # Clean up the state
+            curr_stream_set = self._streams.get(name)
+            if curr_stream_set is not None:
+                curr_stream_set.discard(notify_deferred)
+                if len(curr_stream_set) == 0:
+                    self._streams.pop(name)
+
+            # Cancel the sleep if we were woken up
+            if call.active():
+                call.cancel()
diff --git a/synapse/util/retryutils.py b/synapse/util/retryutils.py
index d81f2527d7..81bfed268e 100644
--- a/synapse/util/retryutils.py
+++ b/synapse/util/retryutils.py
@@ -14,13 +14,17 @@
 import logging
 import random
 from types import TracebackType
-from typing import Any, Optional, Type
+from typing import TYPE_CHECKING, Any, Optional, Type
 
 import synapse.logging.context
 from synapse.api.errors import CodeMessageException
 from synapse.storage import DataStore
 from synapse.util import Clock
 
+if TYPE_CHECKING:
+    from synapse.notifier import Notifier
+    from synapse.replication.tcp.handler import ReplicationCommandHandler
+
 logger = logging.getLogger(__name__)
 
 # the initial backoff, after the first transaction fails
@@ -131,6 +135,8 @@ class RetryDestinationLimiter:
         retry_interval: int,
         backoff_on_404: bool = False,
         backoff_on_failure: bool = True,
+        notifier: Optional["Notifier"] = None,
+        replication_client: Optional["ReplicationCommandHandler"] = None,
     ):
         """Marks the destination as "down" if an exception is thrown in the
         context, except for CodeMessageException with code < 500.
@@ -160,6 +166,9 @@ class RetryDestinationLimiter:
         self.backoff_on_404 = backoff_on_404
         self.backoff_on_failure = backoff_on_failure
 
+        self.notifier = notifier
+        self.replication_client = replication_client
+
     def __enter__(self) -> None:
         pass
 
@@ -239,6 +248,19 @@ class RetryDestinationLimiter:
                     retry_last_ts,
                     self.retry_interval,
                 )
+
+                if self.notifier:
+                    # Inform the relevant places that the remote server is back up.
+                    self.notifier.notify_remote_server_up(self.destination)
+
+                if self.replication_client:
+                    # If we're on a worker we try and inform master about this. The
+                    # replication client doesn't hook into the notifier to avoid
+                    # infinite loops where we send a `REMOTE_SERVER_UP` command to
+                    # master, which then echoes it back to us which in turn pokes
+                    # the notifier.
+                    self.replication_client.send_remote_server_up(self.destination)
+
             except Exception:
                 logger.exception("Failed to store destination_retry_timings")