Fix spinloop during partial state sync when a prev event is in backoff (#15351)

Previously, we would spin in a tight loop until
`update_state_for_partial_state_event` stopped raising
`FederationPullAttemptBackoffError`s. Replace the spinloop with a wait
until the backoff period has expired.

Signed-off-by: Sean Quah <seanq@matrix.org>

1 files changed, 13 insertions, 4 deletions

diff --git a/synapse/api/errors.py b/synapse/api/errors.py

index 8c6822f3c6..f2d6f9ab2d 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -27,7 +27,7 @@ from synapse.util import json_decoder
 
 if typing.TYPE_CHECKING:
     from synapse.config.homeserver import HomeServerConfig
-    from synapse.types import JsonDict
+    from synapse.types import JsonDict, StrCollection
 
 logger = logging.getLogger(__name__)
 
@@ -682,18 +682,27 @@ class FederationPullAttemptBackoffError(RuntimeError):
     Attributes:
         event_id: The event_id which we are refusing to pull
         message: A custom error message that gives more context
+        retry_after_ms: The remaining backoff interval, in milliseconds
     """
 
-    def __init__(self, event_ids: List[str], message: Optional[str]):
-        self.event_ids = event_ids
+    def __init__(
+        self, event_ids: "StrCollection", message: Optional[str], retry_after_ms: int
+    ):
+        event_ids = list(event_ids)
 
         if message:
             error_message = message
         else:
-            error_message = f"Not attempting to pull event_ids={self.event_ids} because we already tried to pull them recently (backing off)."
+            error_message = (
+                f"Not attempting to pull event_ids={event_ids} because we already "
+                "tried to pull them recently (backing off)."
+            )
 
         super().__init__(error_message)
 
+        self.event_ids = event_ids
+        self.retry_after_ms = retry_after_ms
+
 
 class HttpResponseException(CodeMessageException):
     """