Faster joins: don't stall when a user joins during a fast join (#14606)

Fixes #12801.
Complement tests are at
https://github.com/matrix-org/complement/pull/567.

Avoid blocking on full state when handling a subsequent join into a
partial state room.

Also always perform a remote join into partial state rooms, since we do
not know whether the joining user has been banned and want to avoid
leaking history to banned users.

Signed-off-by: Mathieu Velten <mathieuv@matrix.org>
Co-authored-by: Sean Quah <seanq@matrix.org>
Co-authored-by: David Robertson <davidr@element.io>

synapse/api/errors.py

@@ -751,3 +751,25 @@ class ModuleFailedException(Exception):
     Raised when a module API callback fails, for example because it raised an
     exception.
     """
+
+
+class PartialStateConflictError(SynapseError):
+    """An internal error raised when attempting to persist an event with partial state
+    after the room containing the event has been un-partial stated.
+
+    This error should be handled by recomputing the event context and trying again.
+
+    This error has an HTTP status code so that it can be transported over replication.
+    It should not be exposed to clients.
+    """
+
+    @staticmethod
+    def message() -> str:
+        return "Cannot persist partial state event in un-partial stated room"
+
+    def __init__(self) -> None:
+        super().__init__(
+            HTTPStatus.CONFLICT,
+            msg=PartialStateConflictError.message(),
+            errcode=Codes.UNKNOWN,
+        )