diff --git a/changelog.d/7905.bugfix b/changelog.d/7905.bugfix
new file mode 100644
index 000000000000..e60e62441210
--- /dev/null
+++ b/changelog.d/7905.bugfix
@@ -0,0 +1 @@
+Fix a longstanding bug when storing a media file with an empty `upload_name`.
diff --git a/changelog.d/8415.doc b/changelog.d/8415.doc
new file mode 100644
index 000000000000..28b579853364
--- /dev/null
+++ b/changelog.d/8415.doc
@@ -0,0 +1 @@
+Improve description of `server_name` config option in `homserver.yaml`.
\ No newline at end of file
diff --git a/changelog.d/8419.feature b/changelog.d/8419.feature
new file mode 100644
index 000000000000..b363e929ea8c
--- /dev/null
+++ b/changelog.d/8419.feature
@@ -0,0 +1 @@
+Add experimental support for sharding event persister.
diff --git a/changelog.d/8422.misc b/changelog.d/8422.misc
new file mode 100644
index 000000000000..03fba120c6d9
--- /dev/null
+++ b/changelog.d/8422.misc
@@ -0,0 +1 @@
+Typing fixes for `synapse.handlers.federation`.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 76f588fa9fc3..4831484ef2d4 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -33,10 +33,23 @@
 
 ## Server ##
 
-# The domain name of the server, with optional explicit port.
-# This is used by remote servers to connect to this server,
-# e.g. matrix.org, localhost:8080, etc.
-# This is also the last part of your UserID.
+# The public-facing domain of the server
+#
+# The server_name name will appear at the end of usernames and room addresses
+# created on this server. For example if the server_name was example.com,
+# usernames on this server would be in the format @user:example.com
+#
+# In most cases you should avoid using a matrix specific subdomain such as
+# matrix.example.com or synapse.example.com as the server_name for the same
+# reasons you wouldn't use user@email.example.com as your email address.
+# See https://github.com/matrix-org/synapse/blob/master/docs/delegate.md
+# for information on how to host Synapse on a subdomain while preserving
+# a clean server_name.
+#
+# The server_name cannot be changed later so it is important to
+# configure this correctly before you start Synapse. It should be all
+# lowercase and may contain an explicit port.
+# Examples: matrix.org, localhost:8080
 #
 server_name: "SERVERNAME"
 
diff --git a/synapse/config/server.py b/synapse/config/server.py
index eff2a846a47d..9a5c7cb0ab4a 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -647,10 +647,23 @@ def generate_config_section(
             """\
         ## Server ##
 
-        # The domain name of the server, with optional explicit port.
-        # This is used by remote servers to connect to this server,
-        # e.g. matrix.org, localhost:8080, etc.
-        # This is also the last part of your UserID.
+        # The public-facing domain of the server
+        #
+        # The server_name name will appear at the end of usernames and room addresses
+        # created on this server. For example if the server_name was example.com,
+        # usernames on this server would be in the format @user:example.com
+        #
+        # In most cases you should avoid using a matrix specific subdomain such as
+        # matrix.example.com or synapse.example.com as the server_name for the same
+        # reasons you wouldn't use user@email.example.com as your email address.
+        # See https://github.com/matrix-org/synapse/blob/master/docs/delegate.md
+        # for information on how to host Synapse on a subdomain while preserving
+        # a clean server_name.
+        #
+        # The server_name cannot be changed later so it is important to
+        # configure this correctly before you start Synapse. It should be all
+        # lowercase and may contain an explicit port.
+        # Examples: matrix.org, localhost:8080
         #
         server_name: "%(server_name)s"
 
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 688d43fffb4c..302b2f69bcdd 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -24,10 +24,12 @@
     Dict,
     Iterable,
     List,
+    Mapping,
     Optional,
     Sequence,
     Tuple,
     TypeVar,
+    Union,
 )
 
 from prometheus_client import Counter
@@ -501,7 +503,7 @@ async def make_membership_event(
         user_id: str,
         membership: str,
         content: dict,
-        params: Dict[str, str],
+        params: Optional[Mapping[str, Union[str, Iterable[str]]]],
     ) -> Tuple[str, EventBase, RoomVersion]:
         """
         Creates an m.room.member event, with context, without participating in the room.
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 33feb1b2ba91..eb91f2f7620d 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -155,8 +155,9 @@ def __init__(self, hs):
             self._device_list_updater = hs.get_device_handler().device_list_updater
             self._maybe_store_room_on_invite = self.store.maybe_store_room_on_invite
 
-        # When joining a room we need to queue any events for that room up
-        self.room_queues = {}
+        # When joining a room we need to queue any events for that room up.
+        # For each room, a list of (pdu, origin) tuples.
+        self.room_queues = {}  # type: Dict[str, List[Tuple[EventBase, str]]]
         self._room_pdu_linearizer = Linearizer("fed_room_pdu")
 
         self.third_party_event_rules = hs.get_third_party_event_rules()
@@ -817,6 +818,9 @@ async def backfill(self, dest, room_id, limit, extremities):
             dest, room_id, limit=limit, extremities=extremities
         )
 
+        if not events:
+            return []
+
         # ideally we'd sanity check the events here for excess prev_events etc,
         # but it's hard to reject events at this point without completely
         # breaking backfill in the same way that it is currently broken by
@@ -2174,10 +2178,10 @@ async def _check_for_soft_fail(
             # given state at the event. This should correctly handle cases
             # like bans, especially with state res v2.
 
-            state_sets = await self.state_store.get_state_groups(
+            state_sets_d = await self.state_store.get_state_groups(
                 event.room_id, extrem_ids
             )
-            state_sets = list(state_sets.values())
+            state_sets = list(state_sets_d.values())  # type: List[Iterable[EventBase]]
             state_sets.append(state)
             current_states = await self.state_handler.resolve_events(
                 room_version, state_sets, event
@@ -2968,6 +2972,7 @@ async def persist_events_and_notify(
             )
             return result["max_stream_id"]
         else:
+            assert self.storage.persistence
             max_stream_token = await self.storage.persistence.persist_events(
                 event_and_contexts, backfilled=backfilled
             )
diff --git a/synapse/rest/media/v1/media_repository.py b/synapse/rest/media/v1/media_repository.py
index 69f353d46f9e..ae6822d6e742 100644
--- a/synapse/rest/media/v1/media_repository.py
+++ b/synapse/rest/media/v1/media_repository.py
@@ -139,7 +139,7 @@ def mark_recently_accessed(self, server_name, media_id):
     async def create_content(
         self,
         media_type: str,
-        upload_name: str,
+        upload_name: Optional[str],
         content: IO,
         content_length: int,
         auth_user: str,
@@ -147,8 +147,8 @@ async def create_content(
         """Store uploaded content for a local user and return the mxc URL
 
         Args:
-            media_type: The content type of the file
-            upload_name: The name of the file
+            media_type: The content type of the file.
+            upload_name: The name of the file, if provided.
             content: A file like object that is the content to store
             content_length: The length of the content
             auth_user: The user_id of the uploader
@@ -156,6 +156,7 @@ async def create_content(
         Returns:
             The mxc url of the stored content
         """
+
         media_id = random_string(24)
 
         file_info = FileInfo(server_name=None, file_id=media_id)
diff --git a/synapse/rest/media/v1/upload_resource.py b/synapse/rest/media/v1/upload_resource.py
index 3ebf7a68e673..d76f7389e101 100644
--- a/synapse/rest/media/v1/upload_resource.py
+++ b/synapse/rest/media/v1/upload_resource.py
@@ -63,6 +63,10 @@ async def _async_render_POST(self, request):
                     msg="Invalid UTF-8 filename parameter: %r" % (upload_name), code=400
                 )
 
+        # If the name is falsey (e.g. an empty byte string) ensure it is None.
+        else:
+            upload_name = None
+
         headers = request.requestHeaders
 
         if headers.hasHeader(b"Content-Type"):
diff --git a/synapse/storage/databases/state/store.py b/synapse/storage/databases/state/store.py
index 989f0cbc9d3b..0e31cc811a38 100644
--- a/synapse/storage/databases/state/store.py
+++ b/synapse/storage/databases/state/store.py
@@ -24,7 +24,7 @@
 from synapse.storage.state import StateFilter
 from synapse.storage.types import Cursor
 from synapse.storage.util.sequence import build_sequence_generator
-from synapse.types import StateMap
+from synapse.types import MutableStateMap, StateMap
 from synapse.util.caches.descriptors import cached
 from synapse.util.caches.dictionary_cache import DictionaryCache
 
@@ -208,7 +208,7 @@ def _get_state_for_group_using_cache(self, cache, group, state_filter):
 
     async def _get_state_for_groups(
         self, groups: Iterable[int], state_filter: StateFilter = StateFilter.all()
-    ) -> Dict[int, StateMap[str]]:
+    ) -> Dict[int, MutableStateMap[str]]:
         """Gets the state at each of a list of state groups, optionally
         filtering by type/state_key
 
diff --git a/synapse/storage/persist_events.py b/synapse/storage/persist_events.py
index 603cd7d825e0..ded6cf965528 100644
--- a/synapse/storage/persist_events.py
+++ b/synapse/storage/persist_events.py
@@ -197,7 +197,7 @@ def __init__(self, hs, stores: Databases):
 
     async def persist_events(
         self,
-        events_and_contexts: List[Tuple[EventBase, EventContext]],
+        events_and_contexts: Iterable[Tuple[EventBase, EventContext]],
         backfilled: bool = False,
     ) -> RoomStreamToken:
         """
diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 8f68d968f0c8..08a69f2f9667 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -20,7 +20,7 @@
 
 from synapse.api.constants import EventTypes
 from synapse.events import EventBase
-from synapse.types import StateMap
+from synapse.types import MutableStateMap, StateMap
 
 logger = logging.getLogger(__name__)
 
@@ -349,7 +349,7 @@ async def get_state_group_delta(self, state_group: int):
 
     async def get_state_groups_ids(
         self, _room_id: str, event_ids: Iterable[str]
-    ) -> Dict[int, StateMap[str]]:
+    ) -> Dict[int, MutableStateMap[str]]:
         """Get the event IDs of all the state for the state groups for the given events
 
         Args:
@@ -532,7 +532,7 @@ async def get_state_ids_for_event(
 
     def _get_state_for_groups(
         self, groups: Iterable[int], state_filter: StateFilter = StateFilter.all()
-    ) -> Awaitable[Dict[int, StateMap[str]]]:
+    ) -> Awaitable[Dict[int, MutableStateMap[str]]]:
         """Gets the state at each of a list of state groups, optionally
         filtering by type/state_key
 
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 4fd7573e260d..02fbb656e81c 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -273,6 +273,19 @@ def _load_current_ids(
 
         # Load the current positions of all writers for the stream.
         if self._writers:
+            # We delete any stale entries in the positions table. This is
+            # important if we add back a writer after a long time; we want to
+            # consider that a "new" writer, rather than using the old stale
+            # entry here.
+            sql = """
+                DELETE FROM stream_positions
+                WHERE
+                    stream_name = ?
+                    AND instance_name != ALL(?)
+            """
+            sql = self._db.engine.convert_param_style(sql)
+            cur.execute(sql, (self._stream_name, self._writers))
+
             sql = """
                 SELECT instance_name, stream_id FROM stream_positions
                 WHERE stream_name = ?
@@ -453,11 +466,22 @@ def get_current_token_for_writer(self, instance_name: str) -> int:
         """Returns the position of the given writer.
         """
 
+        # If we don't have an entry for the given instance name, we assume it's a
+        # new writer.
+        #
+        # For new writers we assume their initial position to be the current
+        # persisted up to position. This stops Synapse from doing a full table
+        # scan when a new writer announces itself over replication.
         with self._lock:
-            return self._return_factor * self._current_positions.get(instance_name, 0)
+            return self._return_factor * self._current_positions.get(
+                instance_name, self._persisted_upto_position
+            )
 
     def get_positions(self) -> Dict[str, int]:
         """Get a copy of the current positon map.
+
+        Note that this won't necessarily include all configured writers if some
+        writers haven't written anything yet.
         """
 
         with self._lock:
diff --git a/tests/storage/test_id_generators.py b/tests/storage/test_id_generators.py
index 4558bee7be85..392b08832b05 100644
--- a/tests/storage/test_id_generators.py
+++ b/tests/storage/test_id_generators.py
@@ -390,17 +390,28 @@ def test_writer_config_change(self):
         # Initial config has two writers
         id_gen = self._create_id_generator("first", writers=["first", "second"])
         self.assertEqual(id_gen.get_persisted_upto_position(), 3)
+        self.assertEqual(id_gen.get_current_token_for_writer("first"), 3)
+        self.assertEqual(id_gen.get_current_token_for_writer("second"), 5)
 
         # New config removes one of the configs. Note that if the writer is
         # removed from config we assume that it has been shut down and has
         # finished persisting, hence why the persisted upto position is 5.
         id_gen_2 = self._create_id_generator("second", writers=["second"])
         self.assertEqual(id_gen_2.get_persisted_upto_position(), 5)
+        self.assertEqual(id_gen_2.get_current_token_for_writer("second"), 5)
 
         # This config points to a single, previously unused writer.
         id_gen_3 = self._create_id_generator("third", writers=["third"])
         self.assertEqual(id_gen_3.get_persisted_upto_position(), 5)
 
+        # For new writers we assume their initial position to be the current
+        # persisted up to position. This stops Synapse from doing a full table
+        # scan when a new writer comes along.
+        self.assertEqual(id_gen_3.get_current_token_for_writer("third"), 5)
+
+        id_gen_4 = self._create_id_generator("fourth", writers=["third"])
+        self.assertEqual(id_gen_4.get_current_token_for_writer("third"), 5)
+
         # Check that we get a sane next stream ID with this new config.
 
         async def _get_next_async():
@@ -410,6 +421,13 @@ async def _get_next_async():
         self.get_success(_get_next_async())
         self.assertEqual(id_gen_3.get_persisted_upto_position(), 6)
 
+        # If we add back the old "first" then we shouldn't see the persisted up
+        # to position revert back to 3.
+        id_gen_5 = self._create_id_generator("five", writers=["first", "third"])
+        self.assertEqual(id_gen_5.get_persisted_upto_position(), 6)
+        self.assertEqual(id_gen_5.get_current_token_for_writer("first"), 6)
+        self.assertEqual(id_gen_5.get_current_token_for_writer("third"), 6)
+
     def test_sequence_consistency(self):
         """Test that we error out if the table and sequence diverges.
         """