From 440b8845b531db05e7c4f646e48dee7635cf1f0a Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 23 Jul 2018 12:38:46 +0100
Subject: [PATCH 1/6] Make EventContext lazy load state

---
 synapse/events/snapshot.py | 153 +++++++++++++++++++++++++++----------
 synapse/state.py           |   6 +-
 2 files changed, 115 insertions(+), 44 deletions(-)

diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index 5e02ef1a5..f9568638a 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -19,18 +19,12 @@ from frozendict import frozendict
 
 from twisted.internet import defer
 
+from synapse.util.logcontext import make_deferred_yieldable, run_in_background
+
 
 class EventContext(object):
     """
     Attributes:
-        current_state_ids (dict[(str, str), str]):
-            The current state map including the current event.
-            (type, state_key) -> event_id
-
-        prev_state_ids (dict[(str, str), str]):
-            The current state map excluding the current event.
-            (type, state_key) -> event_id
-
         state_group (int|None): state group id, if the state has been stored
             as a state group. This is usually only None if e.g. the event is
             an outlier.
@@ -47,36 +41,71 @@ class EventContext(object):
 
         prev_state_events (?): XXX: is this ever set to anything other than
             the empty list?
+
+        _current_state_ids (dict[(str, str), str]|None):
+            The current state map including the current event. None if outlier
+            or we haven't fetched the state from DB yet.
+            (type, state_key) -> event_id
+
+        _prev_state_ids (dict[(str, str), str]|None):
+            The current state map excluding the current event. None if outlier
+            or we haven't fetched the state from DB yet.
+            (type, state_key) -> event_id
+
+        _fetching_state_deferred (Deferred|None): Resolves when *_state_ids have
+            been calculated. None if we haven't started calculating yet
+
+        _prev_state_id (str|None): If set then the event associated with the
+            context overrode the _prev_state_id
+
+        _event_type (str): The type of the event the context is associated with
+
+        _event_state_key (str|None): The state_key of the event the context is
+            associated with
     """
 
     __slots__ = [
-        "current_state_ids",
-        "prev_state_ids",
         "state_group",
         "rejected",
         "prev_group",
         "delta_ids",
         "prev_state_events",
         "app_service",
+        "_current_state_ids",
+        "_prev_state_ids",
+        "_prev_state_id",
+        "_event_type",
+        "_event_state_key",
+        "_fetching_state_deferred",
     ]
 
-    def __init__(self, state_group, current_state_ids, prev_state_ids,
-                 prev_group=None, delta_ids=None):
+    @staticmethod
+    def with_state(state_group, current_state_ids, prev_state_ids,
+                   prev_group=None, delta_ids=None):
+        context = EventContext()
+
         # The current state including the current event
-        self.current_state_ids = current_state_ids
+        context._current_state_ids = current_state_ids
         # The current state excluding the current event
-        self.prev_state_ids = prev_state_ids
-        self.state_group = state_group
+        context._prev_state_ids = prev_state_ids
+        context.state_group = state_group
+
+        context._prev_state_id = None
+        context._event_type = None
+        context._event_state_key = None
+        context._fetching_state_deferred = defer.succeed(None)
 
         # A previously persisted state group and a delta between that
         # and this state.
-        self.prev_group = prev_group
-        self.delta_ids = delta_ids
+        context.prev_group = prev_group
+        context.delta_ids = delta_ids
 
-        self.prev_state_events = []
+        context.prev_state_events = []
 
-        self.rejected = False
-        self.app_service = None
+        context.rejected = False
+        context.app_service = None
+
+        return context
 
     def serialize(self, event):
         """Converts self to a type that can be serialized as JSON, and then
@@ -123,30 +152,17 @@ class EventContext(object):
         Returns:
             EventContext
         """
+        context = EventContext()
+
         # We use the state_group and prev_state_id stuff to pull the
         # current_state_ids out of the DB and construct prev_state_ids.
-        prev_state_id = input["prev_state_id"]
-        event_type = input["event_type"]
-        event_state_key = input["event_state_key"]
+        context._prev_state_id = input["prev_state_id"]
+        context._event_type = input["event_type"]
+        context._event_state_key = input["event_state_key"]
 
-        state_group = input["state_group"]
-
-        current_state_ids = yield store.get_state_ids_for_group(
-            state_group,
-        )
-        if prev_state_id and event_state_key:
-            prev_state_ids = dict(current_state_ids)
-            prev_state_ids[(event_type, event_state_key)] = prev_state_id
-        else:
-            prev_state_ids = current_state_ids
-
-        context = EventContext(
-            state_group=state_group,
-            current_state_ids=current_state_ids,
-            prev_state_ids=prev_state_ids,
-            prev_group=input["prev_group"],
-            delta_ids=_decode_state_dict(input["delta_ids"]),
-        )
+        context.state_group = input["state_group"]
+        context.prev_group = input["prev_group"]
+        context.delta_ids = _decode_state_dict(input["delta_ids"])
 
         context.rejected = input["rejected"]
         context.prev_state_events = input["prev_state_events"]
@@ -157,6 +173,61 @@ class EventContext(object):
 
         defer.returnValue(context)
 
+    @defer.inlineCallbacks
+    def get_current_state_ids(self, store):
+        """Gets the current state IDs
+
+        Returns:
+            Deferred[dict[(str, str), str]|None]: Returns None if state_group
+            is None, which happens when the associated event is an outlier.
+        """
+
+        if not self._fetching_state_deferred:
+            self._fetching_state_deferred = run_in_background(
+                self._fill_out_state, store,
+            )
+
+        yield make_deferred_yieldable(self._fetching_state_deferred)
+
+        defer.returnValue(self._current_state_ids)
+
+    @defer.inlineCallbacks
+    def get_prev_state_ids(self, store):
+        """Gets the prev state IDs
+
+        Returns:
+            Deferred[dict[(str, str), str]|None]: Returns None if state_group
+            is None, which happens when the associated event is an outlier.
+        """
+
+        if not self._fetching_state_deferred:
+            self._fetching_state_deferred = run_in_background(
+                self._fill_out_state, store,
+            )
+
+        yield make_deferred_yieldable(self._fetching_state_deferred)
+
+        defer.returnValue(self._prev_state_ids)
+
+    @defer.inlineCallbacks
+    def _fill_out_state(self, store):
+        """Called to populate the _current_state_ids and _prev_state_ids
+        attributes by loading from the database.
+        """
+        if self.state_group is None:
+            return
+
+        self._current_state_ids = yield store.get_state_ids_for_group(
+            self.state_group,
+        )
+        if self._prev_state_id and self._event_state_key is not None:
+            self._prev_state_ids = dict(self._current_state_ids)
+
+            key = (self._event_type, self._event_state_key)
+            self._prev_state_ids[key] = self._prev_state_id
+        else:
+            self._prev_state_ids = self._current_state_ids
+
 
 def _encode_state_dict(state_dict):
     """Since dicts of (type, state_key) -> event_id cannot be serialized in
diff --git a/synapse/state.py b/synapse/state.py
index a70869500..32125c95d 100644
--- a/synapse/state.py
+++ b/synapse/state.py
@@ -219,7 +219,7 @@ class StateHandler(object):
 
             # We don't store state for outliers, so we don't generate a state
             # group for it.
-            context = EventContext(
+            context = EventContext.with_state(
                 state_group=None,
                 current_state_ids=current_state_ids,
                 prev_state_ids=prev_state_ids,
@@ -255,7 +255,7 @@ class StateHandler(object):
                 current_state_ids=current_state_ids,
             )
 
-            context = EventContext(
+            context = EventContext.with_state(
                 state_group=state_group,
                 current_state_ids=current_state_ids,
                 prev_state_ids=prev_state_ids,
@@ -322,7 +322,7 @@ class StateHandler(object):
 
             state_group = entry.state_group
 
-        context = EventContext(
+        context = EventContext.with_state(
             state_group=state_group,
             current_state_ids=current_state_ids,
             prev_state_ids=prev_state_ids,

From e42510ba635b3e4d83215e4f5634ca51411996e0 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 23 Jul 2018 13:00:22 +0100
Subject: [PATCH 2/6] Use new getters

---
 synapse/api/auth.py                      |  6 ++++--
 synapse/handlers/_base.py                |  3 ++-
 synapse/handlers/federation.py           | 23 ++++++++++++++-------
 synapse/handlers/message.py              | 26 +++++++++++++++---------
 synapse/handlers/room_member.py          |  9 +++++---
 synapse/push/bulk_push_rule_evaluator.py |  7 ++++---
 synapse/storage/events.py                |  2 +-
 synapse/storage/push_rule.py             |  7 +++++--
 synapse/storage/roommember.py            |  7 +++++--
 9 files changed, 59 insertions(+), 31 deletions(-)

diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index bc629832d..535bdb449 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -65,8 +65,9 @@ class Auth(object):
 
     @defer.inlineCallbacks
     def check_from_context(self, event, context, do_sig_check=True):
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
         auth_events_ids = yield self.compute_auth_events(
-            event, context.prev_state_ids, for_verification=True,
+            event, prev_state_ids, for_verification=True,
         )
         auth_events = yield self.store.get_events(auth_events_ids)
         auth_events = {
@@ -544,7 +545,8 @@ class Auth(object):
 
     @defer.inlineCallbacks
     def add_auth_events(self, builder, context):
-        auth_ids = yield self.compute_auth_events(builder, context.prev_state_ids)
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+        auth_ids = yield self.compute_auth_events(builder, prev_state_ids)
 
         auth_events_entries = yield self.store.add_event_hashes(
             auth_ids
diff --git a/synapse/handlers/_base.py b/synapse/handlers/_base.py
index b6a8b3aa3..704181d2d 100644
--- a/synapse/handlers/_base.py
+++ b/synapse/handlers/_base.py
@@ -112,8 +112,9 @@ class BaseHandler(object):
             guest_access = event.content.get("guest_access", "forbidden")
             if guest_access != "can_join":
                 if context:
+                    current_state_ids = yield context.get_current_state_ids(self.store)
                     current_state = yield self.store.get_events(
-                        list(context.current_state_ids.values())
+                        list(current_state_ids.values())
                     )
                 else:
                     current_state = yield self.state_handler.get_current_state(
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index a6d391c4e..98dd4a7fd 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -486,7 +486,10 @@ class FederationHandler(BaseHandler):
                 # joined the room. Don't bother if the user is just
                 # changing their profile info.
                 newly_joined = True
-                prev_state_id = context.prev_state_ids.get(
+
+                prev_state_ids = yield context.get_prev_state_ids(self.store)
+
+                prev_state_id = prev_state_ids.get(
                     (event.type, event.state_key)
                 )
                 if prev_state_id:
@@ -1106,10 +1109,12 @@ class FederationHandler(BaseHandler):
                 user = UserID.from_string(event.state_key)
                 yield user_joined_room(self.distributor, user, event.room_id)
 
-        state_ids = list(context.prev_state_ids.values())
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+
+        state_ids = list(prev_state_ids.values())
         auth_chain = yield self.store.get_auth_chain(state_ids)
 
-        state = yield self.store.get_events(list(context.prev_state_ids.values()))
+        state = yield self.store.get_events(list(prev_state_ids.values()))
 
         defer.returnValue({
             "state": list(state.values()),
@@ -1635,8 +1640,9 @@ class FederationHandler(BaseHandler):
         )
 
         if not auth_events:
+            prev_state_ids = yield context.get_prev_state_ids(self.store)
             auth_events_ids = yield self.auth.compute_auth_events(
-                event, context.prev_state_ids, for_verification=True,
+                event, prev_state_ids, for_verification=True,
             )
             auth_events = yield self.store.get_events(auth_events_ids)
             auth_events = {
@@ -1876,9 +1882,10 @@ class FederationHandler(BaseHandler):
                         break
 
             if do_resolution:
+                prev_state_ids = yield context.get_prev_state_ids(self.store)
                 # 1. Get what we think is the auth chain.
                 auth_ids = yield self.auth.compute_auth_events(
-                    event, context.prev_state_ids
+                    event, prev_state_ids
                 )
                 local_auth_chain = yield self.store.get_auth_chain(
                     auth_ids, include_given=True
@@ -2222,7 +2229,8 @@ class FederationHandler(BaseHandler):
             event.content["third_party_invite"]["signed"]["token"]
         )
         original_invite = None
-        original_invite_id = context.prev_state_ids.get(key)
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+        original_invite_id = prev_state_ids.get(key)
         if original_invite_id:
             original_invite = yield self.store.get_event(
                 original_invite_id, allow_none=True
@@ -2264,7 +2272,8 @@ class FederationHandler(BaseHandler):
         signed = event.content["third_party_invite"]["signed"]
         token = signed["token"]
 
-        invite_event_id = context.prev_state_ids.get(
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+        invite_event_id = prev_state_ids.get(
             (EventTypes.ThirdPartyInvite, token,)
         )
 
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index abc07ea87..c4bcd9018 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -630,7 +630,8 @@ class EventCreationHandler(object):
         If so, returns the version of the event in context.
         Otherwise, returns None.
         """
-        prev_event_id = context.prev_state_ids.get((event.type, event.state_key))
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+        prev_event_id = prev_state_ids.get((event.type, event.state_key))
         prev_event = yield self.store.get_event(prev_event_id, allow_none=True)
         if not prev_event:
             return
@@ -752,8 +753,8 @@ class EventCreationHandler(object):
         event = builder.build()
 
         logger.debug(
-            "Created event %s with state: %s",
-            event.event_id, context.prev_state_ids,
+            "Created event %s",
+            event.event_id,
         )
 
         defer.returnValue(
@@ -884,9 +885,11 @@ class EventCreationHandler(object):
                         e.sender == event.sender
                     )
 
+                current_state_ids = yield context.get_current_state_ids(self.store)
+
                 state_to_include_ids = [
                     e_id
-                    for k, e_id in iteritems(context.current_state_ids)
+                    for k, e_id in iteritems(current_state_ids)
                     if k[0] in self.hs.config.room_invite_state_types
                     or k == (EventTypes.Member, event.sender)
                 ]
@@ -922,8 +925,9 @@ class EventCreationHandler(object):
                     )
 
         if event.type == EventTypes.Redaction:
+            prev_state_ids = yield context.get_prev_state_ids(self.store)
             auth_events_ids = yield self.auth.compute_auth_events(
-                event, context.prev_state_ids, for_verification=True,
+                event, prev_state_ids, for_verification=True,
             )
             auth_events = yield self.store.get_events(auth_events_ids)
             auth_events = {
@@ -943,11 +947,13 @@ class EventCreationHandler(object):
                         "You don't have permission to redact events"
                     )
 
-        if event.type == EventTypes.Create and context.prev_state_ids:
-            raise AuthError(
-                403,
-                "Changing the room create event is forbidden",
-            )
+        if event.type == EventTypes.Create:
+            prev_state_ids = yield context.get_prev_state_ids(self.store)
+            if prev_state_ids:
+                raise AuthError(
+                    403,
+                    "Changing the room create event is forbidden",
+                )
 
         (event_stream_id, max_stream_id) = yield self.store.persist_event(
             event, context=context
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 00f2e279b..a832d9180 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -201,7 +201,9 @@ class RoomMemberHandler(object):
             ratelimit=ratelimit,
         )
 
-        prev_member_event_id = context.prev_state_ids.get(
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+
+        prev_member_event_id = prev_state_ids.get(
             (EventTypes.Member, target.to_string()),
             None
         )
@@ -496,9 +498,10 @@ class RoomMemberHandler(object):
         if prev_event is not None:
             return
 
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
         if event.membership == Membership.JOIN:
             if requester.is_guest:
-                guest_can_join = yield self._can_guest_join(context.prev_state_ids)
+                guest_can_join = yield self._can_guest_join(prev_state_ids)
                 if not guest_can_join:
                     # This should be an auth check, but guests are a local concept,
                     # so don't really fit into the general auth process.
@@ -517,7 +520,7 @@ class RoomMemberHandler(object):
             ratelimit=ratelimit,
         )
 
-        prev_member_event_id = context.prev_state_ids.get(
+        prev_member_event_id = prev_state_ids.get(
             (EventTypes.Member, event.state_key),
             None
         )
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index bb181d94e..1d14d3639 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -112,7 +112,8 @@ class BulkPushRuleEvaluator(object):
 
     @defer.inlineCallbacks
     def _get_power_levels_and_sender_level(self, event, context):
-        pl_event_id = context.prev_state_ids.get(POWER_KEY)
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+        pl_event_id = prev_state_ids.get(POWER_KEY)
         if pl_event_id:
             # fastpath: if there's a power level event, that's all we need, and
             # not having a power level event is an extreme edge case
@@ -120,7 +121,7 @@ class BulkPushRuleEvaluator(object):
             auth_events = {POWER_KEY: pl_event}
         else:
             auth_events_ids = yield self.auth.compute_auth_events(
-                event, context.prev_state_ids, for_verification=False,
+                event, prev_state_ids, for_verification=False,
             )
             auth_events = yield self.store.get_events(auth_events_ids)
             auth_events = {
@@ -304,7 +305,7 @@ class RulesForRoom(object):
 
                 push_rules_delta_state_cache_metric.inc_hits()
             else:
-                current_state_ids = context.current_state_ids
+                current_state_ids = yield context.get_current_state_ids(self.store)
                 push_rules_delta_state_cache_metric.inc_misses()
 
             push_rules_state_size_counter.inc(len(current_state_ids))
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 4ff0fdc4a..bf4f3ee92 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -549,7 +549,7 @@ class EventsStore(EventsWorkerStore):
             if ctx.state_group in state_groups_map:
                 continue
 
-            state_groups_map[ctx.state_group] = ctx.current_state_ids
+            state_groups_map[ctx.state_group] = yield ctx.get_current_state_ids(self)
 
         # We need to map the event_ids to their state groups. First, let's
         # check if the event is one we're persisting, in which case we can
diff --git a/synapse/storage/push_rule.py b/synapse/storage/push_rule.py
index be655d287..af564b1b4 100644
--- a/synapse/storage/push_rule.py
+++ b/synapse/storage/push_rule.py
@@ -186,6 +186,7 @@ class PushRulesWorkerStore(ApplicationServiceWorkerStore,
 
         defer.returnValue(results)
 
+    @defer.inlineCallbacks
     def bulk_get_push_rules_for_room(self, event, context):
         state_group = context.state_group
         if not state_group:
@@ -195,9 +196,11 @@ class PushRulesWorkerStore(ApplicationServiceWorkerStore,
             # To do this we set the state_group to a new object as object() != object()
             state_group = object()
 
-        return self._bulk_get_push_rules_for_room(
-            event.room_id, state_group, context.current_state_ids, event=event
+        current_state_ids = yield context.get_current_state_ids(self)
+        result = yield self._bulk_get_push_rules_for_room(
+            event.room_id, state_group, current_state_ids, event=event
         )
+        defer.returnValue(result)
 
     @cachedInlineCallbacks(num_args=2, cache_context=True)
     def _bulk_get_push_rules_for_room(self, room_id, state_group, current_state_ids,
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 02a802bed..a27702a7a 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -232,6 +232,7 @@ class RoomMemberWorkerStore(EventsWorkerStore):
 
         defer.returnValue(user_who_share_room)
 
+    @defer.inlineCallbacks
     def get_joined_users_from_context(self, event, context):
         state_group = context.state_group
         if not state_group:
@@ -241,11 +242,13 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             # To do this we set the state_group to a new object as object() != object()
             state_group = object()
 
-        return self._get_joined_users_from_context(
-            event.room_id, state_group, context.current_state_ids,
+        current_state_ids = yield context.get_current_state_ids(self)
+        result = yield self._get_joined_users_from_context(
+            event.room_id, state_group, current_state_ids,
             event=event,
             context=context,
         )
+        defer.returnValue(result)
 
     def get_joined_users_from_state(self, room_id, state_entry):
         state_group = state_entry.state_group

From 027bc01a1bc254fe08140c6e91a9fb945b08486f Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 23 Jul 2018 13:02:09 +0100
Subject: [PATCH 3/6] Add support for updating state

---
 synapse/events/snapshot.py     | 19 +++++++++++++++++++
 synapse/handlers/federation.py | 32 +++++++++++++++++++++++---------
 2 files changed, 42 insertions(+), 9 deletions(-)

diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index f9568638a..b090751bf 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -228,6 +228,25 @@ class EventContext(object):
         else:
             self._prev_state_ids = self._current_state_ids
 
+    @defer.inlineCallbacks
+    def update_state(self, state_group, prev_state_ids, current_state_ids,
+                     delta_ids):
+        """Replace the state in the context
+        """
+
+        # We need to make sure we wait for any ongoing fetching of state
+        # to complete so that the updated state doesn't get clobbered
+        if self._fetching_state_deferred:
+            yield make_deferred_yieldable(self._fetching_state_deferred)
+
+        self.state_group = state_group
+        self._prev_state_ids = prev_state_ids
+        self._current_state_ids = current_state_ids
+        self.delta_ids = delta_ids
+
+        # We need to ensure that that we've marked as having fetched the state
+        self._fetching_state_deferred = defer.succeed(None)
+
 
 def _encode_state_dict(state_dict):
     """Since dicts of (type, state_key) -> event_id cannot be serialized in
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 98dd4a7fd..14654d59f 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1975,21 +1975,35 @@ class FederationHandler(BaseHandler):
             k: a.event_id for k, a in iteritems(auth_events)
             if k != event_key
         }
-        context.current_state_ids = dict(context.current_state_ids)
-        context.current_state_ids.update(state_updates)
+        current_state_ids = yield context.get_current_state_ids(self.store)
+        current_state_ids = dict(current_state_ids)
+
+        current_state_ids.update(state_updates)
+
         if context.delta_ids is not None:
-            context.delta_ids = dict(context.delta_ids)
-            context.delta_ids.update(state_updates)
-        context.prev_state_ids = dict(context.prev_state_ids)
-        context.prev_state_ids.update({
+            delta_ids = dict(context.delta_ids)
+            delta_ids.update(state_updates)
+
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+        prev_state_ids = dict(prev_state_ids)
+
+        prev_state_ids.update({
             k: a.event_id for k, a in iteritems(auth_events)
         })
-        context.state_group = yield self.store.store_state_group(
+
+        state_group = yield self.store.store_state_group(
             event.event_id,
             event.room_id,
             prev_group=context.prev_group,
-            delta_ids=context.delta_ids,
-            current_state_ids=context.current_state_ids,
+            delta_ids=delta_ids,
+            current_state_ids=current_state_ids,
+        )
+
+        yield context.update_state(
+            state_group=state_group,
+            current_state_ids=current_state_ids,
+            prev_state_ids=prev_state_ids,
+            delta_ids=delta_ids,
         )
 
     @defer.inlineCallbacks

From f3182bb1d0ba496bef710a529c4cd7a99da72061 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 23 Jul 2018 13:19:24 +0100
Subject: [PATCH 4/6] Newsfile

---
 changelog.d/3579.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/3579.misc

diff --git a/changelog.d/3579.misc b/changelog.d/3579.misc
new file mode 100644
index 000000000..2374dc0c4
--- /dev/null
+++ b/changelog.d/3579.misc
@@ -0,0 +1 @@
+Lazily load state on master process when using workers to reduce DB consumption

From 8fbe418777a62ea6dd5fd811d63e30c42589650b Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 23 Jul 2018 13:33:49 +0100
Subject: [PATCH 5/6] Fix unit tests

---
 .../replication/slave/storage/test_events.py  |  8 ++--
 tests/test_state.py                           | 47 ++++++++++++++-----
 2 files changed, 40 insertions(+), 15 deletions(-)

diff --git a/tests/replication/slave/storage/test_events.py b/tests/replication/slave/storage/test_events.py
index cea01d93e..f5b47f5ec 100644
--- a/tests/replication/slave/storage/test_events.py
+++ b/tests/replication/slave/storage/test_events.py
@@ -222,9 +222,11 @@ class SlavedEventStoreTestCase(BaseSlavedStoreTestCase):
             state_ids = {
                 key: e.event_id for key, e in state.items()
             }
-            context = EventContext()
-            context.current_state_ids = state_ids
-            context.prev_state_ids = state_ids
+            context = EventContext.with_state(
+                state_group=None,
+                current_state_ids=state_ids,
+                prev_state_ids=state_ids
+            )
         else:
             state_handler = self.hs.get_state_handler()
             context = yield state_handler.compute_event_context(event)
diff --git a/tests/test_state.py b/tests/test_state.py
index c0f2d1152..429a18cbf 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -204,7 +204,8 @@ class StateTestCase(unittest.TestCase):
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
-        self.assertEqual(2, len(context_store["D"].prev_state_ids))
+        prev_state_ids = yield context_store["D"].get_prev_state_ids(self.store)
+        self.assertEqual(2, len(prev_state_ids))
 
     @defer.inlineCallbacks
     def test_branch_basic_conflict(self):
@@ -255,9 +256,11 @@ class StateTestCase(unittest.TestCase):
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
+        prev_state_ids = yield context_store["D"].get_prev_state_ids(self.store)
+
         self.assertSetEqual(
             {"START", "A", "C"},
-            {e_id for e_id in context_store["D"].prev_state_ids.values()}
+            {e_id for e_id in prev_state_ids.values()}
         )
 
     @defer.inlineCallbacks
@@ -318,9 +321,11 @@ class StateTestCase(unittest.TestCase):
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
+        prev_state_ids = yield context_store["E"].get_prev_state_ids(self.store)
+
         self.assertSetEqual(
             {"START", "A", "B", "C"},
-            {e for e in context_store["E"].prev_state_ids.values()}
+            {e for e in prev_state_ids.values()}
         )
 
     @defer.inlineCallbacks
@@ -398,9 +403,11 @@ class StateTestCase(unittest.TestCase):
             self.store.register_event_context(event, context)
             context_store[event.event_id] = context
 
+        prev_state_ids = yield context_store["D"].get_prev_state_ids(self.store)
+
         self.assertSetEqual(
             {"A1", "A2", "A3", "A5", "B"},
-            {e for e in context_store["D"].prev_state_ids.values()}
+            {e for e in prev_state_ids.values()}
         )
 
     def _add_depths(self, nodes, edges):
@@ -429,8 +436,10 @@ class StateTestCase(unittest.TestCase):
             event, old_state=old_state
         )
 
+        current_state_ids = yield context.get_current_state_ids(self.store)
+
         self.assertEqual(
-            set(e.event_id for e in old_state), set(context.current_state_ids.values())
+            set(e.event_id for e in old_state), set(current_state_ids.values())
         )
 
         self.assertIsNotNone(context.state_group)
@@ -449,8 +458,10 @@ class StateTestCase(unittest.TestCase):
             event, old_state=old_state
         )
 
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+
         self.assertEqual(
-            set(e.event_id for e in old_state), set(context.prev_state_ids.values())
+            set(e.event_id for e in old_state), set(prev_state_ids.values())
         )
 
     @defer.inlineCallbacks
@@ -475,9 +486,11 @@ class StateTestCase(unittest.TestCase):
 
         context = yield self.state.compute_event_context(event)
 
+        current_state_ids = yield context.get_current_state_ids(self.store)
+
         self.assertEqual(
             set([e.event_id for e in old_state]),
-            set(context.current_state_ids.values())
+            set(current_state_ids.values())
         )
 
         self.assertEqual(group_name, context.state_group)
@@ -504,9 +517,11 @@ class StateTestCase(unittest.TestCase):
 
         context = yield self.state.compute_event_context(event)
 
+        prev_state_ids = yield context.get_prev_state_ids(self.store)
+
         self.assertEqual(
             set([e.event_id for e in old_state]),
-            set(context.prev_state_ids.values())
+            set(prev_state_ids.values())
         )
 
         self.assertIsNotNone(context.state_group)
@@ -545,7 +560,9 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2,
         )
 
-        self.assertEqual(len(context.current_state_ids), 6)
+        current_state_ids = yield context.get_current_state_ids(self.store)
+
+        self.assertEqual(len(current_state_ids), 6)
 
         self.assertIsNotNone(context.state_group)
 
@@ -585,7 +602,9 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2,
         )
 
-        self.assertEqual(len(context.current_state_ids), 6)
+        current_state_ids = yield context.get_current_state_ids(self.store)
+
+        self.assertEqual(len(current_state_ids), 6)
 
         self.assertIsNotNone(context.state_group)
 
@@ -642,8 +661,10 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2,
         )
 
+        current_state_ids = yield context.get_current_state_ids(self.store)
+
         self.assertEqual(
-            old_state_2[3].event_id, context.current_state_ids[("test1", "1")]
+            old_state_2[3].event_id, current_state_ids[("test1", "1")]
         )
 
         # Reverse the depth to make sure we are actually using the depths
@@ -670,8 +691,10 @@ class StateTestCase(unittest.TestCase):
             event, prev_event_id1, old_state_1, prev_event_id2, old_state_2,
         )
 
+        current_state_ids = yield context.get_current_state_ids(self.store)
+
         self.assertEqual(
-            old_state_1[3].event_id, context.current_state_ids[("test1", "1")]
+            old_state_1[3].event_id, current_state_ids[("test1", "1")]
         )
 
     def _get_context(self, event, prev_event_id_1, old_state_1, prev_event_id_2,

From 4797ed000e612b68c418ce8c342bdd7ecc16b198 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 23 Jul 2018 15:05:40 +0100
Subject: [PATCH 6/6] Update docstrings to make sense

---
 synapse/events/snapshot.py | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index b090751bf..a6d7bf570 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -55,13 +55,16 @@ class EventContext(object):
         _fetching_state_deferred (Deferred|None): Resolves when *_state_ids have
             been calculated. None if we haven't started calculating yet
 
-        _prev_state_id (str|None): If set then the event associated with the
-            context overrode the _prev_state_id
-
-        _event_type (str): The type of the event the context is associated with
+        _event_type (str): The type of the event the context is associated with.
+            Only set when state has not been fetched yet.
 
         _event_state_key (str|None): The state_key of the event the context is
-            associated with
+            associated with. Only set when state has not been fetched yet.
+
+        _prev_state_id (str|None): If the event associated with the context is
+            a state event, then `_prev_state_id` is the event_id of the state
+            that was replaced.
+            Only set when state has not been fetched yet.
     """
 
     __slots__ = [