Track and deduplicate in-flight requests to `_get_state_for_groups`. (#10870)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

1 files changed, 178 insertions, 25 deletions

diff --git a/synapse/storage/databases/state/store.py b/synapse/storage/databases/state/store.py
index 7614d76ac6..3af69a2076 100644
--- a/synapse/storage/databases/state/store.py
+++ b/synapse/storage/databases/state/store.py
@@ -13,11 +13,23 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Collection, Dict, Iterable, List, Optional, Set, Tuple
+from typing import (
+    TYPE_CHECKING,
+    Collection,
+    Dict,
+    Iterable,
+    Optional,
+    Sequence,
+    Set,
+    Tuple,
+)
 
 import attr
 
+from twisted.internet import defer
+
 from synapse.api.constants import EventTypes
+from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import (
     DatabasePool,
@@ -29,6 +41,12 @@ 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 MutableStateMap, StateKey, StateMap
+from synapse.util import unwrapFirstError
+from synapse.util.async_helpers import (
+    AbstractObservableDeferred,
+    ObservableDeferred,
+    yieldable_gather_results,
+)
 from synapse.util.caches.descriptors import cached
 from synapse.util.caches.dictionary_cache import DictionaryCache
 
@@ -37,7 +55,6 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
-
 MAX_STATE_DELTA_HOPS = 100
 
 
@@ -106,6 +123,12 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
             500000,
         )
 
+        # Current ongoing get_state_for_groups in-flight requests
+        # {group ID -> {StateFilter -> ObservableDeferred}}
+        self._state_group_inflight_requests: Dict[
+            int, Dict[StateFilter, AbstractObservableDeferred[StateMap[str]]]
+        ] = {}
+
         def get_max_state_group_txn(txn: Cursor) -> int:
             txn.execute("SELECT COALESCE(max(id), 0) FROM state_groups")
             return txn.fetchone()[0]  # type: ignore
@@ -157,7 +180,7 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
         )
 
     async def _get_state_groups_from_groups(
-        self, groups: List[int], state_filter: StateFilter
+        self, groups: Sequence[int], state_filter: StateFilter
     ) -> Dict[int, StateMap[str]]:
         """Returns the state groups for a given set of groups from the
         database, filtering on types of state events.
@@ -228,6 +251,150 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
 
         return state_filter.filter_state(state_dict_ids), not missing_types
 
+    def _get_state_for_group_gather_inflight_requests(
+        self, group: int, state_filter_left_over: StateFilter
+    ) -> Tuple[Sequence[AbstractObservableDeferred[StateMap[str]]], StateFilter]:
+        """
+        Attempts to gather in-flight requests and re-use them to retrieve state
+        for the given state group, filtered with the given state filter.
+
+        Used as part of _get_state_for_group_using_inflight_cache.
+
+        Returns:
+            Tuple of two values:
+                A sequence of ObservableDeferreds to observe
+                A StateFilter representing what else needs to be requested to fulfill the request
+        """
+
+        inflight_requests = self._state_group_inflight_requests.get(group)
+        if inflight_requests is None:
+            # no requests for this group, need to retrieve it all ourselves
+            return (), state_filter_left_over
+
+        # The list of ongoing requests which will help narrow the current request.
+        reusable_requests = []
+        for (request_state_filter, request_deferred) in inflight_requests.items():
+            new_state_filter_left_over = state_filter_left_over.approx_difference(
+                request_state_filter
+            )
+            if new_state_filter_left_over == state_filter_left_over:
+                # Reusing this request would not gain us anything, so don't bother.
+                continue
+
+            reusable_requests.append(request_deferred)
+            state_filter_left_over = new_state_filter_left_over
+            if state_filter_left_over == StateFilter.none():
+                # we have managed to collect enough of the in-flight requests
+                # to cover our StateFilter and give us the state we need.
+                break
+
+        return reusable_requests, state_filter_left_over
+
+    async def _get_state_for_group_fire_request(
+        self, group: int, state_filter: StateFilter
+    ) -> StateMap[str]:
+        """
+        Fires off a request to get the state at a state group,
+        potentially filtering by type and/or state key.
+
+        This request will be tracked in the in-flight request cache and automatically
+        removed when it is finished.
+
+        Used as part of _get_state_for_group_using_inflight_cache.
+
+        Args:
+            group: ID of the state group for which we want to get state
+            state_filter: the state filter used to fetch state from the database
+        """
+        cache_sequence_nm = self._state_group_cache.sequence
+        cache_sequence_m = self._state_group_members_cache.sequence
+
+        # Help the cache hit ratio by expanding the filter a bit
+        db_state_filter = state_filter.return_expanded()
+
+        async def _the_request() -> StateMap[str]:
+            group_to_state_dict = await self._get_state_groups_from_groups(
+                (group,), state_filter=db_state_filter
+            )
+
+            # Now let's update the caches
+            self._insert_into_cache(
+                group_to_state_dict,
+                db_state_filter,
+                cache_seq_num_members=cache_sequence_m,
+                cache_seq_num_non_members=cache_sequence_nm,
+            )
+
+            # Remove ourselves from the in-flight cache
+            group_request_dict = self._state_group_inflight_requests[group]
+            del group_request_dict[db_state_filter]
+            if not group_request_dict:
+                # If there are no more requests in-flight for this group,
+                # clean up the cache by removing the empty dictionary
+                del self._state_group_inflight_requests[group]
+
+            return group_to_state_dict[group]
+
+        # We don't immediately await the result, so must use run_in_background
+        # But we DO await the result before the current log context (request)
+        # finishes, so don't need to run it as a background process.
+        request_deferred = run_in_background(_the_request)
+        observable_deferred = ObservableDeferred(request_deferred, consumeErrors=True)
+
+        # Insert the ObservableDeferred into the cache
+        group_request_dict = self._state_group_inflight_requests.setdefault(group, {})
+        group_request_dict[db_state_filter] = observable_deferred
+
+        return await make_deferred_yieldable(observable_deferred.observe())
+
+    async def _get_state_for_group_using_inflight_cache(
+        self, group: int, state_filter: StateFilter
+    ) -> MutableStateMap[str]:
+        """
+        Gets the state at a state group, potentially filtering by type and/or
+        state key.
+
+        1. Calls _get_state_for_group_gather_inflight_requests to gather any
+           ongoing requests which might overlap with the current request.
+        2. Fires a new request, using _get_state_for_group_fire_request,
+           for any state which cannot be gathered from ongoing requests.
+
+        Args:
+            group: ID of the state group for which we want to get state
+            state_filter: the state filter used to fetch state from the database
+        Returns:
+            state map
+        """
+
+        # first, figure out whether we can re-use any in-flight requests
+        # (and if so, what would be left over)
+        (
+            reusable_requests,
+            state_filter_left_over,
+        ) = self._get_state_for_group_gather_inflight_requests(group, state_filter)
+
+        if state_filter_left_over != StateFilter.none():
+            # Fetch remaining state
+            remaining = await self._get_state_for_group_fire_request(
+                group, state_filter_left_over
+            )
+            assembled_state: MutableStateMap[str] = dict(remaining)
+        else:
+            assembled_state = {}
+
+        gathered = await make_deferred_yieldable(
+            defer.gatherResults(
+                (r.observe() for r in reusable_requests), consumeErrors=True
+            )
+        ).addErrback(unwrapFirstError)
+
+        # assemble our result.
+        for result_piece in gathered:
+            assembled_state.update(result_piece)
+
+        # Filter out any state that may be more than what we asked for.
+        return state_filter.filter_state(assembled_state)
+
     async def _get_state_for_groups(
         self, groups: Iterable[int], state_filter: Optional[StateFilter] = None
     ) -> Dict[int, MutableStateMap[str]]:
@@ -269,31 +436,17 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
         if not incomplete_groups:
             return state
 
-        cache_sequence_nm = self._state_group_cache.sequence
-        cache_sequence_m = self._state_group_members_cache.sequence
-
-        # Help the cache hit ratio by expanding the filter a bit
-        db_state_filter = state_filter.return_expanded()
-
-        group_to_state_dict = await self._get_state_groups_from_groups(
-            list(incomplete_groups), state_filter=db_state_filter
-        )
+        async def get_from_cache(group: int, state_filter: StateFilter) -> None:
+            state[group] = await self._get_state_for_group_using_inflight_cache(
+                group, state_filter
+            )
 
-        # Now lets update the caches
-        self._insert_into_cache(
-            group_to_state_dict,
-            db_state_filter,
-            cache_seq_num_members=cache_sequence_m,
-            cache_seq_num_non_members=cache_sequence_nm,
+        await yieldable_gather_results(
+            get_from_cache,
+            incomplete_groups,
+            state_filter,
         )
 
-        # And finally update the result dict, by filtering out any extra
-        # stuff we pulled out of the database.
-        for group, group_state_dict in group_to_state_dict.items():
-            # We just replace any existing entries, as we will have loaded
-            # everything we need from the database anyway.
-            state[group] = state_filter.filter_state(group_state_dict)
-
         return state
 
     def _get_state_for_groups_using_cache(