Merge pull request #139 from matrix-org/bugs/SYN-369

Fix race with cache invalidation. SYN-369

4 files changed, 80 insertions, 16 deletions

diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index e01c61d08d..a2da3dd1b1 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -31,7 +31,9 @@ import functools
 import simplejson as json
 import sys
 import time
+import threading
 
+DEBUG_CACHES = False
 
 logger = logging.getLogger(__name__)
 
@@ -68,9 +70,20 @@ class Cache(object):
 
         self.name = name
         self.keylen = keylen
-
+        self.sequence = 0
+        self.thread = None
         caches_by_name[name] = self.cache
 
+    def check_thread(self):
+        expected_thread = self.thread
+        if expected_thread is None:
+            self.thread = threading.current_thread()
+        else:
+            if expected_thread is not threading.current_thread():
+                raise ValueError(
+                    "Cache objects can only be accessed from the main thread"
+                )
+
     def get(self, *keyargs):
         if len(keyargs) != self.keylen:
             raise ValueError("Expected a key to have %d items", self.keylen)
@@ -82,6 +95,13 @@ class Cache(object):
         cache_counter.inc_misses(self.name)
         raise KeyError()
 
+    def update(self, sequence, *args):
+        self.check_thread()
+        if self.sequence == sequence:
+            # Only update the cache if the caches sequence number matches the
+            # number that the cache had before the SELECT was started (SYN-369)
+            self.prefill(*args)
+
     def prefill(self, *args):  # because I can't  *keyargs, value
         keyargs = args[:-1]
         value = args[-1]
@@ -96,9 +116,12 @@ class Cache(object):
         self.cache[keyargs] = value
 
     def invalidate(self, *keyargs):
+        self.check_thread()
         if len(keyargs) != self.keylen:
             raise ValueError("Expected a key to have %d items", self.keylen)
-
+        # Increment the sequence number so that any SELECT statements that
+        # raced with the INSERT don't update the cache (SYN-369)
+        self.sequence += 1
         self.cache.pop(keyargs, None)
 
 
@@ -128,11 +151,26 @@ def cached(max_entries=1000, num_args=1, lru=False):
         @defer.inlineCallbacks
         def wrapped(self, *keyargs):
             try:
-                defer.returnValue(cache.get(*keyargs))
+                cached_result = cache.get(*keyargs)
+                if DEBUG_CACHES:
+                    actual_result = yield orig(self, *keyargs)
+                    if actual_result != cached_result:
+                        logger.error(
+                            "Stale cache entry %s%r: cached: %r, actual %r",
+                            orig.__name__, keyargs,
+                            cached_result, actual_result,
+                        )
+                        raise ValueError("Stale cache entry")
+                defer.returnValue(cached_result)
             except KeyError:
+                # Get the sequence number of the cache before reading from the
+                # database so that we can tell if the cache is invalidated
+                # while the SELECT is executing (SYN-369)
+                sequence = cache.sequence
+
                 ret = yield orig(self, *keyargs)
 
-                cache.prefill(*keyargs + (ret,))
+                cache.update(sequence, *keyargs + (ret,))
 
                 defer.returnValue(ret)
 
@@ -147,12 +185,20 @@ class LoggingTransaction(object):
     """An object that almost-transparently proxies for the 'txn' object
     passed to the constructor. Adds logging and metrics to the .execute()
     method."""
-    __slots__ = ["txn", "name", "database_engine"]
+    __slots__ = ["txn", "name", "database_engine", "after_callbacks"]
 
-    def __init__(self, txn, name, database_engine):
+    def __init__(self, txn, name, database_engine, after_callbacks):
         object.__setattr__(self, "txn", txn)
         object.__setattr__(self, "name", name)
         object.__setattr__(self, "database_engine", database_engine)
+        object.__setattr__(self, "after_callbacks", after_callbacks)
+
+    def call_after(self, callback, *args):
+        """Call the given callback on the main twisted thread after the
+        transaction has finished. Used to invalidate the caches on the
+        correct thread.
+        """
+        self.after_callbacks.append((callback, args))
 
     def __getattr__(self, name):
         return getattr(self.txn, name)
@@ -294,6 +340,8 @@ class SQLBaseStore(object):
 
         start_time = time.time() * 1000
 
+        after_callbacks = []
+
         def inner_func(conn, *args, **kwargs):
             with LoggingContext("runInteraction") as context:
                 if self.database_engine.is_connection_closed(conn):
@@ -318,10 +366,10 @@ class SQLBaseStore(object):
                     while True:
                         try:
                             txn = conn.cursor()
-                            return func(
-                                LoggingTransaction(txn, name, self.database_engine),
-                                *args, **kwargs
+                            txn = LoggingTransaction(
+                                txn, name, self.database_engine, after_callbacks
                             )
+                            return func(txn, *args, **kwargs)
                         except self.database_engine.module.OperationalError as e:
                             # This can happen if the database disappears mid
                             # transaction.
@@ -370,6 +418,8 @@ class SQLBaseStore(object):
             result = yield self._db_pool.runWithConnection(
                 inner_func, *args, **kwargs
             )
+        for after_callback, after_args in after_callbacks:
+            after_callback(*after_args)
         defer.returnValue(result)
 
     def cursor_to_dict(self, cursor):
diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index 68f39bd684..893344eff3 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -330,7 +330,9 @@ class EventFederationStore(SQLBaseStore):
             )
             txn.execute(query)
 
-            self.get_latest_event_ids_in_room.invalidate(room_id)
+            txn.call_after(
+                self.get_latest_event_ids_in_room.invalidate, room_id
+            )
 
     def get_backfill_events(self, room_id, event_list, limit):
         """Get a list of Events for a given topic that occurred before (and
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index a3c260ddc4..17f9d27289 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -93,7 +93,7 @@ class EventsStore(SQLBaseStore):
                            current_state=None):
 
         # Remove the any existing cache entries for the event_id
-        self._invalidate_get_event_cache(event.event_id)
+        txn.call_after(self._invalidate_get_event_cache, event.event_id)
 
         if stream_ordering is None:
             with self._stream_id_gen.get_next_txn(txn) as stream_ordering:
@@ -114,6 +114,13 @@ class EventsStore(SQLBaseStore):
             )
 
             for s in current_state:
+                if s.type == EventTypes.Member:
+                    txn.call_after(
+                        self.get_rooms_for_user.invalidate, s.state_key
+                    )
+                    txn.call_after(
+                        self.get_joined_hosts_for_room.invalidate, s.room_id
+                    )
                 self._simple_insert_txn(
                     txn,
                     "current_state_events",
@@ -281,7 +288,9 @@ class EventsStore(SQLBaseStore):
         )
 
         if context.rejected:
-            self._store_rejections_txn(txn, event.event_id, context.rejected)
+            self._store_rejections_txn(
+                txn, event.event_id, context.rejected
+            )
 
         for hash_alg, hash_base64 in event.hashes.items():
             hash_bytes = decode_base64(hash_base64)
@@ -293,7 +302,8 @@ class EventsStore(SQLBaseStore):
             for alg, hash_base64 in prev_hashes.items():
                 hash_bytes = decode_base64(hash_base64)
                 self._store_prev_event_hash_txn(
-                    txn, event.event_id, prev_event_id, alg, hash_bytes
+                    txn, event.event_id, prev_event_id, alg,
+                    hash_bytes
                 )
 
         for auth_id, _ in event.auth_events:
@@ -356,9 +366,11 @@ class EventsStore(SQLBaseStore):
                     }
                 )
 
+        return
+
     def _store_redaction(self, txn, event):
         # invalidate the cache for the redacted event
-        self._invalidate_get_event_cache(event.redacts)
+        txn.call_after(self._invalidate_get_event_cache, event.redacts)
         txn.execute(
             "INSERT INTO redactions (event_id, redacts) VALUES (?,?)",
             (event.event_id, event.redacts)
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 09fb77a194..839c74f63a 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -64,8 +64,8 @@ class RoomMemberStore(SQLBaseStore):
             }
         )
 
-        self.get_rooms_for_user.invalidate(target_user_id)
-        self.get_joined_hosts_for_room.invalidate(event.room_id)
+        txn.call_after(self.get_rooms_for_user.invalidate, target_user_id)
+        txn.call_after(self.get_joined_hosts_for_room.invalidate, event.room_id)
 
     def get_room_member(self, user_id, room_id):
         """Retrieve the current state of a room member.