Fix perf when streams don't change often (#17767)

There is a bug with the `StreamChangeCache` where it would incorrectly
return that all entities had changed if asked for entities changed
*since* the earliest stream position.

Note that for streams we use the inequalities: `$min_stream_id <
stream_id <= $max_stream_id`, i.e. when we ask the stream change cache
for all things that have changed since `$stream_id` we don't care for
events that happened *at* `$stream_id`.

Specifically: `_earliest_known_stream_pos` is the position at which we
know that we'll have entries for all changes since that point, we can
use the cache for any stream IDs that equal
`_earliest_known_stream_pos`.

`_earliest_known_stream_pos` is set in three places:
- On startup we set it either to:
  - the current maximum stream ID, with not prefilled values; or
  - the minimum of the latest N values we pulled from the DB
- When we evict items from the bottom, we set it to the stream ID of the
evicted items.

This was changed in https://github.com/matrix-org/synapse/pull/14435,
but I think we were overly conservative there.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/17767.misc

@@ -0,0 +1 @@
+Fix performance of streams that don't change often.

synapse/util/caches/stream_change_cache.py

@@ -142,9 +142,9 @@ class StreamChangeCache:
         """
         assert isinstance(stream_pos, int)
 
-        # _cache is not valid at or before the earliest known stream position, so
+        # _cache is not valid before the earliest known stream position, so
         # return that the entity has changed.
-        if stream_pos <= self._earliest_known_stream_pos:
+        if stream_pos < self._earliest_known_stream_pos:
             self.metrics.inc_misses()
             return True
 
@@ -186,7 +186,7 @@ class StreamChangeCache:
             This will be all entities if the given stream position is at or earlier
             than the earliest known stream position.
         """
-        if not self._cache or stream_pos <= self._earliest_known_stream_pos:
+        if not self._cache or stream_pos < self._earliest_known_stream_pos:
             self.metrics.inc_misses()
             return set(entities)
 
@@ -238,9 +238,9 @@ class StreamChangeCache:
         """
         assert isinstance(stream_pos, int)
 
-        # _cache is not valid at or before the earliest known stream position, so
+        # _cache is not valid before the earliest known stream position, so
         # return that an entity has changed.
-        if stream_pos <= self._earliest_known_stream_pos:
+        if stream_pos < self._earliest_known_stream_pos:
             self.metrics.inc_misses()
             return True
 
@@ -270,9 +270,9 @@ class StreamChangeCache:
         """
         assert isinstance(stream_pos, int)
 
-        # _cache is not valid at or before the earliest known stream position, so
+        # _cache is not valid before the earliest known stream position, so
         # return None to mark that it is unknown if an entity has changed.
-        if stream_pos <= self._earliest_known_stream_pos:
+        if stream_pos < self._earliest_known_stream_pos:
             return AllEntitiesChangedResult(None)
 
         changed_entities: List[EntityType] = []

tests/util/test_stream_change_cache.py

@@ -53,8 +53,8 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
         # return True, whether it's a known entity or not.
         self.assertTrue(cache.has_entity_changed("user@foo.com", 0))
         self.assertTrue(cache.has_entity_changed("not@here.website", 0))
-        self.assertTrue(cache.has_entity_changed("user@foo.com", 3))
-        self.assertTrue(cache.has_entity_changed("not@here.website", 3))
+        self.assertTrue(cache.has_entity_changed("user@foo.com", 2))
+        self.assertTrue(cache.has_entity_changed("not@here.website", 2))
 
     def test_entity_has_changed_pops_off_start(self) -> None:
         """
@@ -76,9 +76,11 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
         self.assertTrue("user@foo.com" not in cache._entity_to_key)
 
         self.assertEqual(
-            cache.get_all_entities_changed(3).entities, ["user@elsewhere.org"]
+            cache.get_all_entities_changed(2).entities,
+            ["bar@baz.net", "user@elsewhere.org"],
         )
-        self.assertFalse(cache.get_all_entities_changed(2).hit)
+        self.assertFalse(cache.get_all_entities_changed(1).hit)
+        self.assertTrue(cache.get_all_entities_changed(2).hit)
 
         # If we update an existing entity, it keeps the two existing entities
         cache.entity_has_changed("bar@baz.net", 5)
@@ -89,7 +91,8 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
             cache.get_all_entities_changed(3).entities,
             ["user@elsewhere.org", "bar@baz.net"],
         )
-        self.assertFalse(cache.get_all_entities_changed(2).hit)
+        self.assertFalse(cache.get_all_entities_changed(1).hit)
+        self.assertTrue(cache.get_all_entities_changed(2).hit)
 
     def test_get_all_entities_changed(self) -> None:
         """
@@ -114,7 +117,8 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
         self.assertEqual(
             cache.get_all_entities_changed(3).entities, ["user@elsewhere.org"]
         )
-        self.assertFalse(cache.get_all_entities_changed(1).hit)
+        self.assertFalse(cache.get_all_entities_changed(0).hit)
+        self.assertTrue(cache.get_all_entities_changed(1).hit)
 
         # ... later, things gest more updates
         cache.entity_has_changed("user@foo.com", 5)
@@ -149,7 +153,7 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
         # With no entities, it returns True for the past, present, and False for
         # the future.
         self.assertTrue(cache.has_any_entity_changed(0))
-        self.assertTrue(cache.has_any_entity_changed(1))
+        self.assertFalse(cache.has_any_entity_changed(1))
         self.assertFalse(cache.has_any_entity_changed(2))
 
         # We add an entity