Compare to the earliest known stream pos in the stream change cache. (#14435)

The internal methods of the StreamChangeCache were inconsistently
treating the earliest known stream position as valid. It is now treated as
invalid, meaning the cache cannot determine if an entity at the earliest
known stream position has changed or not.	

1 files changed, 13 insertions, 25 deletions

diff --git a/tests/util/test_stream_change_cache.py b/tests/util/test_stream_change_cache.py
index 1b0fa52ad1..a29cc872f9 100644
--- a/tests/util/test_stream_change_cache.py
+++ b/tests/util/test_stream_change_cache.py
@@ -51,6 +51,8 @@ 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))
 
     def test_entity_has_changed_pops_off_start(self) -> None:
         """
@@ -65,15 +67,14 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
 
         # The cache is at the max size, 2
         self.assertEqual(len(cache._cache), 2)
+        # The cache's earliest known position is 2.
+        self.assertEqual(cache._earliest_known_stream_pos, 2)
 
         # The oldest item has been popped off
         self.assertTrue("user@foo.com" not in cache._entity_to_key)
 
-        self.assertEqual(
-            cache.get_all_entities_changed(2),
-            ["bar@baz.net", "user@elsewhere.org"],
-        )
-        self.assertIsNone(cache.get_all_entities_changed(1))
+        self.assertEqual(cache.get_all_entities_changed(3), ["user@elsewhere.org"])
+        self.assertIsNone(cache.get_all_entities_changed(2))
 
         # If we update an existing entity, it keeps the two existing entities
         cache.entity_has_changed("bar@baz.net", 5)
@@ -81,10 +82,10 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
             {"bar@baz.net", "user@elsewhere.org"}, set(cache._entity_to_key)
         )
         self.assertEqual(
-            cache.get_all_entities_changed(2),
+            cache.get_all_entities_changed(3),
             ["user@elsewhere.org", "bar@baz.net"],
         )
-        self.assertIsNone(cache.get_all_entities_changed(1))
+        self.assertIsNone(cache.get_all_entities_changed(2))
 
     def test_get_all_entities_changed(self) -> None:
         """
@@ -99,28 +100,15 @@ class StreamChangeCacheTests(unittest.HomeserverTestCase):
         cache.entity_has_changed("anotheruser@foo.com", 3)
         cache.entity_has_changed("user@elsewhere.org", 4)
 
-        r = cache.get_all_entities_changed(1)
+        r = cache.get_all_entities_changed(2)
 
-        # either of these are valid
-        ok1 = [
-            "user@foo.com",
-            "bar@baz.net",
-            "anotheruser@foo.com",
-            "user@elsewhere.org",
-        ]
-        ok2 = [
-            "user@foo.com",
-            "anotheruser@foo.com",
-            "bar@baz.net",
-            "user@elsewhere.org",
-        ]
+        # Results are ordered so either of these are valid.
+        ok1 = ["bar@baz.net", "anotheruser@foo.com", "user@elsewhere.org"]
+        ok2 = ["anotheruser@foo.com", "bar@baz.net", "user@elsewhere.org"]
         self.assertTrue(r == ok1 or r == ok2)
 
-        r = cache.get_all_entities_changed(2)
-        self.assertTrue(r == ok1[1:] or r == ok2[1:])
-
         self.assertEqual(cache.get_all_entities_changed(3), ["user@elsewhere.org"])
-        self.assertEqual(cache.get_all_entities_changed(0), None)
+        self.assertEqual(cache.get_all_entities_changed(1), None)
 
         # ... later, things gest more updates
         cache.entity_has_changed("user@foo.com", 5)