Test and fix find_first_stream_ordering_after_ts

It seemed to suffer from a bunch of off-by-one errors.

tests/storage/test_event_push_actions.py

@@ -127,3 +127,70 @@ class EventPushActionsStoreTestCase(tests.unittest.TestCase):
         yield _assert_counts(1, 1)
         yield _rotate(10)
         yield _assert_counts(1, 1)
+
+    @tests.unittest.DEBUG
+    @defer.inlineCallbacks
+    def test_find_first_stream_ordering_after_ts(self):
+        def add_event(so, ts):
+            return self.store._simple_insert("events", {
+                "stream_ordering": so,
+                "received_ts": ts,
+                "event_id": "event%i" % so,
+                "type": "",
+                "room_id": "",
+                "content": "",
+                "processed": True,
+                "outlier": False,
+                "topological_ordering": 0,
+                "depth": 0,
+            })
+
+        # start with the base case where there are no events in the table
+        r = yield self.store.find_first_stream_ordering_after_ts(11)
+        self.assertEqual(r, 0)
+
+        # now with one event
+        yield add_event(2, 10)
+        r = yield self.store.find_first_stream_ordering_after_ts(9)
+        self.assertEqual(r, 2)
+        r = yield self.store.find_first_stream_ordering_after_ts(10)
+        self.assertEqual(r, 2)
+        r = yield self.store.find_first_stream_ordering_after_ts(11)
+        self.assertEqual(r, 3)
+
+        # add a bunch of dummy events to the events table
+        for (stream_ordering, ts) in (
+                (3, 110),
+                (4, 120),
+                (5, 120),
+                (10, 130),
+                (20, 140),
+        ):
+            yield add_event(stream_ordering, ts)
+
+        r = yield self.store.find_first_stream_ordering_after_ts(110)
+        self.assertEqual(r, 3,
+                         "First event after 110ms should be 3, was %i" % r)
+
+        # 4 and 5 are both after 12: we want 4 rather than 5
+        r = yield self.store.find_first_stream_ordering_after_ts(120)
+        self.assertEqual(r, 4,
+                         "First event after 120ms should be 4, was %i" % r)
+
+        r = yield self.store.find_first_stream_ordering_after_ts(129)
+        self.assertEqual(r, 10,
+                         "First event after 129ms should be 10, was %i" % r)
+
+        # check we can get the last event
+        r = yield self.store.find_first_stream_ordering_after_ts(140)
+        self.assertEqual(r, 20,
+                         "First event after 14ms should be 20, was %i" % r)
+
+        # off the end
+        r = yield self.store.find_first_stream_ordering_after_ts(160)
+        self.assertEqual(r, 21)
+
+        # check we can find an event at ordering zero
+        yield add_event(0, 5)
+        r = yield self.store.find_first_stream_ordering_after_ts(1)
+        self.assertEqual(r, 0)