From ab7dd39028f27962c4dd2db1ecfca74bb2c79744 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 19 Feb 2021 02:44:33 -0600
Subject: [PATCH] Add endpoint to get an event at a given timestamp

---
 synapse/rest/client/v1/room.py                | 25 ++++++++++++
 .../storage/databases/main/events_worker.py   | 40 +++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 9a1df30c2999..b5384bd6c929 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -980,6 +980,30 @@ def register_txn_path(servlet, regex_string, http_server, with_get=False):
             servlet.__class__.__name__,
         )
 
+class TimestampLookupRestServlet(ClientV1RestServlet):
+    PATTERNS = client_path_patterns("/rooms/(?P<room_id>[^/]*)/timestamp_to_event$")
+
+    def __init__(self, hs):
+        super(TimestampLookupRestServlet, self).__init__(hs)
+        self.store = hs.get_datastore()
+
+    @defer.inlineCallbacks
+    def on_GET(self, request, room_id):
+        requester = yield self.auth.get_user_by_req(request)
+        yield self.auth.check_joined_room(room_id, requester..to_string())
+
+        timestamp = parse_integer(request, "ts")
+        thread_id = parse_integer(request, "thread_id", 0)
+
+        event_id = yield self.store.get_event_for_timestamp(
+            room_id, thread_id, timestamp,
+        )
+
+        .returnValue((200, {
+            "event_id": event_id,
+        }))
+
+
 
 def register_servlets(hs, http_server, is_worker=False):
     RoomStateEventRestServlet(hs).register(http_server)
@@ -994,6 +1018,7 @@ def register_servlets(hs, http_server, is_worker=False):
     RoomRedactEventRestServlet(hs).register(http_server)
     RoomTypingRestServlet(hs).register(http_server)
     RoomEventContextServlet(hs).register(http_server)
+    TimestampLookupRestServlet(hs).register(http_server)
 
     # Some servlets only get registered for the main process.
     if not is_worker:
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index c8850a4707f6..65b3670c4769 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -1444,3 +1444,43 @@ def _cleanup_old_transaction_ids_txn(txn):
             "_cleanup_old_transaction_ids",
             _cleanup_old_transaction_ids_txn,
         )
+
+    def get_event_for_timestamp(self, room_id, thread_id, timestamp):
+        sql_template = """
+            SELECT event_id, origin_server_ts FROM 
+            WHERE
+                origin_server_ts %s ?
+                AND room_id = ?
+                AND thread_id = ?
+            ORDER BY origin_server_ts
+            LIMIT 1;
+        """
+
+        def f(txn):
+            txn.execute(sql_template % ("<=",), (timestamp, room_id, thread_id))
+            row = txn.fetchone()
+            if row:
+                event_id_before, ts_before = row
+            else:
+                event_id_before, ts_before = None, None
+
+            txn.execute(sql_template % (">=",), (timestamp, room_id, thread_id))
+            row = txn.fetchone()
+            if row:
+                event_id_after, ts_after = row
+            else:
+                event_id_after, ts_after = None, None
+
+            if event_id_before and event_id_after:
+                # Return the closest
+                if (timestamp - ts_before) < (ts_after - timestamp):
+                    return event_id_before
+                else:
+                    return event_id_after
+
+            if event_id_before:
+                return event_id_before
+
+            return event_id_after
+
+        return self.runInteraction("get_event_for_timestamp", f)