From f52565de50ee311ac154d138f8234950aeeed309 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 15 Jul 2016 14:23:15 +0100
Subject: [PATCH] Fix /purge_history bug

This was caused by trying to insert duplicate backward extremeties
---
 synapse/storage/events.py | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 59f806133..9d74fd159 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -1343,7 +1343,7 @@ class EventsStore(SQLBaseStore):
         # We calculate the new entries for the backward extremeties by finding
         # all events that point to events that are to be purged
         txn.execute(
-            "SELECT e.event_id FROM events as e"
+            "SELECT DISTINCT e.event_id FROM events as e"
             " INNER JOIN event_edges as ed ON e.event_id = ed.prev_event_id"
             " INNER JOIN events as e2 ON e2.event_id = ed.event_id"
             " WHERE e.room_id = ? AND e.topological_ordering < ?"
@@ -1352,6 +1352,20 @@ class EventsStore(SQLBaseStore):
         )
         new_backwards_extrems = txn.fetchall()
 
+        txn.execute(
+            "DELETE FROM event_backward_extremities WHERE room_id = ?",
+            (room_id,)
+        )
+
+        # Update backward extremeties
+        txn.executemany(
+            "INSERT INTO event_backward_extremities (room_id, event_id)"
+            " VALUES (?, ?)",
+            [
+                (room_id, event_id) for event_id, in new_backwards_extrems
+            ]
+        )
+
         # Get all state groups that are only referenced by events that are
         # to be deleted.
         txn.execute(
@@ -1404,30 +1418,12 @@ class EventsStore(SQLBaseStore):
             "event_search",
             "event_signatures",
             "rejections",
-            "event_backward_extremities",
         ):
             txn.executemany(
                 "DELETE FROM %s WHERE event_id = ?" % (table,),
                 to_delete
             )
 
-        txn.execute(
-            "SELECT event_id FROM event_backward_extremities WHERE room_id = ?",
-            (room_id,)
-        )
-
-        cur_back_event_ids = [event_id for event_id, in txn.fetchall()]
-
-        # Update backward extremeties
-        txn.executemany(
-            "INSERT INTO event_backward_extremities (room_id, event_id)"
-            " VALUES (?, ?)",
-            [
-                (room_id, event_id) for event_id, in new_backwards_extrems
-                if event_id not in cur_back_event_ids
-            ]
-        )
-
         txn.executemany(
             "DELETE FROM events WHERE event_id = ?",
             to_delete