From 4d10a8fb1807b3ab742e17a562bcae377057be50 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@element.io>
Date: Mon, 8 Apr 2024 14:55:19 +0100
Subject: [PATCH] Fixups to #17064 (#17065)

Forget a line, and an empty batch is trivially linear.

c.f. #17064
---
 changelog.d/17065.bugfix | 1 +
 synapse/handlers/sync.py | 3 +++
 2 files changed, 4 insertions(+)
 create mode 100644 changelog.d/17065.bugfix

diff --git a/changelog.d/17065.bugfix b/changelog.d/17065.bugfix
new file mode 100644
index 000000000..99ed435d7
--- /dev/null
+++ b/changelog.d/17065.bugfix
@@ -0,0 +1 @@
+Fix various long-standing bugs which could cause incorrect state to be returned from `/sync` in certain situations.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 7c29c1554..410805e80 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1272,8 +1272,11 @@ class SyncHandler:
             for e in batch.events[1:]:
                 if e.prev_event_ids() != [prev_event_id]:
                     break
+                prev_event_id = e.event_id
             else:
                 is_linear_timeline = True
+        else:
+            is_linear_timeline = True
 
         if is_linear_timeline and not batch.limited:
             state_ids: StateMap[str] = {}