From 5fd284cf3ac6c874d44abaa7983726fd4fb4e158 Mon Sep 17 00:00:00 2001
From: Alan Jeffrey <ajeffrey@mozilla.com>
Date: Thu, 5 Jan 2017 12:08:03 +0000
Subject: [PATCH] Index the session past correctly when discarding.

---
 components/constellation/constellation.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index f5a5983fa9b..76537234f09 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -2110,7 +2110,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         } else if let Some(frame) = self.frames.get_mut(&frame_change.frame_id) {
             debug!("Adding pipeline to existing frame.");
             frame.load(frame_change.new_pipeline_id, frame_change.url.clone());
-            let evicted_id = frame.prev.get_mut(opts::get().max_session_history)
+            let evicted_id = frame.prev.len().checked_sub(opts::get().max_session_history)
+                .and_then(|index| frame.prev.get_mut(index))
                 .and_then(|entry| entry.pipeline_id.take());
             (evicted_id, false, true, true)
         } else {