From cf3f19501220ab859910417520b530c26ea84389 Mon Sep 17 00:00:00 2001
From: Mark Nauwelaerts <mnauw@users.sourceforge.net>
Date: Mon, 11 Sep 2017 20:24:27 +0200
Subject: [PATCH] matroskademux: search_cluster should find preceding cluster
 before target

... since failing this constraint takes search_pos by surprise which might
then end up in an infinite loop.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=787538
---
 gst/matroska/matroska-demux.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/gst/matroska/matroska-demux.c b/gst/matroska/matroska-demux.c
index a8bac1b406..fe528a5508 100644
--- a/gst/matroska/matroska-demux.c
+++ b/gst/matroska/matroska-demux.c
@@ -1703,15 +1703,23 @@ gst_matroska_demux_search_cluster (GstMatroskaDemux * demux, gint64 * pos,
   while (1) {
     GstByteReader reader;
     gint cluster_pos;
+    guint toread = chunk;
 
-    if (!forward)
-      newpos = MAX (0, newpos - chunk);
+    if (!forward) {
+      /* never read beyond the requested target */
+      if (G_UNLIKELY (newpos < chunk)) {
+        toread = newpos;
+        newpos = 0;
+      } else {
+        newpos -= chunk;
+      }
+    }
     if (buf != NULL) {
       gst_buffer_unmap (buf, &map);
       gst_buffer_unref (buf);
       buf = NULL;
     }
-    ret = gst_pad_pull_range (demux->common.sinkpad, newpos, chunk, &buf);
+    ret = gst_pad_pull_range (demux->common.sinkpad, newpos, toread, &buf);
     if (ret != GST_FLOW_OK)
       break;
     GST_DEBUG_OBJECT (demux,