From 73ef16864e79cf9c24891eff1f573e2d7bf18c94 Mon Sep 17 00:00:00 2001
From: Branko Subasic <branko@axis.com>
Date: Fri, 23 May 2025 09:41:51 +0200
Subject: [PATCH] matroskamux: Write stream headers before finishing file

In the rare case when we get EOS on all pads before we get any buffer we
would finish the file without writing headers, i.e. the file would be
corrupt. This patch makes sure that a header is always written.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9061>
---
 .../gst/matroska/matroska-mux.c               | 29 +++++++++++++++----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/subprojects/gst-plugins-good/gst/matroska/matroska-mux.c b/subprojects/gst-plugins-good/gst/matroska/matroska-mux.c
index 3ce759c874..a110b319b1 100644
--- a/subprojects/gst-plugins-good/gst/matroska/matroska-mux.c
+++ b/subprojects/gst-plugins-good/gst/matroska/matroska-mux.c
@@ -4439,12 +4439,25 @@ gst_matroska_mux_all_pads_have_codec_id (GstMatroskaMux * mux)
   return result;
 }
 
+static gboolean
+gst_matroska_mux_write_streams_header (GstMatroskaMux * mux)
+{
+  mux->state = GST_MATROSKA_MUX_STATE_HEADER;
+  gst_ebml_start_streamheader (mux->ebml_write);
+  if (!gst_matroska_mux_start_file (mux)) {
+    GST_ERROR_OBJECT (mux, "Failed to write headers");
+    return FALSE;
+  }
+  gst_matroska_mux_stop_streamheader (mux);
+
+  return TRUE;
+}
+
 static GstFlowReturn
 gst_matroska_mux_aggregate (GstAggregator * agg, gboolean timeout)
 {
   GstMatroskaMux *mux = GST_MATROSKA_MUX (agg);
   GstClockTime buffer_timestamp, end_ts = GST_CLOCK_TIME_NONE;
-  GstEbmlWrite *ebml = mux->ebml_write;
   GstMatroskaMuxPad *best = NULL;
   GstBuffer *buf;
   GstFlowReturn ret = GST_FLOW_OK;
@@ -4458,6 +4471,15 @@ gst_matroska_mux_aggregate (GstAggregator * agg, gboolean timeout)
   if (best == NULL) {
     if (gst_matroska_mux_all_pads_eos (mux)) {
       GST_DEBUG_OBJECT (mux, "All pads EOS. Finishing...");
+      /* write headers if not already done */
+      if (mux->state == GST_MATROSKA_MUX_STATE_START) {
+        GST_WARNING_OBJECT (mux, "All pads EOS before any buffers received. "
+            "Writing stream headers before finishing");
+        if (!gst_matroska_mux_write_streams_header (mux)) {
+          ret = GST_FLOW_ERROR;
+          goto exit;
+        }
+      }
       if (!mux->ebml_write->streamable) {
         gst_matroska_mux_finish (mux);
       } else {
@@ -4476,13 +4498,10 @@ gst_matroska_mux_aggregate (GstAggregator * agg, gboolean timeout)
       ret = GST_AGGREGATOR_FLOW_NEED_DATA;
       goto exit;
     }
-    mux->state = GST_MATROSKA_MUX_STATE_HEADER;
-    gst_ebml_start_streamheader (ebml);
-    if (!gst_matroska_mux_start_file (mux)) {
+    if (!gst_matroska_mux_write_streams_header (mux)) {
       ret = GST_FLOW_ERROR;
       goto exit;
     }
-    gst_matroska_mux_stop_streamheader (mux);
     mux->state = GST_MATROSKA_MUX_STATE_DATA;
   }