GstEnginePipeline: Fix possible race condition in pipeline destructor

Wait for ongoing state changes to complete before setting pipeline to NULL.
This prevents race conditions with async state transitions that can cause crashes in GStreamer elements.

Fixes #1875

src/engine/gstenginepipeline.cpp

@@ -26,6 +26,7 @@
 #include <cstdint>
 #include <cstring>
 #include <cmath>
+#include <algorithm>
 
 #include <glib.h>
 #include <glib-object.h>
@@ -215,6 +216,23 @@ GstEnginePipeline::~GstEnginePipeline() {
 
   if (pipeline_) {
 
+    // Wait for any ongoing state changes for this pipeline to complete before setting to NULL.
+    // This prevents race conditions with async state transitions.
+    {
+      // Copy futures to local list to avoid holding mutex during waitForFinished()
+      QList<QFuture<GstStateChangeReturn>> futures_to_wait;
+      {
+        QMutexLocker locker(&mutex_pending_state_changes_);
+        futures_to_wait = pending_state_changes_;
+        pending_state_changes_.clear();
+      }
+
+      // Wait for all pending futures to complete
+      for (QFuture<GstStateChangeReturn> &future : futures_to_wait) {
+        future.waitForFinished();
+      }
+    }
+
     gst_element_set_state(pipeline_, GST_STATE_NULL);
 
     GstElement *audiobin = nullptr;
@@ -1869,6 +1887,12 @@ QFuture<GstStateChangeReturn> GstEnginePipeline::SetState(const GstState state)
   QFuture<GstStateChangeReturn> future = QtConcurrent::run(shared_state_threadpool(), &gst_element_set_state, pipeline_, state);
   watcher->setFuture(future);
 
+  // Track this future so destructor can wait for it
+  {
+    QMutexLocker locker(&mutex_pending_state_changes_);
+    pending_state_changes_.append(future);
+  }
+
   return future;
 
 }
@@ -1878,6 +1902,12 @@ void GstEnginePipeline::SetStateFinishedSlot(const GstState state, const GstStat
   last_set_state_in_progress_ = GST_STATE_VOID_PENDING;
   --set_state_in_progress_;
 
+  // Remove finished futures from tracking list to prevent unbounded growth
+  {
+    QMutexLocker locker(&mutex_pending_state_changes_);
+    pending_state_changes_.erase(std::remove_if(pending_state_changes_.begin(), pending_state_changes_.end(), [](const QFuture<GstStateChangeReturn> &f) { return f.isFinished(); }), pending_state_changes_.end());
+  }
+
   switch (state_change_return) {
     case GST_STATE_CHANGE_SUCCESS:
     case GST_STATE_CHANGE_ASYNC: