imagefreeze: Handle flushing correctly

First of all get rid of the atomic seeking boolean, which was only ever
set and never read. Replace it with a flushing boolean that is used in
the loop function to distinguish no buffer because of flushing and no
buffer because of an error as otherwise we could end up in a
GST_FLOW_ERROR case during flushing.

Also only reset the state of imagefreeze in flush-stop when all
processing is stopped instead of doing it as part of flush-start.

And last, get a reference to the imagefreeze buffer in the loop function
in the very beginning and work from that as otherwise it could in theory
be replaced or set to NULL in the meantime as we release and re-take the
mutex a couple of times during the loop function.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/580>

gst/imagefreeze/gstimagefreeze.c

@@ -190,14 +190,13 @@ gst_image_freeze_reset (GstImageFreeze * self)
 
   gst_segment_init (&self->segment, GST_FORMAT_TIME);
   self->need_segment = TRUE;
+  self->flushing = TRUE;
 
   self->negotiated_framerate = FALSE;
   self->fps_n = self->fps_d = 0;
   self->offset = 0;
   self->seqnum = 0;
   g_mutex_unlock (&self->lock);
-
-  g_atomic_int_set (&self->seeking, 0);
 }
 
 static gboolean
@@ -659,7 +658,10 @@ gst_image_freeze_src_event (GstPad * pad, GstObject * parent, GstEvent * event)
       if (flush) {
         GstEvent *e;
 
-        g_atomic_int_set (&self->seeking, 1);
+        g_mutex_lock (&self->lock);
+        self->flushing = TRUE;
+        g_mutex_unlock (&self->lock);
+
         e = gst_event_new_flush_start ();
         gst_event_set_seqnum (e, seqnum);
         gst_pad_push_event (self->srcpad, e);
@@ -677,6 +679,7 @@ gst_image_freeze_src_event (GstPad * pad, GstObject * parent, GstEvent * event)
       last_stop = self->segment.position;
 
       start_task = self->buffer != NULL;
+      self->flushing = FALSE;
       g_mutex_unlock (&self->lock);
 
       if (flush) {
@@ -685,7 +688,6 @@ gst_image_freeze_src_event (GstPad * pad, GstObject * parent, GstEvent * event)
         e = gst_event_new_flush_stop (TRUE);
         gst_event_set_seqnum (e, seqnum);
         gst_pad_push_event (self->srcpad, e);
-        g_atomic_int_set (&self->seeking, 0);
       }
 
       if (flags & GST_SEEK_FLAG_SEGMENT) {
@@ -715,8 +717,18 @@ gst_image_freeze_src_event (GstPad * pad, GstObject * parent, GstEvent * event)
       break;
     }
     case GST_EVENT_FLUSH_START:
+      g_mutex_lock (&self->lock);
+      self->flushing = TRUE;
+      g_mutex_unlock (&self->lock);
+      ret = gst_pad_push_event (self->sinkpad, event);
+      break;
+    case GST_EVENT_FLUSH_STOP:
       gst_image_freeze_reset (self);
-      /* fall through */
+      g_mutex_lock (&self->lock);
+      self->flushing = FALSE;
+      g_mutex_unlock (&self->lock);
+      ret = gst_pad_push_event (self->sinkpad, event);
+      break;
     default:
       ret = gst_pad_push_event (self->sinkpad, event);
       break;
@@ -815,18 +827,31 @@ gst_image_freeze_src_loop (GstPad * pad)
   gboolean first = FALSE;
 
   g_mutex_lock (&self->lock);
-  if (!self->buffer) {
+  if (self->flushing) {
+    GST_DEBUG_OBJECT (pad, "Flushing");
+    flow_ret = GST_FLOW_FLUSHING;
+    g_mutex_unlock (&self->lock);
+    goto pause_task;
+  } else if (!self->buffer) {
     GST_ERROR_OBJECT (pad, "Have no buffer yet");
     flow_ret = GST_FLOW_ERROR;
     g_mutex_unlock (&self->lock);
     goto pause_task;
   }
 
+  g_assert (self->buffer);
+
+  /* Take a new reference of the buffer here so we're guaranteed to have one
+   * in all the following code even if it disappears while we temporarily
+   * unlock the mutex */
+  buffer = gst_buffer_ref (self->buffer);
+
   if (gst_pad_check_reconfigure (self->srcpad)) {
     GstCaps *buffer_caps = gst_caps_ref (self->buffer_caps);
     g_mutex_unlock (&self->lock);
     if (!gst_image_freeze_sink_setcaps (self, buffer_caps)) {
       gst_caps_unref (buffer_caps);
+      gst_buffer_unref (buffer);
       gst_pad_mark_reconfigure (self->srcpad);
       flow_ret = GST_FLOW_NOT_NEGOTIATED;
       goto pause_task;
@@ -840,13 +865,14 @@ gst_image_freeze_src_loop (GstPad * pad)
     GST_DEBUG_OBJECT (pad, "Buffers left %d", self->num_buffers_left);
     if (self->num_buffers_left == 0) {
       flow_ret = GST_FLOW_EOS;
+      gst_buffer_unref (buffer);
       g_mutex_unlock (&self->lock);
       goto pause_task;
     } else {
       self->num_buffers_left--;
     }
   }
-  buffer = gst_buffer_copy (self->buffer);
+  buffer = gst_buffer_make_writable (buffer);
   g_mutex_unlock (&self->lock);
 
   if (self->need_segment) {
@@ -1001,6 +1027,9 @@ gst_image_freeze_change_state (GstElement * element, GstStateChange transition)
   switch (transition) {
     case GST_STATE_CHANGE_READY_TO_PAUSED:
       gst_image_freeze_reset (self);
+      g_mutex_lock (&self->lock);
+      self->flushing = FALSE;
+      g_mutex_unlock (&self->lock);
       break;
     case GST_STATE_CHANGE_PAUSED_TO_READY:
       gst_pad_stop_task (self->srcpad);

gst/imagefreeze/gstimagefreeze.h

@@ -65,9 +65,7 @@ struct _GstImageFreeze
 
   guint64 offset;
 
-  /* TRUE if currently doing a flushing seek, protected
-   * by srcpad's stream lock */
-  gint seeking;
+  gboolean flushing;
 };
 
 struct _GstImageFreezeClass