pad: allow probes to remove the data item whilst returning PROBE_OK

Use case: we want to block the source pad of a leaky queue and
drop the buffer that causes the block. If we return PROBE_DROP
then the buffer gets dropped, but we get called again. If we
return PROBE_OK we can't easily drop the buffer. If we just
replace the item into the GstPadProbeInfo structure with NULL,
GStreamer will push a NULL buffer to the next element when we
unblock the pad probe. This patch ensures it doesn't do that.

https://bugzilla.gnome.org/show_bug.cgi?id=734342

gst/gstpad.c

@@ -3270,6 +3270,7 @@ probe_hook_marshal (GHook * hook, ProbeMarshall * data)
   GstPadProbeType type, flags;
   GstPadProbeCallback callback;
   GstPadProbeReturn ret;
+  gpointer original_data;
 
   /* if we have called this callback, do nothing */
   if (PROBE_COOKIE (hook) == data->cookie) {
@@ -3283,6 +3284,7 @@ probe_hook_marshal (GHook * hook, ProbeMarshall * data)
 
   flags = hook->flags >> G_HOOK_FLAG_USER_SHIFT;
   type = info->type;
+  original_data = info->data;
 
   /* one of the data types for non-idle probes */
   if ((type & GST_PAD_PROBE_TYPE_IDLE) == 0
@@ -3321,6 +3323,12 @@ probe_hook_marshal (GHook * hook, ProbeMarshall * data)
 
   GST_OBJECT_LOCK (pad);
 
+  if (original_data != NULL && info->data == NULL) {
+    GST_DEBUG_OBJECT (pad, "data item in pad probe info was dropped");
+    info->type = GST_PAD_PROBE_TYPE_INVALID;
+    data->dropped = TRUE;
+  }
+
   switch (ret) {
     case GST_PAD_PROBE_REMOVE:
       /* remove the probe */
@@ -4265,7 +4273,8 @@ probe_stopped:
     GST_OBJECT_UNLOCK (pad);
     pad->ABI.abi.last_flowret =
         ret == GST_FLOW_CUSTOM_SUCCESS ? GST_FLOW_OK : ret;
-    gst_mini_object_unref (GST_MINI_OBJECT_CAST (data));
+    if (data != NULL)
+      gst_mini_object_unref (GST_MINI_OBJECT_CAST (data));
 
     switch (ret) {
       case GST_FLOW_CUSTOM_SUCCESS:

tests/check/gst/gstpad.c

@@ -1191,6 +1191,83 @@ GST_START_TEST (test_pad_probe_remove)
 
 GST_END_TEST;
 
+typedef struct
+{
+  gulong probe_id;
+  GstPad *probe_pad;
+  GThread *thread;
+} BlockReplaceProbeHelper;
+
+static gpointer
+unblock_probe_thread (gpointer user_data)
+{
+  BlockReplaceProbeHelper *helper = user_data;
+
+  GST_INFO_OBJECT (helper->probe_pad, "removing probe to unblock pad");
+  gst_pad_remove_probe (helper->probe_pad, helper->probe_id);
+  return NULL;
+}
+
+static GstPadProbeReturn
+block_and_replace_buffer_probe_cb (GstPad * pad, GstPadProbeInfo * info,
+    gpointer user_data)
+{
+  BlockReplaceProbeHelper *helper = user_data;
+
+  GST_INFO_OBJECT (pad, "about to block pad, replacing buffer");
+
+  /* we want to block, but also drop this buffer */
+  gst_buffer_unref (GST_BUFFER (info->data));
+  info->data = NULL;
+
+  helper->thread =
+      g_thread_new ("gst-pad-test-thread", unblock_probe_thread, helper);
+
+  return GST_PAD_PROBE_OK;
+}
+
+GST_START_TEST (test_pad_probe_block_and_drop_buffer)
+{
+  BlockReplaceProbeHelper helper;
+  GstFlowReturn flow;
+  GstPad *src, *sink;
+
+  src = gst_pad_new ("src", GST_PAD_SRC);
+  gst_pad_set_active (src, TRUE);
+  sink = gst_pad_new ("sink", GST_PAD_SINK);
+  gst_pad_set_chain_function (sink, gst_check_chain_func);
+  gst_pad_set_active (sink, TRUE);
+
+  fail_unless (gst_pad_push_event (src,
+          gst_event_new_stream_start ("test")) == TRUE);
+  fail_unless (gst_pad_push_event (src,
+          gst_event_new_segment (&dummy_segment)) == TRUE);
+
+  fail_unless_equals_int (gst_pad_link (src, sink), GST_PAD_LINK_OK);
+
+  helper.probe_id = gst_pad_add_probe (src,
+      GST_PAD_PROBE_TYPE_BLOCK | GST_PAD_PROBE_TYPE_BUFFER,
+      block_and_replace_buffer_probe_cb, &helper, NULL);
+  helper.probe_pad = src;
+
+  /* push a buffer so the events are propagated downstream */
+  flow = gst_pad_push (src, gst_buffer_new ());
+
+  g_thread_join (helper.thread);
+
+  fail_unless_equals_int (flow, GST_FLOW_OK);
+
+  /* no buffer should have made it through to the sink pad, and especially
+   * not a NULL pointer buffer */
+  fail_if (buffers && buffers->data == NULL);
+  fail_unless (buffers == NULL);
+
+  gst_object_unref (src);
+  gst_object_unref (sink);
+}
+
+GST_END_TEST;
+
 static GstPadProbeReturn
 probe_block_a (GstPad * pad, GstPadProbeInfo * info, gpointer user_data)
 {
@@ -2085,6 +2162,7 @@ gst_pad_suite (void)
   tcase_add_test (tc_chain, test_pad_blocking_with_probe_type_blocking);
   tcase_add_test (tc_chain, test_pad_probe_remove);
   tcase_add_test (tc_chain, test_pad_probe_block_add_remove);
+  tcase_add_test (tc_chain, test_pad_probe_block_and_drop_buffer);
   tcase_add_test (tc_chain, test_pad_probe_flush_events);
   tcase_add_test (tc_chain, test_queue_src_caps_notify_linked);
   tcase_add_test (tc_chain, test_queue_src_caps_notify_not_linked);