gstpad: specialize gst_pad_chain_list_default

In the case where the bufferlist is writable, send the
buffers immediately without adding to the refcount. This
allows writable buffers to maintain their writability, even
without implementing a chain_list function on the element.

Adds a test to verify this property, where a writable list
maintains refcount 1, but a readonly list increases it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7894>

subprojects/gstreamer/gst/gstpad.c

@@ -195,6 +195,15 @@ static GstFlowReturn gst_pad_push_event_unchecked (GstPad * pad,
 static gboolean activate_mode_internal (GstPad * pad, GstObject * parent,
     GstPadMode mode, gboolean active);
 
+typedef struct
+{
+  GstPad *pad;
+  GstFlowReturn flow_res;
+} PadChainListData;
+
+static gboolean list_process_buffer_writable (GstBuffer ** buffer, guint idx,
+    gpointer userdata);
+
 static guint gst_pad_signals[LAST_SIGNAL] = { 0 };
 
 static GParamSpec *pspec_caps = NULL;
@@ -4656,6 +4665,16 @@ gst_pad_chain (GstPad * pad, GstBuffer * buffer)
       GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_PUSH, buffer);
 }
 
+static gboolean
+list_process_buffer_writable (GstBuffer ** buffer, guint idx, gpointer userdata)
+{
+  PadChainListData *data = (PadChainListData *) (userdata);
+  data->flow_res = gst_pad_chain_data_unchecked (data->pad,
+      GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_PUSH, *buffer);
+  *buffer = NULL;
+  return data->flow_res == GST_FLOW_OK;
+}
+
 static GstFlowReturn
 gst_pad_chain_list_default (GstPad * pad, GstObject * parent,
     GstBufferList * list)
@@ -4666,9 +4685,14 @@ gst_pad_chain_list_default (GstPad * pad, GstObject * parent,
 
   GST_LOG_OBJECT (pad, "chaining each buffer in list individually");
 
-  len = gst_buffer_list_length (list);
-
   ret = GST_FLOW_OK;
+
+  if (gst_buffer_list_is_writable (list)) {
+    PadChainListData data = {.pad = pad,.flow_res = GST_FLOW_OK };
+    gst_buffer_list_foreach (list, list_process_buffer_writable, &data);
+    ret = data.flow_res;
+  } else {
+    len = gst_buffer_list_length (list);
     for (i = 0; i < len; i++) {
       buffer = gst_buffer_list_get (list, i);
       ret =
@@ -4678,6 +4702,8 @@ gst_pad_chain_list_default (GstPad * pad, GstObject * parent,
       if (ret != GST_FLOW_OK)
         break;
     }
+  }
+
   gst_buffer_list_unref (list);
 
   return ret;

subprojects/gstreamer/tests/check/gst/gstpad.c

@@ -3390,6 +3390,84 @@ GST_START_TEST (test_pad_offset_src)
 
 GST_END_TEST;
 
+/* Pushing a writable list should not increase refcount. */
+static GstFlowReturn
+verify_buffer_writable (GstPad * pad, GstObject * parent, GstBuffer * buffer)
+{
+  fail_unless (gst_buffer_is_writable (buffer));
+  gst_buffer_unref (buffer);
+  return GST_FLOW_OK;
+}
+
+static GstFlowReturn
+verify_buffer_refcount_two (GstPad * pad, GstObject * parent,
+    GstBuffer * buffer)
+{
+  fail_unless_equals_int (GST_MINI_OBJECT_REFCOUNT (buffer), 2);
+  gst_buffer_unref (buffer);
+  return GST_FLOW_OK;
+}
+
+GST_START_TEST (test_pad_chain_list_writable)
+{
+  GstPad *src, *sink;
+  GstPadLinkReturn plr;
+  GstCaps *caps;
+
+  /* setup */
+  sink = gst_pad_new ("sink", GST_PAD_SINK);
+  fail_if (sink == NULL);
+  gst_pad_set_chain_function (sink, verify_buffer_writable);
+
+  src = gst_pad_new ("src", GST_PAD_SRC);
+  fail_if (src == NULL);
+
+  caps = gst_caps_from_string ("foo/bar");
+
+  gst_pad_set_active (src, TRUE);
+
+  gst_pad_push_event (src, gst_event_new_stream_start ("test"));
+
+  gst_pad_set_active (sink, TRUE);
+
+  gst_pad_set_caps (src, caps);
+  plr = gst_pad_link (src, sink);
+
+  fail_unless (gst_pad_push_event (src,
+          gst_event_new_segment (&dummy_segment)) == TRUE);
+
+  fail_unless (GST_PAD_LINK_SUCCESSFUL (plr));
+
+  /* Create a buffer list, that moves into gst_pad_push_list */
+  GstBufferList *list = gst_buffer_list_new_sized (1);
+
+  gst_buffer_list_add (list, gst_buffer_new ());
+  gst_buffer_list_add (list, gst_buffer_new ());
+
+  fail_unless (gst_buffer_list_is_writable (list));
+  /* this takes ownership of list, and will free it */
+  gst_pad_push_list (src, list);
+
+  gst_pad_set_chain_function (sink, verify_buffer_refcount_two);
+  list = gst_buffer_list_new_sized (1);
+
+  gst_buffer_list_add (list, gst_buffer_new ());
+
+  /* ref the list one more time, so it is no longer writable */
+  gst_buffer_list_ref (list);
+
+  fail_unless (!gst_buffer_list_is_writable (list));
+  gst_pad_push_list (src, list);
+
+  gst_buffer_list_unref (list);
+
+  gst_object_unref (sink);
+  gst_object_unref (src);
+  gst_caps_unref (caps);
+}
+
+GST_END_TEST;
+
 static Suite *
 gst_pad_suite (void)
 {
@@ -3451,6 +3529,7 @@ gst_pad_suite (void)
   tcase_add_test (tc_chain, test_proxy_accept_caps_with_proxy);
   tcase_add_test (tc_chain, test_proxy_accept_caps_with_incompatible_proxy);
   tcase_add_test (tc_chain, test_pad_offset_src);
+  tcase_add_test (tc_chain, test_pad_chain_list_writable);
 
   return s;
 }