sctpenc: Prohibit sending of interleaved message parts

Apparently we cannot start sending messages from another datachannel
before the previous message was completely sent. usrsctplib will
complain about being locked on another stream id and set
errno=EINVAL.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2454>

subprojects/gst-plugins-bad/ext/sctp/gstsctpenc.c

@@ -103,6 +103,7 @@ struct _GstSctpEncPad
   GMutex lock;
   GCond cond;
   gboolean flushing;
+  gboolean clear_to_send;
 };
 
 G_DEFINE_TYPE (GstSctpEncPad, gst_sctp_enc_pad, GST_TYPE_PAD);
@@ -132,6 +133,7 @@ gst_sctp_enc_pad_init (GstSctpEncPad * self)
   g_mutex_init (&self->lock);
   g_cond_init (&self->cond);
   self->flushing = FALSE;
+  self->clear_to_send = FALSE;
 }
 
 static void gst_sctp_enc_finalize (GObject * object);
@@ -563,6 +565,7 @@ gst_sctp_enc_sink_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
 {
   GstSctpEnc *self = GST_SCTP_ENC (parent);
   GstSctpEncPad *sctpenc_pad = GST_SCTP_ENC_PAD (pad);
+  GstSctpEncPad *sctpenc_pad_next = NULL;
   GstMapInfo map;
   guint32 ppid;
   gboolean ordered;
@@ -574,6 +577,7 @@ gst_sctp_enc_sink_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
   GstFlowReturn flow_ret = GST_FLOW_ERROR;
   const guint8 *data;
   guint32 length;
+  gboolean clear_to_send;
 
   GST_OBJECT_LOCK (self);
   if (self->src_ret != GST_FLOW_OK) {
@@ -629,7 +633,21 @@ gst_sctp_enc_sink_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
   data = map.data;
   length = map.size;
 
+  GST_OBJECT_LOCK (self);
+  clear_to_send = g_queue_is_empty (&self->pending_pads);
+  g_queue_push_tail (&self->pending_pads, sctpenc_pad);
+  GST_OBJECT_UNLOCK (self);
+
   g_mutex_lock (&sctpenc_pad->lock);
+
+  if (clear_to_send) {
+    sctpenc_pad->clear_to_send = TRUE;
+  }
+
+  while (!sctpenc_pad->flushing && !sctpenc_pad->clear_to_send) {
+    g_cond_wait (&sctpenc_pad->cond, &sctpenc_pad->lock);
+  }
+
   while (!sctpenc_pad->flushing) {
     guint32 bytes_sent;
 
@@ -658,15 +676,8 @@ gst_sctp_enc_sink_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
       length -= bytes_sent;
 
       /* The buffer was probably full. Retry in a while */
-      GST_OBJECT_LOCK (self);
-      g_queue_push_tail (&self->pending_pads, sctpenc_pad);
-      GST_OBJECT_UNLOCK (self);
-
       g_cond_wait_until (&sctpenc_pad->cond, &sctpenc_pad->lock, end_time);
 
-      GST_OBJECT_LOCK (self);
-      g_queue_remove (&self->pending_pads, sctpenc_pad);
-      GST_OBJECT_UNLOCK (self);
     } else if (bytes_sent == length) {
       GST_DEBUG_OBJECT (pad, "Successfully sent buffer");
       sctpenc_pad->bytes_sent += bytes_sent;
@@ -676,8 +687,21 @@ gst_sctp_enc_sink_chain (GstPad * pad, GstObject * parent, GstBuffer * buffer)
   flow_ret = sctpenc_pad->flushing ? GST_FLOW_FLUSHING : GST_FLOW_OK;
 
 out:
+  sctpenc_pad->clear_to_send = FALSE;
   g_mutex_unlock (&sctpenc_pad->lock);
 
+  GST_OBJECT_LOCK (self);
+  g_queue_remove (&self->pending_pads, sctpenc_pad);
+  sctpenc_pad_next = g_queue_peek_head (&self->pending_pads);
+  GST_OBJECT_UNLOCK (self);
+
+  if (sctpenc_pad_next) {
+    g_mutex_lock (&sctpenc_pad_next->lock);
+    sctpenc_pad_next->clear_to_send = TRUE;
+    g_cond_signal (&sctpenc_pad_next->cond);
+    g_mutex_unlock (&sctpenc_pad_next->lock);
+  }
+
   gst_buffer_unmap (buffer, &map);
 error:
   gst_buffer_unref (buffer);
@@ -890,7 +914,6 @@ on_sctp_packet_out (GstSctpAssociation * _association, const guint8 * buf,
   GstSctpEnc *self = user_data;
   GstBuffer *gstbuf;
   GstDataQueueItem *item;
-  GList *pending_pads, *l;
   GstSctpEncPad *sctpenc_pad;
 
   GST_DEBUG_OBJECT (self, "Received output packet of size %" G_GSIZE_FORMAT,
@@ -909,21 +932,22 @@ on_sctp_packet_out (GstSctpAssociation * _association, const guint8 * buf,
     GST_DEBUG_OBJECT (self, "Failed to push item because we're flushing");
   }
 
-  /* Wake up pads in the order they waited, oldest pad first */
+  /* Wake up the oldest pad which is the one that needs to finish first */
   GST_OBJECT_LOCK (self);
-  pending_pads = NULL;
+  sctpenc_pad = g_queue_peek_head (&self->pending_pads);
-  while ((sctpenc_pad = g_queue_pop_tail (&self->pending_pads))) {
+  if (sctpenc_pad) {
-    pending_pads = g_list_prepend (pending_pads, sctpenc_pad);
+    gst_object_ref (sctpenc_pad);
-  }
+
-  GST_OBJECT_UNLOCK (self);
+    GST_OBJECT_UNLOCK (self);
 
-  for (l = pending_pads; l; l = l->next) {
-    sctpenc_pad = l->data;
     g_mutex_lock (&sctpenc_pad->lock);
     g_cond_signal (&sctpenc_pad->cond);
     g_mutex_unlock (&sctpenc_pad->lock);
+
+    gst_object_unref (sctpenc_pad);
+  } else {
+    GST_OBJECT_UNLOCK (self);
   }
-  g_list_free (pending_pads);
 }
 
 static void