decodebin3: Make usage of static sink pad optional

There is no reason why we should mandate people to "at least" use the static
sink pad. This caused issues, like mandating that it should always have valid
content linked to it (problematic in case of upstream stream changes).

Instead we only use it if it's actually linked to, in which case it gets added
to the list of inputs.

This actually simplifies the code too.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7949>

subprojects/gst-plugins-base/gst/playback/gstdecodebin3.c

@@ -253,10 +253,14 @@ struct _GstDecodebin3
 
   /* input_lock protects the following variables */
   GMutex input_lock;
-  /* Main input (static sink pad) */
+  /* Main input connected to the static sink pad.
+   *
+   * Initially it is not in the list of `inputs`. It will only actually be used
+   * if something is linked to it.
+   */
   DecodebinInput *main_input;
-  /* Supplementary input (request sink pads) */
+  /* All active inputs (request sink pads, and sink pad if linked) */
-  GList *other_inputs;
+  GList *inputs;
   /* counter for input */
   guint32 input_counter;
   /* Current stream group_id (default : GST_GROUP_ID_INVALID) */
@@ -749,8 +753,7 @@ gst_decodebin3_reset (GstDecodebin3 * dbin)
   SELECTION_UNLOCK (dbin);
 
   /* Reset the inputs */
-  gst_decodebin_input_reset (dbin->main_input);
+  for (tmp = dbin->inputs; tmp; tmp = tmp->next) {
-  for (tmp = dbin->other_inputs; tmp; tmp = tmp->next) {
     gst_decodebin_input_reset (tmp->data);
   }
 
@@ -791,14 +794,9 @@ gst_decodebin3_dispose (GObject * object)
   g_mutex_unlock (&dbin->factories_lock);
 
   INPUT_LOCK (dbin);
-  if (dbin->main_input) {
+  g_list_free_full (dbin->inputs, (GDestroyNotify) gst_decodebin_input_free);
-    gst_decodebin_input_free (dbin->main_input);
+  dbin->inputs = NULL;
   dbin->main_input = NULL;
-  }
-
-  g_list_free_full (dbin->other_inputs,
-      (GDestroyNotify) gst_decodebin_input_free);
-  dbin->other_inputs = NULL;
   INPUT_UNLOCK (dbin);
 
   gst_clear_caps (&dbin->caps);
@@ -1238,16 +1236,11 @@ gst_decodebin_input_unblock_streams (DecodebinInput * input,
   if (unblock_other_inputs) {
     GList *tmp;
     /* If requrested, unblock inputs which are targetting the same collection */
-    if (dbin->main_input != input) {
+    for (tmp = dbin->inputs; tmp; tmp = tmp->next) {
-      if (dbin->main_input->collection == input->collection) {
-        GST_DEBUG_OBJECT (dbin, "Unblock main input");
-        gst_decodebin_input_unblock_streams (dbin->main_input, FALSE);
-      }
-    }
-    for (tmp = dbin->other_inputs; tmp; tmp = tmp->next) {
       DecodebinInput *other = tmp->data;
       if (other->collection == input->collection) {
-        GST_DEBUG_OBJECT (dbin, "Unblock other input");
+        GST_DEBUG_OBJECT (dbin, "Unblock other input %s:%s",
+            GST_DEBUG_PAD_NAME (input->ghost_sink));
         gst_decodebin_input_unblock_streams (other, FALSE);
       }
     }
@@ -1444,14 +1437,13 @@ static void
 parsebin_drained_cb (GstElement * parsebin, DecodebinInput * input)
 {
   GstDecodebin3 *dbin = input->dbin;
-  gboolean all_drained;
+  gboolean all_drained = TRUE;
   GList *tmp;
 
   GST_INFO_OBJECT (dbin, "input %p drained", input);
   input->drained = TRUE;
 
-  all_drained = dbin->main_input->drained;
+  for (tmp = dbin->inputs; tmp; tmp = tmp->next) {
-  for (tmp = dbin->other_inputs; tmp; tmp = tmp->next) {
     DecodebinInput *data = (DecodebinInput *) tmp->data;
 
     all_drained &= data->drained;
@@ -1565,11 +1557,18 @@ gst_decodebin3_input_pad_link (GstPad * pad, GstObject * parent, GstPad * peer)
 
   GST_DEBUG_OBJECT (dbin, "Upstream can do pull-based : %d", pull_mode);
 
+  INPUT_LOCK (dbin);
+  /* If this is the sinkpad and it's the first time it is used, add it to the
+   * list of inputs */
+  if (input->is_main && !g_list_find (dbin->inputs, input)) {
+    GST_DEBUG_OBJECT (dbin,
+        "Main input now used, adding to list of all inputs");
+    dbin->inputs = g_list_prepend (dbin->inputs, input);
+  }
   /* If upstream *can* do pull-based we always use a parsebin. If not, we will
    * delay that decision to a later stage (caps/stream/collection event
    * processing) to figure out if one is really needed or whether an identity
    * element will be enough */
-  INPUT_LOCK (dbin);
   if (pull_mode) {
     if (!gst_decodebin_input_ensure_parsebin (input))
       res = GST_PAD_LINK_REFUSED;
@@ -1626,16 +1625,14 @@ query_duration_drop_probe (GstPad * pad, GstPadProbeInfo * info,
 static void
 recalculate_group_id (GstDecodebin3 * dbin)
 {
-  guint32 common_group_id;
+  guint32 common_group_id = GST_GROUP_ID_INVALID;
   GList *iter;
 
   GST_DEBUG_OBJECT (dbin,
       "recalculating, current global group_id: %" G_GUINT32_FORMAT,
       dbin->current_group_id);
 
-  common_group_id = dbin->main_input->group_id;
+  for (iter = dbin->inputs; iter; iter = iter->next) {
-
-  for (iter = dbin->other_inputs; iter; iter = iter->next) {
     DecodebinInput *input = iter->data;
 
     if (input->group_id != common_group_id) {
@@ -1758,7 +1755,7 @@ gst_decodebin3_release_pad (GstElement * element, GstPad * pad)
   }
 
   if (!input->is_main) {
-    dbin->other_inputs = g_list_remove (dbin->other_inputs, input);
+    dbin->inputs = g_list_remove (dbin->inputs, input);
     gst_decodebin_input_free (input);
   } else
     gst_decodebin_input_reset (input);
@@ -2211,7 +2208,7 @@ gst_decodebin3_request_new_pad (GstElement * element, GstPadTemplate * temp,
   input = gst_decodebin_input_new (dbin, FALSE);
   if (input) {
     INPUT_LOCK (dbin);
-    dbin->other_inputs = g_list_append (dbin->other_inputs, input);
+    dbin->inputs = g_list_append (dbin->inputs, input);
     res = input->ghost_sink;
     INPUT_UNLOCK (dbin);
   }
@@ -2478,9 +2475,7 @@ get_merged_collection (GstDecodebin3 * dbin)
   guint i, nb_stream;
 
   /* First check if we need to do a merge or just return the only collection */
-  res = dbin->main_input->collection;
+  for (tmp = dbin->inputs; tmp; tmp = tmp->next) {
-
-  for (tmp = dbin->other_inputs; tmp; tmp = tmp->next) {
     DecodebinInput *input = (DecodebinInput *) tmp->data;
     GST_LOG_OBJECT (dbin, "Comparing res %p input->collection %p", res,
         input->collection);
@@ -2501,17 +2496,7 @@ get_merged_collection (GstDecodebin3 * dbin)
   /* We really need to create a new collection */
   /* FIXME : Some numbering scheme maybe ?? */
   res = gst_stream_collection_new ("decodebin3");
-  if (dbin->main_input->collection) {
+  for (tmp = dbin->inputs; tmp; tmp = tmp->next) {
-    nb_stream = gst_stream_collection_get_size (dbin->main_input->collection);
-    GST_DEBUG_OBJECT (dbin, "main input %p %d", dbin->main_input, nb_stream);
-    for (i = 0; i < nb_stream; i++) {
-      GstStream *stream =
-          gst_stream_collection_get_stream (dbin->main_input->collection, i);
-      unsorted_streams = g_list_append (unsorted_streams, stream);
-    }
-  }
-
-  for (tmp = dbin->other_inputs; tmp; tmp = tmp->next) {
     DecodebinInput *input = (DecodebinInput *) tmp->data;
     GST_DEBUG_OBJECT (dbin, "input %p , collection %p", input,
         input->collection);
@@ -2558,11 +2543,7 @@ find_message_parsebin (GstDecodebin3 * dbin, GstElement * child)
     GST_DEBUG_OBJECT (dbin, "parent %s",
         parent ? GST_ELEMENT_NAME (parent) : "<NONE>");
 
-    if (parent == dbin->main_input->parsebin) {
+    for (tmp = dbin->inputs; tmp; tmp = tmp->next) {
-      input = dbin->main_input;
-      break;
-    }
-    for (tmp = dbin->other_inputs; tmp; tmp = tmp->next) {
       DecodebinInput *cur = (DecodebinInput *) tmp->data;
       if (parent == cur->parsebin) {
         input = cur;