playbin3: Move gapless to uridecodebin3

This was the intention from the start, just took me a few years *cough* to
actually implement it properly.

Gapless is handled by re-using as much as possible the same decoders and sinks
if present, and only pre-rolling switching at the sources level (with buffering
if/when needed).

In order to enable "gapless" playback, the "next" uri should be set at any time
between the moment the `about-to-finish` signal is emitted and the moment the
current play item is done. Previously this could only be done with the signal
emission.

This new implementation also allows "Instantaneous URI switching". This allows a
much faster way of switching playback entries while re-using as many elements as
possible. To enable this set `instant-uri` property to TRUE, the default being
FALSE.

API: instant-uri properties
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2784>

subprojects/gst-plugins-base/docs/plugins/gst_plugins_cache.json

@@ -10364,6 +10364,18 @@
                         "type": "gboolean",
                         "writable": true
                     },
+                    "instant-uri": {
+                        "blurb": "When enabled, URI changes are applied immediately",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "false",
+                        "mutable": "null",
+                        "readable": true,
+                        "type": "gboolean",
+                        "writable": true
+                    },
                     "mute": {
                         "blurb": "Mute the audio channel without changing the volume",
                         "conditionally-available": false,
@@ -11415,6 +11427,18 @@
                         "type": "gboolean",
                         "writable": true
                     },
+                    "instant-uri": {
+                        "blurb": "When enabled, URI changes are applied immediately",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "false",
+                        "mutable": "null",
+                        "readable": true,
+                        "type": "gboolean",
+                        "writable": true
+                    },
                     "ring-buffer-max-size": {
                         "blurb": "Max. amount of data in the ring buffer (bytes, 0 = ring buffer disabled)",
                         "conditionally-available": false,

subprojects/gst-plugins-base/tools/gst-play.c

@@ -104,6 +104,7 @@ typedef struct
 
   gboolean buffering;
   gboolean is_live;
+  gboolean initial_file;
 
   GstState desired_state;       /* as per user interaction, PAUSED or PLAYING */
 
@@ -111,6 +112,7 @@ typedef struct
 
   /* configuration */
   gboolean gapless;
+  gboolean instant_uri;
 
   GstPlayTrickMode trick_mode;
   gdouble rate;
@@ -166,8 +168,9 @@ gst_play_printf (const gchar * format, ...)
 
 static GstPlay *
 play_new (gchar ** uris, const gchar * audio_sink, const gchar * video_sink,
-    gboolean gapless, gdouble initial_volume, gboolean verbose,
-    const gchar * flags_string, gboolean use_playbin3, gdouble start_position)
+    gboolean gapless, gboolean instant_uri, gdouble initial_volume,
+    gboolean verbose, const gchar * flags_string, gboolean use_playbin3,
+    gdouble start_position)
 {
   GstElement *sink, *playbin;
   GstPlay *play;
@@ -262,6 +265,11 @@ play_new (gchar ** uris, const gchar * audio_sink, const gchar * video_sink,
         G_CALLBACK (play_about_to_finish), play);
   }
 
+  play->initial_file = TRUE;
+  if (use_playbin3) {
+    play->instant_uri = instant_uri;
+    g_object_set (G_OBJECT (play->playbin), "instant-uri", instant_uri, NULL);
+  }
   if (initial_volume != -1)
     play_set_relative_volume (play, initial_volume - 1.0);
 
@@ -745,7 +753,8 @@ play_uri (GstPlay * play, const gchar * next_uri)
 {
   gchar *loc;
 
-  gst_element_set_state (play->playbin, GST_STATE_READY);
+  if (!play->instant_uri || play->initial_file)
+    gst_element_set_state (play->playbin, GST_STATE_READY);
   play_reset (play);
 
   loc = play_uri_get_display_name (play, next_uri);
@@ -754,23 +763,26 @@ play_uri (GstPlay * play, const gchar * next_uri)
 
   g_object_set (play->playbin, "uri", next_uri, NULL);
 
-  switch (gst_element_set_state (play->playbin, GST_STATE_PAUSED)) {
-    case GST_STATE_CHANGE_FAILURE:
-      /* ignore, we should get an error message posted on the bus */
-      break;
-    case GST_STATE_CHANGE_NO_PREROLL:
-      gst_print ("Pipeline is live.\n");
-      play->is_live = TRUE;
-      break;
-    case GST_STATE_CHANGE_ASYNC:
-      gst_print ("Prerolling...\r");
-      break;
-    default:
-      break;
-  }
+  if (!play->instant_uri || play->initial_file) {
+    switch (gst_element_set_state (play->playbin, GST_STATE_PAUSED)) {
+      case GST_STATE_CHANGE_FAILURE:
+        /* ignore, we should get an error message posted on the bus */
+        break;
+      case GST_STATE_CHANGE_NO_PREROLL:
+        gst_print ("Pipeline is live.\n");
+        play->is_live = TRUE;
+        break;
+      case GST_STATE_CHANGE_ASYNC:
+        gst_print ("Prerolling...\r");
+        break;
+      default:
+        break;
+    }
 
-  if (play->desired_state != GST_STATE_PAUSED)
-    gst_element_set_state (play->playbin, play->desired_state);
+    if (play->desired_state != GST_STATE_PAUSED)
+      gst_element_set_state (play->playbin, play->desired_state);
+  }
+  play->initial_file = FALSE;
 }
 
 /* returns FALSE if we have reached the end of the playlist */
@@ -1589,6 +1601,7 @@ main (int argc, char **argv)
   gboolean print_version = FALSE;
   gboolean interactive = TRUE;
   gboolean gapless = FALSE;
+  gboolean instant_uri = FALSE;
   gboolean shuffle = FALSE;
   gdouble volume = -1;
   gdouble start_position = 0;
@@ -1619,6 +1632,8 @@ main (int argc, char **argv)
         N_("Audio sink to use (default is autoaudiosink)"), NULL},
     {"gapless", 0, 0, G_OPTION_ARG_NONE, &gapless,
         N_("Enable gapless playback"), NULL},
+    {"instant-uri", 0, 0, G_OPTION_ARG_NONE, &instant_uri,
+        N_("Enable instantaneous uri changes (only with playbin3)"), NULL},
     {"shuffle", 0, 0, G_OPTION_ARG_NONE, &shuffle,
         N_("Shuffle playlist"), NULL},
     {"no-interactive", 0, G_OPTION_FLAG_REVERSE, G_OPTION_ARG_NONE,
@@ -1754,8 +1769,9 @@ main (int argc, char **argv)
     shuffle_uris (uris, num);
 
   /* prepare */
-  play = play_new (uris, audio_sink, video_sink, gapless, volume, verbose,
-      flags, use_playbin3, start_position);
+  play =
+      play_new (uris, audio_sink, video_sink, gapless, instant_uri, volume,
+      verbose, flags, use_playbin3, start_position);
 
   if (play == NULL) {
     gst_printerr