flvmux: add 'enforce-increasing-timestamps' property

The hack enforcing strictly increasing timestamps was, according to the
code comments, because librtmp was confused with backwards timestamps.

rtmp2sink is not using librtmp as rtmpsink did, so this is no longer
required.
Also changing the timestamps is causing audio glitches when streaming to
Youtube.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5212>

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

@@ -7258,6 +7258,18 @@
                         "type": "gchararray",
                         "writable": true
                     },
+                    "enforce-increasing-timestamps": {
+                        "blurb": "If set to true, flvmux will modify buffers timestamps to ensure they are always strictly increasing, inside one stream and also between the audio and video streams",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "true",
+                        "mutable": "null",
+                        "readable": true,
+                        "type": "gboolean",
+                        "writable": true
+                    },
                     "metadatacreator": {
                         "blurb": "The value of metadatacreator in the meta packet.",
                         "conditionally-available": false,

subprojects/gst-plugins-good/gst/flv/gstflvmux.c

@@ -59,12 +59,14 @@ enum
   PROP_METADATACREATOR,
   PROP_ENCODER,
   PROP_SKIP_BACKWARDS_STREAMS,
+  PROP_ENFORCE_INCREASING_TIMESTAMPS,
 };
 
 #define DEFAULT_STREAMABLE FALSE
 #define MAX_INDEX_ENTRIES 128
 #define DEFAULT_METADATACREATOR "GStreamer {VERSION} FLV muxer"
 #define DEFAULT_SKIP_BACKWARDS_STREAMS FALSE
+#define DEFAULT_ENFORCE_INCREASING_TIMESTAMPS TRUE
 
 static GstStaticPadTemplate src_templ = GST_STATIC_PAD_TEMPLATE ("src",
     GST_PAD_SRC,
@@ -293,6 +295,23 @@ gst_flv_mux_class_init (GstFlvMuxClass * klass)
           DEFAULT_SKIP_BACKWARDS_STREAMS,
           G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
 
+ /**
+   * GstFlvMux:enforce-increasing-timestamps:
+   *
+   * If set to true, flvmux will modify buffers timestamps to ensure they are always
+   * strictly increasing, inside one stream and also between the audio and video streams.
+   *
+   * Since: 1.24
+   */
+  g_object_class_install_property (gobject_class,
+      PROP_ENFORCE_INCREASING_TIMESTAMPS,
+      g_param_spec_boolean ("enforce-increasing-timestamps",
+          "Enforce increasing timestamps",
+          "If set to true, flvmux will modify buffers timestamps to ensure they are always "
+          "strictly increasing, inside one stream and also between the audio and video streams",
+          DEFAULT_ENFORCE_INCREASING_TIMESTAMPS,
+          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
+
   gstaggregator_class->create_new_pad =
       GST_DEBUG_FUNCPTR (gst_flv_mux_create_new_pad);
   gstelement_class->release_pad = GST_DEBUG_FUNCPTR (gst_flv_mux_release_pad);
@@ -330,6 +349,7 @@ gst_flv_mux_init (GstFlvMux * mux)
   mux->streamable = DEFAULT_STREAMABLE;
   mux->metadatacreator = g_strdup (DEFAULT_METADATACREATOR);
   mux->encoder = g_strdup (DEFAULT_METADATACREATOR);
+  mux->enforce_increasing_timestamps = DEFAULT_ENFORCE_INCREASING_TIMESTAMPS;
 
   mux->new_metadata = FALSE;
 
@@ -1263,7 +1283,7 @@ gst_flv_mux_buffer_to_tag_internal (GstFlvMux * mux, GstBuffer * buffer,
    * it expects timestamps to go forward not only inside one stream, but
    * also between the audio & video streams.
    */
-  if (dts < mux->last_dts) {
+  if (dts < mux->last_dts && mux->enforce_increasing_timestamps) {
     GST_WARNING_OBJECT (pad, "Got backwards dts! (%" GST_TIME_FORMAT
         " < %" GST_TIME_FORMAT ")", GST_TIME_ARGS (dts * GST_MSECOND),
         GST_TIME_ARGS (mux->last_dts * GST_MSECOND));
@@ -2139,6 +2159,9 @@ gst_flv_mux_get_property (GObject * object,
     case PROP_SKIP_BACKWARDS_STREAMS:
       g_value_set_boolean (value, mux->skip_backwards_streams);
       break;
+    case PROP_ENFORCE_INCREASING_TIMESTAMPS:
+      g_value_set_boolean (value, mux->enforce_increasing_timestamps);
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -2182,6 +2205,9 @@ gst_flv_mux_set_property (GObject * object,
     case PROP_SKIP_BACKWARDS_STREAMS:
       mux->skip_backwards_streams = g_value_get_boolean (value);
       break;
+    case PROP_ENFORCE_INCREASING_TIMESTAMPS:
+      mux->enforce_increasing_timestamps = g_value_get_boolean (value);
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;

subprojects/gst-plugins-good/gst/flv/gstflvmux.h

@@ -96,6 +96,7 @@ struct _GstFlvMux {
   gchar *metadatacreator;
   gchar *encoder;
   gboolean skip_backwards_streams;
+  gboolean enforce_increasing_timestamps;
 
   GstTagList *tags;
   gboolean new_metadata;