From d628ccf0e5193de683296f725818cd8d66e1bab5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Tue, 2 Nov 2021 11:21:34 +0200
Subject: [PATCH] webrtcbin: Don't require fixed caps when querying caps for a
 transceiver pad to match it with a media

Upstream caps might for example be
  application/x-rtp,media=audio,encoding-name={OPUS, X-GST-OPUS-DRAFT-SPITTKA-00, multiopus}
and while that is not fixed caps it is enough to match it with a media.

Only caps structures that have the correct structure name and that have
the media and encoding-name field are preserved, but if both are present
then these caps are used as "codec preferences".

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1291>
---
 .../gst-plugins-bad/ext/webrtc/gstwebrtcbin.c | 33 +++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c b/subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c
index 4dad46696b..f67533098b 100644
--- a/subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c
+++ b/subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c
@@ -1663,17 +1663,46 @@ _query_pad_caps (GstWebRTCBin * webrtc, GstWebRTCRTPTransceiver * rtp_trans,
     GstWebRTCBinPad * pad, GstCaps * filter, GError ** error)
 {
   GstCaps *caps;
+  guint i, n;
 
   caps = gst_pad_peer_query_caps (GST_PAD (pad), filter);
   GST_LOG_OBJECT (webrtc, "Using peer query caps: %" GST_PTR_FORMAT, caps);
 
+  /* Only return an error if actual empty caps were returned from the query. */
   if (gst_caps_is_empty (caps)) {
     g_set_error (error, GST_WEBRTC_BIN_ERROR,
         GST_WEBRTC_BIN_ERROR_CAPS_NEGOTIATION_FAILED,
         "Caps negotiation on pad %s failed", GST_PAD_NAME (pad));
     gst_clear_caps (&caps);
-  } else if (!gst_caps_is_fixed (caps) || gst_caps_is_equal (caps, filter)
-      || gst_caps_is_empty (caps) || gst_caps_is_any (caps)) {
+    gst_caps_unref (filter);
+    return NULL;
+  }
+
+  n = gst_caps_get_size (caps);
+  if (n > 0) {
+    /* Make sure the caps are complete enough to figure out the media type and
+     * encoding-name, otherwise they would match with basically any media. */
+    caps = gst_caps_make_writable (caps);
+    for (i = n; i > 0; i--) {
+      const GstStructure *s = gst_caps_get_structure (caps, i - 1);
+
+      if (!gst_structure_has_name (s, "application/x-rtp") ||
+          !gst_structure_has_field (s, "media") ||
+          !gst_structure_has_field (s, "encoding-name")) {
+        gst_caps_remove_structure (caps, i - 1);
+      }
+    }
+  }
+
+  /* If the filtering above resulted in empty caps, or the caps were ANY to
+   * begin with, then don't report and error but just NULL.
+   *
+   * This would be the case if negotiation would not fail but the peer does
+   * not have any specific enough preferred caps that would allow us to
+   * use them further.
+   */
+  if (gst_caps_is_any (caps) || gst_caps_is_empty (caps)) {
+    GST_DEBUG_OBJECT (webrtc, "Peer caps not specific enough");
     gst_clear_caps (&caps);
   }