audio: Fix memory leak when converting GST sample tags. (Fixes mopi…

…dy#1827)

Embedded cover art, and other tags exposed to us as type `Gst.Sample`, were
causing memory leaks when converted to plain Python types using
`GstBuffer.extract_dup()`. `extract_dup()` expects the caller to free the
memory it allocates but older versions of the python3-gi don't seem to do
this. The workaround is to access samples using `GstMemory` methods instead.

This issue was present on Buster 10 systems (python-gi 3.30.4) but not on
Ubuntu 20.04 (python-gi 3.36.0).

Mopidy memory usage after scanning 2861 tracks:
Without fix: 163.2MB
With fix: 54.1MB

docs/changelog.rst

@@ -16,6 +16,9 @@ v3.1.0 (UNRELEASED)
   implemented by playback providers that want to use GStreamer's download
   buffering strategy for their URIs. (PR: :issue:`1888`)
 
+- Audio: Fix memory leak when converting GStreamer `sample` type tags. (Fixes:
+  :issue:`1827`, PR: :issue:`1929`)
+
 
 v3.0.2 (2020-04-02)
 ===================

mopidy/audio/tags.py

@@ -78,7 +78,16 @@ def _extract_sample_data(sample):
     buf = sample.get_buffer()
     if not buf:
         return None
-    return buf.extract_dup(0, buf.get_size())
+    # Fix for https://github.com/mopidy/mopidy/issues/1827
+    mem = buf.get_all_memory()
+    if not mem:
+        return None
+    success, info = mem.map(Gst.MapFlags.READ)
+    if not success:
+        return None
+    data = info.data
+    mem.unmap(info)
+    return data
 
 
 # TODO: split based on "stream" and "track" based conversion? i.e. handle data