Add FFmpegOpusAudio and other voice improvements

Rework FFmpeg player and add FFmpegOpusAudio

I have extracted some of the base FFmpeg source code into its own
base class and reimplemented the PCM and the new Opus variants.

Support avconv probing

Also fix a few things

Update `__all__`

Fix the bugs

Rework probe functions and add factory function

Probing involves subprocess so it has been reworked into an async
factory function.

Add docs + a few tweaks

* Removed unnecessary read() and is_opus() functions from FFmpegAudio
* Clear self._stdout in cleanup()
* Add 20 second process communication timeout to probe functions
* Capped probe function bitrate values at 512

Change AudioPlayer to use more accurate, monotonic time.perf_counter()

Add lazy opus loading

The library now no longer loads libopus on import, only on
opus.Encoder creation or manually.

Fix review nits

1 files changed, 21 insertions, 14 deletions

diff --git a/discord/opus.py b/discord/opus.py
index 9ad6b11c..f958cb3e 100644
--- a/discord/opus.py
+++ b/discord/opus.py
@@ -38,6 +38,8 @@ c_int_ptr = ctypes.POINTER(ctypes.c_int)
 c_int16_ptr = ctypes.POINTER(ctypes.c_int16)
 c_float_ptr = ctypes.POINTER(ctypes.c_float)
 
+_lib = None
+
 class EncoderStruct(ctypes.Structure):
     pass
 
@@ -100,25 +102,29 @@ def libopus_loader(name):
 
     return lib
 
-try:
-    if sys.platform == 'win32':
-        _basedir = os.path.dirname(os.path.abspath(__file__))
-        _bitness = 'x64' if sys.maxsize > 2**32 else 'x86'
-        _filename = os.path.join(_basedir, 'bin', 'libopus-0.{}.dll'.format(_bitness))
-        _lib = libopus_loader(_filename)
-    else:
-        _lib = libopus_loader(ctypes.util.find_library('opus'))
-except Exception:
-    _lib = None
+def _load_default():
+    global _lib
+    try:
+        if sys.platform == 'win32':
+            _basedir = os.path.dirname(os.path.abspath(__file__))
+            _bitness = 'x64' if sys.maxsize > 2**32 else 'x86'
+            _filename = os.path.join(_basedir, 'bin', 'libopus-0.{}.dll'.format(_bitness))
+            _lib = libopus_loader(_filename)
+        else:
+            _lib = libopus_loader(ctypes.util.find_library('opus'))
+    except Exception:
+        _lib = None
+
+    return _lib is not None
 
 def load_opus(name):
     """Loads the libopus shared library for use with voice.
 
     If this function is not called then the library uses the function
-    :func:`ctypes.util.find_library` and then loads that one
-    if available.
+    :func:`ctypes.util.find_library` and then loads that one if available.
 
-    Not loading a library leads to voice not working.
+    Not loading a library and attempting to use PCM based AudioSources will
+    lead to voice not working.
 
     This function propagates the exceptions thrown.
 
@@ -221,7 +227,8 @@ class Encoder:
         self.application = application
 
         if not is_loaded():
-            raise OpusNotLoaded()
+            if not _load_default():
+                raise OpusNotLoaded()
 
         self._state = self._create_state()
         self.set_bitrate(128)