Add a naive loopback implementation using select.poll()

It does work, though.

loopback.py

@@ -0,0 +1,105 @@
+#!/usr/bin/env python3
+# -*- mode: python; indent-tabs-mode: t; c-basic-offset: 4; tab-width: 4 -*-
+
+import sys
+import select
+import logging
+from alsaaudio import PCM, pcms, PCM_PLAYBACK, PCM_CAPTURE, PCM_FORMAT_S16_LE, PCM_NONBLOCK
+from argparse import ArgumentParser
+
+poll_names = {
+	select.POLLIN: 'POLLIN',
+	select.POLLPRI: 'POLLPRI',
+	select.POLLOUT: 'POLLOUT',
+	select.POLLERR: 'POLLERR',
+	select.POLLHUP: 'POLLHUP',
+	select.POLLRDHUP: 'POLLRDHUP',
+	select.POLLNVAL: 'POLLNVAL'
+}
+
+def poll_desc(mask):
+	return '|'.join([poll_names[bit] for bit, name in poll_names.items() if mask & bit])
+
+class Loopback(object):
+
+	def __init__(self, playback, capture):
+
+		self.playback = playback
+		self.playback_fd, self.playback_mask = playback.polldescriptors()[0]
+
+		self.capture = capture
+		self.capture_fd, self.capture_mask = capture.polldescriptors()[0]
+
+		self.poll = select.poll()
+
+		self.poll.register(self.playback_fd, self.playback_mask)
+		logging.debug(f'registered playback: {poll_desc(self.playback_mask)}')
+
+		self.poll.register(self.capture_fd, self.capture_mask)
+		logging.debug(f'registered capture: {poll_desc(self.capture_mask)}')
+
+	def fd_desc(self, fd):
+		if fd == self.capture_fd:
+			return 'capture'
+
+		if fd == self.playback_fd:
+			return 'playback'
+
+		return 'unknown'
+
+	def run(self):
+
+		# start reading
+		self.capture.read()
+
+		while True:
+			events = self.poll.poll()
+			for fd, ev in events:
+				logging.debug(f'{self.fd_desc(fd)}: {poll_desc(ev)} ({ev})')
+
+				# This is very basic. We just write data as soon as we read it
+				# and don't care for errors or the playback device not being ready
+				if fd == self.capture_fd:
+					size, data = self.capture.read()
+					written = self.playback.write(data)
+					logging.debug(f'wrote {size}: {written}')
+
+if __name__ == '__main__':
+
+	logging.basicConfig(format='%(asctime)s %(message)s', level=logging.INFO)
+
+	parser = ArgumentParser(description='ALSA loopback')
+
+	playback_pcms = pcms(pcmtype=PCM_PLAYBACK)
+	capture_pcms = pcms(pcmtype=PCM_CAPTURE)
+
+	if not playback_pcms:
+		log.error('no playback PCM found')
+		sys.exit(2)
+
+	if not capture_pcms:
+		log.error('no capture PCM found')
+		sys.exit(2)
+
+	parser.add_argument('-d', '--debug', action='store_true')
+	parser.add_argument('-i', '--input', default=capture_pcms[0])
+	parser.add_argument('-o', '--output', default=playback_pcms[0])
+	parser.add_argument('-r', '--rate', type=int, default=48000)
+	parser.add_argument('-c', '--channels', type=int, default=2)
+	parser.add_argument('-p', '--periodsize', type=int, default=480)
+	parser.add_argument('-P', '--periods', type=int, default=4)
+
+	args = parser.parse_args()
+
+	if args.debug:
+		logging.getLogger().setLevel(logging.DEBUG)
+
+	playback = PCM(type=PCM_PLAYBACK, mode=PCM_NONBLOCK, device=args.output, rate=args.rate,
+		channels=args.channels, periodsize=args.periodsize, periods=args.periods)
+
+	capture = PCM(type=PCM_CAPTURE, mode=PCM_NONBLOCK, device=args.input, rate=args.rate,
+		channels=args.channels, periodsize=args.periodsize, periods=args.periods)
+
+	loopback = Loopback(playback, capture)
+
+	loopback.run()