Start processing data from pipe

ffmpeg is being weird. Wave files start with a header consisting of the
text "RIFF", followed by 4 bytes for the length of the data chunk. But
because we're streaming to a pipe, that value isn't known yet, so it
just fills in 0xFFFFFFFF. libsndfile apparently doesn't like this and
quits with the message "Error in WAV file. No 'data' chunk marker." I
think I might need to drop libsndfile and manually process the data.

src/python/rpitx/_hidden.py

@@ -8,6 +8,9 @@ import os
 import subprocess
 import threading
 
+PIPE = 'pipe:1'
+DUMMY_FILE = 'dummy-file'
+
 
 def broadcast_fm(media_file_name, frequency):
     """Play a media file's audio over FM."""
@@ -29,36 +32,38 @@ def broadcast_fm(media_file_name, frequency):
         VideoCodec = libavwrapper.VideoCodec
         AudioCodec = libavwrapper.AudioCodec
         Stream = libavwrapper.AVConv
+        Stream.add_option = Stream.add_parameter
         command = 'avconv'
     else:
         raise NotImplementedError(
-            'Broadcasting audio requires either avconv or ffmpeg to be installed'
+            'Broadcasting audio requires either avconv or ffmpeg to be installed\n'
+            'sudo apt install libav-tools'
         )
 
-    logger.debug('Using {}'.format(command))
+    logger.debug('Using %s', command)
 
-    pipe_name = '/tmp/rpitx-fifo.wav'
-    if not os.path.exists(pipe_name):
-        os.mkfifo(pipe_name)
+    input_media = Input(media_file_name)
+    codec = AudioCodec('pcm_s16le').frequence(48000).channels(1)
+    output_audio = Output(DUMMY_FILE, codec)
+    stream = Stream(command, input_media, output_audio)
+    command_line = list(stream)
+    # The format needs to be specified manually because we're writing to
+    # stderr and not a named file. Normally, it would infer the format from
+    # the file name extension. Also, ffmpeg will only write to a pipe if it's
+    # the last named argument.
+    command_line.remove(DUMMY_FILE)
+    command_line += ('-f', 'wav', PIPE)
+    logger.debug('Running command "%s"', ' '.join(command_line))
+    stream_process = subprocess.Popen(command_line, stdout=subprocess.PIPE)
 
-    def convert():
-        """Runs the conversion command and writes to a FIFO."""
-        input_media = Input(media_file_name)
-        codec = AudioCodec('pcm_s16le').frequence(48000).channels(1)
-        output_audio = Output(pipe_name, codec)
-        stream = Stream(command, input_media, output_audio)
-        # Force overwriting existing file (it's a FIFO, that's what we want)
-        if hasattr(stream, 'add_option'):
-            stream.add_option('-y', '')
-        else:
-            stream.add_parameter('-y', '')
-        logger.debug(str(stream).replace("'", '').replace(',', ''))
+    def log_stdout():
+        """Log stdout from the stream process."""
         lines = stream.run()
         for line in lines:
             logger.debug(line)
 
-    thread = threading.Thread(target=convert)
-    thread.start()
+    #thread = threading.Thread(target=log_stdout).start()
+
     logger.debug('Calling broadcast_fm')
-    _rpitx.broadcast_fm(pipe_name, frequency)
+    _rpitx.broadcast_fm(stream_process.stdout.fileno(), frequency)
     thread.join()