Replace decoder_control.notify with decoder_control.mutex and
decoder_control.cond.  Lock the mutex on all accesses to
decoder_control.command and decoder_control.state.

Don't abort the whole MPD process when the conversion fails.  This has
been a denial-of-service attack vector for years.

Database update was broken due to the dc.pipe!=NULL assertion.  This
assertion is only valid while MPD decodes a song, not during database
update.

dc.pipe must be non-NULL while the decoder thread is running.  Ensure
that with a load of assertions.

When a new song starts playing, send its tag (song->tag) to the music
pipe.  This allows output plugins to render tags for all songs, not
only those with embedded tags understood by the decoder plugin.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

Added music_pipe_allocate(), music_pipe_push() and
music_pipe_cancel().  Those functions allow the caller (decoder thread
in this case) to do its own chunk management.  The functions
music_pipe_flush() and music_pipe_tag() can now be removed.

To aid debugging, print the audio format of the decoder plugin in a
debug message, and print information about PCM conversion.

The parameter name "wait" overlaps with the POSIX wait() function.
Rename it.

Now that I've found this nice function in the GLib docs, we can
finally remove our custom sleep function.  Still all those callers of
g_usleep() have to be migrated one day to use events, instead of
regular polling.

The decoder_plugin struct is used by both the MPD core and the decoder
plugin implementations.  Move it to a shared header file, to minimize
header dependencies.

One of the previous patches made MPD consume 100% CPU in a busy wait:
when the music_pipe was full, it did not wait (with notify_wait()) for
free chunks, because a variable has a different meaning now.  Always
pass "true" as the "wait" parameter.

Removed yet another superfluous buffer layer: return the PCM buffer
from pcm_convert() instead of copying PCM data into the
caller-supplied buffer.

Copy PCM data to the music_pipe_write() buffer, and apply replay gain
/ normalization to it, instead of manipulating the source buffer.

Decoder plugins must not send partial frames.

In NDEBUG, the parameter "decoder" is not used.

When the decoder thread is waiting for free chunks in the music pipe,
don't ignore the STOP command.  Just return dc.command without further
checks.

Free memory allocated by libsamplerate when the output or the decoder
is closed.

The tag() method reads a tag from the stream.  This replaces the
meta_name and meta_title attributes.

This patch fixes a minor memory leak: when decoder_tag() attempted to
send a merged tag object (created by tag_add_stream_tags()), and was
interrupted by a decoder command, it did not free the temporary merged
tag object.

It is illegal to pass an empty audio buffer around.  pcm_resample()
sometimes seems to result in 0 samples, maybe related to
libsamplerate.  To work around that problem, add special checks after
both pcm_convert() invocations.  Removed the pcm_resample()==0 checks
from pcm_convert().

Refuse to play audio formats which are not supported by MPD.

Pushing buffers with a zero length into the MPD core causes failures;
don't let them pass beyond the decoder plugi API.

When a global audio format is configured (setting
"audio_output_format"), decoder_data() overwrote the "length"
parameter with the size of the output buffer (result of
pcm_convert_size()).  Declare a separate variable for the output
buffer length.

When the caller passes length==0, decoder_read() entered an endless
loop.  Check that condition before entering the "while" loop.

When a command is sent while the decoder waits for a free chunk in the
music pipe, it was not returned by decoder_data().

Prevent superfluous wakeups and a deadlock condition.

Seeking was somewhat broken in some decoder plugins because they sent
empty chunks, and never got a command.  Check the decoder command
before doing anything else in decoder_data().

When the decoder receives a SEEK during initialization, it should
ignore that for now.  The old code made most decoders abort.

Renamed functions and variables.

The assertion on dc.state in decoder_read() was too strict: when a
decoder tried to call decoder_read() from tag_dup(), the decoder state
was NONE.  Allow this special case.

Added a lot of assertions checking the state of the decoder plugin.

Renamed variables and functions.

Before passing the first chunk to the audio output device, send the
current song's tag.