free(NULL) isn't explicitly forbidden, but isn't exactly good style.
Check the rare case that the audio buffer isn't initialized yet in
closeAudioDevice().  In this case, we also don't have to call
flushAudioBuffer().

To make openAudioDevice() smaller and more readable, move code to a
static function.  Also don't use realloc(), since the old value of the
buffer isn't needed anymore, saving a memcpy().

There are too many static variables in audio.c - organize all
properties of the audio buffer in a struct.  The current audio format
is also a property of the buffer, since it describes the buffer's
data format.

Passing NULL to this function doesn't make sense, and complicates its
implementation.  The one caller which might pass NULL should rather
check this.

The function isAudioDeviceOpen() is never used.

audio_format_clear() sets an audio_format struct to an cleared
(undefined) state, which is both faster and smaller than memset(0).
audio_format_defined() checks if the audio_format struct actually has
a defined value (i.e. non-zero).  Both can be used to avoid pointers
to audio_format, replacing the "NULL" value with an "undefined"
audio_format.

Remove one comparison by changing branch order.

Since the caller chain doesn't care about the return value (except for
COMMAND_RETURN_KILL, COMMAND_RETURN_CLOSE), just return 0 if there is
nothing special.  This saves one local variable initialization, and
one access to it.

Also remove one unreachable "return 1" from client_read().

Don't close the client within client_process_line(), return
COMMAND_RETURN_CLOSE instead.  This is the signal for the caller chain
to actually close it.  This makes dealing with the client pointer a
lot safer, since the caller always knows whether it is still valid.

It's easier to reuse the variable if it has a more generic name.

Don't update client data if it is going to be closed anyway.

Rename it to audio_format_equals() and return "true" if they are
equal.

The "!src" check in copyAudioFormat() used to hide bugs - one should
never pass NULL to it.  There is one caller which might pass NULL, add
a check in this caller.

Instead of doing mempcy(), we can simply assign the structures, which
looks more natural.

Getting rid of CamcelCase, again.

Similar to decoder_control.c, output_control.c will provide functions
for controlling the output thread (which will be implemented later).

No CamelCase.  Also don't declare typedefs for the methods.

This function is declared, but is neither used nor implemented.

The audio_format argument to timer_new() should be constant, because
it is not modified.  The same is true for ShoutData.audioFormat.

Otherwise we'd be writing to whatever directory that mpd is
running in.

The way we used non-blocking mode was HORRIBLE.

It was non-blocking to ALSA, but we end up blocking in a busy
loop that does absolutely NOTHING but retry.  We don't check
for playback cancellation (like we do in decoders) or anything.

This is seriously broken and I can imagine it affects people on
fast CPUs more because we do asynchronous output buffering and
our ALSA device will always have data ready.

Lets not use deprecated functions. It's apparently
possible to not care about the sw_params stuff at all!

This is safer than the patch in
  http://www.musicpd.org/mantis/view.php?id=1542
with multiple audio outputs enabled.

Sadly, I only noticed that patch/problem when I googled for
"snd_config_update_free_global"

We never use it for anything anyways as we release the device
entirely on pause.

That's the name of this project.

Apparently snd_pcm_hw_params_can_resume() can return false even
though my hardware does in fact support resuming.  So stop
carrying that value in the canResume flag and just try to resume
when we're in the suspended state; falling back to
snd_pcm_prepare only if resuming fails.  libao does something
similar on resume, too.

While we're at it, use the E() macro which will enable us to
have better error reporting.

[mk: remove the E() macro stuff]

Due to a minor typo, the string set had duplicate values, because
strset_add() didn't check the base slot properly.

With a large music database, the linear string collection in
tagTracker.c becomes very slow.  We implemented that in a
quick'n'dirty fashion when we removed tree.c, and now we rewrite it
using the fast hashed string set.

"struct strset" is a hashed string set: you can add strings to this
library, and it stores them as a set of unique strings.  You can get
the size of the set, and you can enumerate through all values.

This will be used to replace the linear tagTracker library.

Since the plugin struct is never modified, we should store it in
constant locations.

Instead of having to register each output plugin, store them
statically in an array.  This eliminates the need for the List library
here, and saves some small allocations during startup.

Instead of copying all that stuff from the audio output plugin to the
audio output structure, store a pointer to the plugin.

Also rename AudioOutputPlugin to struct audio_output_plugin, and use
forward declarations to reduce include dependencies.

Just like decoder_api.h, output_api.h provides the audio output API
which is used by the plugins.

Due to clumsy layout, the audio_format struct took 12 bytes.  Move the
"channels" to the end, so it can be merged into the same 32 bit slot
as "bits", which reduces the struct size to 8 bytes.

Get rid of CamelCase, and don't use a typedef, so we can
forward-declare it, and unclutter the include dependencies.

print_playlist_result() had an assert(0) at the end, in case there was
an invalid result value.  With NDEBUG, this resulted in a function not
returning a value - add a dummy "return -1" at the end to keep gcc
quiet.

Since all callers of song_id_exists() will map it to a song position
after the check, introduce a new function called song_id_to_position()
which performs both the check and the map lookup, including nice
assertions.

Don't pass a pointer to client->permission to processCommand(), better
let the code in command.c use the new permission getter/setter
functions.