Again, a data type which can be forward-declared.

The struct can be forward-declared by other headers, which relaxes the
header dependencies.

exploreDirectory() duplicates some code in updateDirectory().  Merge
both functions, and use directory_is_empty() to determine whether
update or explore mode should be used.

directory_is_empty() is a tiny inline function which determine if a
directory has any child objects (sub directories or songs).

The source directory.c mixes several libraries: directory object
management, database management and database update, resulting in a
1000+ line monster.  Move the whole database update code to update.c.

Having all functions as static (non-inline) functions generates GCC
warnings, and duplicates binary code across several object files.
Most of dirvec's methods are too complex for becoming inline
functions.  Move them all to dirvec.c and publish the prototypes in
dirvec.h.

pthread_cond_wait() may wake up spuriously.  To prevent superfluous
state checks, loop until the "pending" flag becomes true.  Removed the
dangerous assertion.

Somehow I must have missed this in commit 22e40b61.

oops :x

This makes the update code thread-safe and doesn't penalize
the playlist code by complicating it with complicated and
error-prone locks (and the associated overhead, not everybody
has a thread-implementation as good as NPTL).

The update task blocks during the delete; but the update task is
a slow task anyways so we can block w/o people caring too much.

This was also our only freeSong call site, so remove that
function.

Note that deleting entire directories is not fully thread-safe,
yet; as their traversals are not yet locked.

Get rid of songvec_write so we can enforce proper locking

songvec_for_each() has locking, use it instead of manually iterating
over the songvec items.

Pass const songvec pointers to songvec_find() and songvec_for_each().

Being consistent with most UNIX functions...

Only one lock is used for all songvec traversals since
they're rarely changed.  Also, minimize lock time and
release it before calling iterator functions since they
may block (updateSongInfo => stat/open/seek/read).

This lock only protects songvecs (and all of them) during
traversals; not the individual song structures themselves.

This is so we can more consistently deal with locking
needed for thread-safety in iterator functions.

This make argument order more consistent for iterators.
Additionally, these now return ssize_t results for error
checking.

song objects cannot exist without a path or URL

* Add missing headers in Makefile.am
* remove mp4ff.dsp (Win32 crap)
* Add scripts, m4, bs, autogen.sh to allow for hotfixes by the
  SCM-challenged.  (downloading the source via git is NOT a
  lightweight operation for everybody).

This is needed for people that don't use any of the following:
JACK, ALSA, libmikmod, Shout.

These patterns have been sitting in my private .git/info/exclude
files since 2005 because we used to use SVN.

We already know if a song is a URL or not based on whether it
has parentDir defined or not.  Hopefully one day in the future
we can drop HTTP support from MPD entirely when an HTTP
filesystem comes along and we can access streams via open(2).

The "packed" attribute may have negative side effects on performance.
Remove the "packed" attribute, and increase the size of "song.url" to
a multiple of the machine word size.

Reduce the number of allocations we make, so there's less
pressure on the allocator and less overhead to keep track
of the allocations in.

It didn't save us any lines of code nor did it do anything
useful since we would overwrite everything anyways.

There's no reason to scan the playlist for a song we
just allocated.

This got broken when listHandlerFunc was removed.  Since we no
longer need it and it's confusing, remove processCommandInternal
and just use process_command.

Instead of allocating a new one, just reuse an existing
one if one is found when rereading the DB.  This is a small
makes the previous commit work on subdirectories
of the root music directory.

[1] "song: better handling of existing songs when rereading DB"

avoid some redundant clearing logic as well, since the tag is
getting freed.

commands should really not behave differently if they're issued
inside a command list or not; so stop having special handler
functions to deal with them.  "update" was the only command
that used this functionality and I changed that in the last
commit to serialize access.

Now the "update" command can be issued multiple times regardless
of whether the client is in list mode or not.

We serialize the update tasks to prevent updates from trampling
over each other and will spawn another update task
once the current one is finished updating and reaped.

Right now we cap the queue size to 32 which is probably enough (I
bet most people usually run update with no argument anyways);
but we can make it grow/shrink dynamically if needed.  There'll
still be a hard-coded limit to prevent DoS attacks, though.

It'll be easier to keep track of what code runs in what
task/thread this way.

Instead of relying on the shortname, just pass the song pointer
to prevent redundant lookups during deletes.

Add support for 24 bit PCM samples to all functions.  Note that
pcm_convertAudioFormat() converts 24 bit samples to 16 bit; to
preserve full quality, support for "real" 24 bit conversion should be
added.

Moved code into separate bit specific functions:
- pcm_volumeChange() -> pcm_volume_change_X()
- pcm_add() -> pcm_add_X()
- pcm_convertTo16bit() -> pcm_convert_8_to_16()

pcm_mix() might overflow the destination buffer if it is smaller than
the second buffer.  This is ok because the physical buffer size passed
by cross_fade_apply() is always big enough, but clutters pcm_mix()
with complicated length checks and contains a dangerous buffer
overflow pitfall.  Simplify pcm_mix()/pcm_add() and pass only the
smaller buffer size; let cross_fade_apply() do the memcpy().

pause() puts the audio output into pause mode: if supported, it may
perform a special action, which keeps the device open, but does not
play anything.  Output plugins like "shout" might want to play silence
during pause, so their clients won't be disconnected.  Plugins which
do not support pausing will simply be closed, and have to be reopened
when unpaused.

This pach includes an implementation for the shout plugin, which
sends silence chunks.

The function audio_output_is_pending() returns whether there is a
pending command.  This is useful for output plugins as a break
condition for longer loops.

The old struct initializers are error prone and don't allow moving
elements around.  Since we are going to overhaul some of the APIs
soon, it's easier to have all implementations use C99 initializers.

Why have a "_func" prefix on all method names?  Also don't typedef the
methods, there is no advantage in that.