"volume" was passed as an unsigned integer, which is correct.  It's
just that when it was multiplied with the sample value, the whole
operation was changed to unsigned, breaking the algorithm (and Qball's
ears).  Internally change "volume" to signed.

With commit 6dcd7fea (if I am not mistaken) the error returned when
you try to save to an existing playlist is wrong.  Instead of
MPD_ACK_ERROR_EXIST, MPD_ACK_ERROR_NO_EXIST is returned.  This is
obviously wrong and breaks gmpc.

Commit 0bfe7802 broke update for new files in the root directory,
because music_root->path was an empty string and not NULL.  There were
some NULL tests missing.  Change them to !isRootDirectory(path)
instead of path!=NULL.

Due to a merge error, reap_update_task() called cond_signal_async()
with a locked mutex.  That always fails.  Use cond_signal_sync()
instead.

ino_t and dev_t are declared in sys/types.h, not sys/stat.h.

For the root directory, let's set path to an empty string.  This saves
a few checks.

Also convert directory_get_path() to an inline function, which returns
a constant string.

CamelCase is ugly, rename the functions.

Yet another CamelCase removal patch.

The same can be achieved with directory_print(db_get_directory()).

Taming the directory.c monster, part II: move the database management
stuff to database.  directory.c should only contain code which works
on directory objects.

The function isRootDirectory() is tiny and can be converted to an
inline function.  Don't allow name==NULL.

The runtime check suggests that the author has somehow thought
song_get_url(NULL) might be valid.  It should not be.  Replace it with
an assertion.

Eliminate duplicated code.

Instead of returning 0 or -1, return true on success and false on
failure.  This seems more natural, and when the C library was
designed, there was no "bool" data type.

This function was never used on remote songs.  Replace the runtime
check with an assertion.

CamelCase is ugly...  rename all functions.

Provide separate constructors for creating a remote song, a local
song, and one for loading data from a song file.  This way, we can add
more assertions.

Check the old status before assigning.  This saves a temporary
variable.

"bool" should be used in C99 programs for boolean values.

When there are standardized headers, use these instead of the bloated
os_compat.h.

Include only headers which are really used.

Typedefs shouldn't be used, use the bare struct names instead.

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.