Moved code from db_get_song().

Renamed directory_get_directory() to directory_lookup_directory().
Added API documentation.

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.

[mk: fixed whitespace errors; use delete_song() instead of
songvec_delete()]

Remember the modification time of each directory.  This is important
for archives (which are virtual directories right now), but may also
be useful for an automatic update mechanism.

Moved some of them to to directory_save.c, and others to database.c.

directory_is_root() is cheaper than
isRootDirectory(directory_get_path()).

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

directory_get_name() returns the base name of the directory.

The inline functions directory_is_empty() and directory_get_path()
don't modify the object - pass constant object pointers to them.

This way we avoid unnecessary heap allocations.

Some tiny utilities... wrappers like these may become helpful when we
introduce locking.

No idea why it was created in directory.h, but it should be in
dirvec.h.

Remove clutter from directory.c.  Everything which saves or loads
to/from the hard disk goes to directory_save.c, and code which sends
directory information to the client is moved into directory_print.c.

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.

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.

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

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

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

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.

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.

MPD has supported more audio formats than just MP3
for over five years...

We no longer fork for directory updates, so we
no longer have children to reap.

Small memory reduction compared to songvec since most users have
much fewer dirs than songs, but still nice to have.

SongList has been superseded by struct songvec.

A lot of the preparation was needed (and done in previous
months) in making update thread-safe, but here it is.

This was the first thing I made work inside a thread when I
started mpd-uclinux many years ago, and also the last thing I've
done in mainline mpd to work inside a thread, go figure.

Our linked-list implementation is wasteful and the
SongList isn't modified enough to benefit from being a linked
list.  So use a more compact array of song pointers which
saves ~200K on a library with ~9K songs (on x86-32).

It hasn't been used in many years

  commit 3a89afdd
  Author: Warren Dukes <warren.dukes@gmail.com>
  Date:   Sat Nov 20 20:28:32 2004 +0000

      remove --update-db option

      (SVN r2719)

This should save a few thousand ops.  Not worth it to malloc
for such a small (3-words on 32-bit ARM and x86) structures.
Signed-off-by: Eric Wong <normalperson@yhbt.net>

Don't pass the raw file descriptor around.  This migration patch is
rather large, because all of the sources have inter dependencies - we
have to change all of them at the same time.

Again, move error handling to command.c.

This patch continues the work of the previous patch: don't pass a file
descriptor at all to traverseAllIn().  Since this fd was only used to
report "directory not found" errors, we can easily move that check to
the caller.  This is a great relief, since it removes the dependency
on a client connection from a lot of enumeration functions.

Database traversal should be generic, and not bound to a client
connection.  This is the first step: no file descriptor for the
callback functions forEachSong() and forEachDir().  If a callback
needs the file descriptor, it has to be passed in the void*data
pointer somehow; some callbacks might need a new struct for passing
more than one parameter.  This might look a bit cumbersome right now,
but our goal is to have a clean API.

The usual bunch of const pointer conversions.