GLib's g_path_get_basename() is much more reliable than
mpd_basename().  The latter could be tricked into an assertion
failure.

Don't use db_get_directory() and traverse the full path with every
directory being loaded.  Just see if the current parent contains the
entry.  Everything else would be invalid anyway..

A manipulated database could trigger an assertion failure, because the
parent didn't match.  Do a proper check if the new directory is within
the parent's.  This uses FATAL() to bail out, so MPD still dies, but
it doesn't crash.

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.

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

CamelCase is ugly, rename the functions.

Yet another CamelCase removal patch.

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.

CamelCase is ugly...  rename all functions.

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.

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

Being consistent with most UNIX functions...

song objects cannot exist without a path or URL

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).

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.

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"

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.

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

Improving the signal to noise ratio...

If a write failed, it's a good sign subsequent writes will fail,
too, so propgate errors all the way up the stack.

A long time ago in an mpd far away...

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

It was a huge confusing mess of parameter passing around
and around.  Add a few extra assertions to ensure we're
handling parent/child relationships properly.

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

We no longer for for updates

"free" implies the songvec structure itself is freed,
which is not the case.

If we updated the mpd metadata database; then there's a chance
some of those songs in the playlist will have updated metadata.
So be on the safe side and increment the playlist version number
if _any_ song changed (this is how all released versions of mpd
did it, too).

This bug was introduced recently when making "update" threaded.

Thanks to stonecrest for the bug report.

Use freeList() instead of free() to free all elements in
the list.

open(2) should only interrupt on "slow" devices, afaik...

[mk: still using fopen()]

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.