When a song file was not modified, MPD printed the debug message "not
a directory or music", because the first "if" branch did not return.

When the update queue is full, directory_update_init() did not free
the path argument.

Since the return value cannot be -1 anymore, we can make it unsigned.

The documentation for directory_update_init() was incorrect: a job ID
must be positive, not non-negative.  If the update queue is full and
no job was created, it makes more sense to return 0 instead of -1,
because it is more consistent with the return value of isUpdatingDB().

When the update task is idle, there is no need to check for deleted
songs.  Return early from reap_update_task().

pthread_join() expects a "pointer to a pointer" parameter, but it got
a "pointer to an enum".  On AMD64, an enum is smaller than a pointer,
leading to a buffer overflow.

Removed the local variable "was_empty": don't remember if the
directory is new.  Always call removeDeletedFromDirectory().

In updateInDirectory(), add new directories immediately and
delete them when they turn out to be empty.  This simplifies the code
and allows us to eliminate addSubDirectoryToDirectory().

The recursive checks were performed in several functions, and
sometimes a directory was checked twice.

When reading a new directory, copy the stat data (which we have
anyway) to the directory struct.  This may save a stat() in the
future.

Pass a pointer to the stat struct to more functions.

updatePath() duplicated a lot of code from the more generic
updateInDirectory().  Eliminate most of updatePath() and call
updateInDirectory().

If the user requests database update during startup, call
directory_update_init().  This should be changed to fully asynchronous
update later.

For this to work, main_notify has to be initialized before db_init().

The stat struct isn't going to be modified, make it const.

update_task() already checks if it has got a root path.  Extend this
check and in turn remove a check in the inner function updatePath().

Search'n'replace typo..

directory_update_init() has to be called with a path that is already
sanitized.  Don't call sanitizePathDup() again in updatePath().

The algorithm in addDirectoryPathToDB() can be simplified further if
it is combined with the function addParentPathToDB().  Since there is
no other caller of addDirectoryPathToDB(), we can do that.  This saves
another large stack buffer.

This recursive function is very dangerous because it allocates a large
buffer on the stack in every iteration.  That may be misused to
generate a stack overflow.

When a directory cannot be updated, there must be something wrong with
it, and the database contains stale data.  Remove it.

The branching looks a bit complicated in addDirectoryPathToDB() -
improve its readability by moving code to a simplified separate
function.

When the root directory fails to update, its contents are invalid.
Clear it then.

When a directory failed to update, it was removed from the database,
without freeing all children and songs (memory leak), and without
locking (race condition).  Introduce the functions clear_directory()
and delete_directory(), which do both.

Use updateInDirectory() instead of addToDirectory().  Eliminate a
duplicate stat() in updateInDirectory() by calling song_file_update()
directly.

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.

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

dirvec_find() does not modify the object, thus it should get a const
pointer.

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.

Use sizeof(buffer) instead.

Removed a superfluous closure.

Having an array with disabled entries sucks.  Removed that
DISABLED_SHOUT_ENCODER_PLUGIN macro, and fill the plugin list only
with plugins which are actually enabled.  This should be done for all
plugin types.

In the libsamplerate fallback code, a "const" attribute was missing.

"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.