Fixes hide_playlist_targets not working after server restart
Currently, `hide_playlists_targets` works by skipping songs with
`in_playlist` value set to true in
[`Directory::Walk`](a57bcd0238/src/db/plugins/simple/Directory.cxx (L237)). But
`in_playlist` is not stored and only updated in
[`UpdateWalk::PurgeDanglingFromPlaylists`](a57bcd0238/src/db/update/Playlist.cxx (L139)),
which will only be executed while updating DB.
This causes the problem that playlist target songs are correctly
hidden after database update, but will remain visible after mpd server
restarted. This pr solves the problem by storing `in_playlist` value
of songs into the `SimpleDatabase` file.
Boost makes building a piece of software much more difficult than
necessary. It's a huge library, and just uncompressing it takes a
considerable amount of time. MPD only used a tiny fraction of it, yet
its header bloat made the MPD build very slow. Locating Boost was
difficult due to its arcane build system and its resistance to use
pkg-config; it's always a special case. MPD could never use features
of newer Boost versions because Linux distributions always shipped old
Boost versions. Boost made everything complicated and slow.
So, finally, after getting rid of GLib (commit ccdb94b06c), switching
to C++ and using Boost (commit 0801b3f495), we've finally get rid of
it 8 years later.
Unfortunately, I had to reimplement parts of it along the way
(e.g. IntrusiveList). Kind of NIH, but on the other hand, compiling
MPD has become much easier for users.
SonarLint reports the latter to be better:
std::scoped_lock basically provides the same feature as std::lock_guard,
but is more generic: It can lock several mutexes at the same time, with a
deadlock prevention mechanism (see {rule:cpp:S5524}). The equivalent code
to perform simultaneous locking with std::lock_guard is significantly more
complex. Therefore, it is simpler to use std::scoped_lock all the time,
even when locking only one mutex (there will be no performance impact).
Signed-off-by: Rosen Penev <rosenp@gmail.com>
