The shutil.move() function attempts to copy metadata (e.g., permissions and
mtime) when copying a file across filesystems. This always fails on Samba shares
because the utime() call is never permitted by normal users. We don't care about
preserving mtimes across moves, though, so this commit eschews shutil and
reimplements the move algorithm.
Previously, exceptions while communicating with the MusicBrainz API would bring
down the entire autotagging process. These errors are depressingly common, so we
now handle them and log errors to the console (much as we already do with any
exception raised by the Mutagen module). Fault isolation!
This has the added side-effect of giving better context for MB errors when they
do happen -- the logged errors now show the query that was running when MB
failed.
In an attempt to finally address the longstanding SQLite locking issues, I'm
introducing a way to explicitly, lexically scope transactions. The Transaction
class is a context manager that always fully fetches after SELECTs and
automatically commits on exit. No direct access to the library is allowed, so
all changes will eventually be committed and all queries will be completed. This
will also provide a debugging mechanism to show where concurrent transactions
are beginning and ending.
To support composition (transaction reentrancy), an internal, per-Library stack
of transactions is maintained. Commits only happen when the outermost
transaction exits. This means that, while it's possible to introduce atomicity
bugs by invoking Library methods outside of a transaction, you can conveniently
call them *without* a currently-active transaction to get a single atomic
action.
Note that this "transaction stack" concepts assumes a single Library object per
thread. Because we need to duplicate Library objects for concurrent access due
to sqlite3 limitation already, this is fine for now. Later, the interface should
provide one transaction stack per thread for shared Library objects.
Previously, all files would be moved/copied/deleted before the corresponding
Items and Albums were added to the database. Now, the in-place items are added
to the database; the files are moved; and then the new paths are saved to the
DB. The apply_choices coroutine now executes two database transactions per task.
This has a couple of benefits:
- %aunique{} requires album structures to be in place before the destination()
call, so this now works as expected.
- As an added bonus, the "in_album" parameter to move() and destination() --
along with its associated ugly hacks -- is no longer required.
