Major stuff:
- Created a FileCache for managing user cache files in Spack. Currently just
handles virtuals.
- Moved virtual cache from the repository to the home directory so that users do
not need write access to Spack repositories to use them.
- Refactored `Transaction` class in `database.py` -- moved it to
`LockTransaction` in `lock.py` and made it reusable by other classes.
Other additions:
- Added tests for file cache and transactions.
- Added a few more tests for database
- Fixed bug in DB where writes could happen even if exceptions were raised
during a transaction.
- `spack uninstall` now attempts to repair the database when it discovers that a
prefix doesn't exist but a DB record does.
- Code simplification ignored case where exception was raised.
- If LockError was raised, read and write counts were incremented erroneously.
- updated lock test.
- llnl.util.lock now uses fcntl.lockf instead of flock
- purported to have more NFS compatibility.
- Added an extensive test case for locks.
- tests acquiring, releasing, upgrading, timeouts, shared, & exclusive cases.
1. Database stores a file version, so we can add to it in the future.
2. Database indexed by hashes and not numerical indexes.
3. Specs built by database have consistent hashes and it's checked.
4. minor naming and whitespace changes.
Most importantly wrote the Lock, Read_Lock_Instance, and Write_Lock_Instance classes in lock.py
Updated the locking in database.py
TODO: Lock on larger areas
