* feat(snapshots): support restoring sparse files
This commit implements basic support for restoring sparse files from
a snapshot. When specifying "--mode=sparse" in a snapshot restore
command, Kopia will make a best effort to make sure the underlying
filesystem allocates the minimum amount of blocks needed to persist
restored files. In other words, enabling this feature will "force"
all restored files to be sparse-blocks of zero bytes in the source
file should not be allocated.
* Address review comments
- Separate sparse option into its own bool flag
- Implement sparsefile packagewith copySparse method
- Truncate once before writing sparse file
- Check error from Truncate
- Add unit test for copySparse
- Invoke GetBlockSize once per file copy
- Remove support for Windows and explain why
- Add unit test for stat package
Co-authored-by: Dave Smith-Uchida <dave@kasten.io>
From https://github.com/google/gvisor/tree/master/tools/checklocks
This will perform static verification that we're using
`sync.Mutex`, `sync.RWMutex` and `atomic` correctly to guard access
to certain fields.
This was mostly just a matter of adding annotations to indicate which
fields are guarded by which mutex.
In a handful of places the code had to be refactored to allow static
analyzer to do its job better or to not be confused by some
constructs.
In one place this actually uncovered a bug where a function was not
releasing a lock properly in an error case.
The check is part of `make lint` but can also be invoked by
`make check-locks`.
