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`.
* sftp: fixed performance reggression due to connection management
The previous pooling was causing serialization of all requests, which
was too slow.
This change effectively reverts connection pooling but adds
automatic reconnection + unit tests.
* sftp: fixed unwanted retry on initial connection
