`solana-test-validator` creates uncompressed snapshots and it can be
useful to attach another validator to a `solana-test-validator` cluster
from time to time
Convert to use type alias for the callback and cascade the changes to callers. Thanks @jeffwashington for the help making it possible.
Changed the closure for the progress update in the validator main to FnMut and modify the abort count in the closure which is more reliable.
1. Allow the validator bootstrap code to specify the minimal snapshot download speed. If the snapshot download speed is detected below that, a different RPC can be retried. The default is 10MB/sec.
2. To prevent spinning on a number of sub-optimal choices and not making progress, the abort/retry logic is implemented with the following safe guards:
2.1 at maximum we do this retry for 5 times -- this number is configurable with default 5.
2.2 if the download in one notification round (5 second) is more than 2%, do not do retry -- it is not as bad anyway.
2.3 if the remaining estimate time is less than 1 minutes, do not abort retry as it will be done quickly anyway.
2.4 We do this abort/retry logic only at the first notification to avoid wasting download efforts -- the reasoning is being opportunistic and too greedy may not achieve overall shorter download time.
3. The download_snapshot and download_file is modified with the option allowing caller to notified of download progress via a callback. This allows the business logic of retrying to the place it belongs.
* purge_old_snapshot_archives is changed to take an extra argument 'maximum_snapshots_to_retain' to control the max number of latest snapshot archives to retain. Note the oldest snapshot is always retained as before and is not subjected to this new options.
* The validator and ledger-tool executables are modified with a CLI argument --maximum-snapshots-to-retain. And the options are propagated down the call chains. Their corresponding shell scripts were changed accordingly.
* SnapshotConfig is modified to have an extra field for the maximum_snapshots_to_retain
* Unit tests are developed to cover purge_old_snapshot_archives