feat(mine): Add an internal Zcash miner to Zebra

[teor2345]

Motivation

It would be easier to test Zebra if we could mine blocks without external mining software.

This PR depends on changes to equihash:
zcash/librustzcash#1088
zcash/librustzcash#1083

This is an experimental "free sprint" project.

PR Author Checklist

Check before marking the PR as ready for review:

• Will the PR name make sense to users?
• Does the PR have a priority label?
• Have you added or updated tests?
• Is the documentation up to date?
  • List of features in zebrad/src/lib.rs
  • List of tasks in zebrad/src/commands/start.rs

For significant changes:

• Is there a summary in the CHANGELOG?
• Can these changes be split into multiple PRs?

If a checkbox isn't relevant to the PR, mark it as done.

Specifications

This code obeys the consensus rules by generating valid blocks, but it uses the existing verifiers. So there are no consensus-critical changes here.

Complex Code or Requirements

This code launches concurrent async tasks and OS threads. We should check that panics and shutdowns are handled correctly. In particular, miner threads should restart mining with each new block template.

We should check that any common or expected errors are handled by retrying rather than panicking.

We should check that the threads are actually doing different work, they should have nonces with large gaps between them.

Solution

Rust Features:

• add an internal-miner feature and dependencies

Solver:

• depend on the new solver code in equihash
• add an equihash::Solution::solve() method with difficulty checks

Miner:

• add a miner task to the start command
• wait until a valid template is available
• spawn to a blocking async executor thread
• spawn to a low-priority OS thread
• handle shutdowns by cancelling the miner

Parallel Mining:

• use a different nonce for each thread
• spawn multiple miner threads based on a new config, use available_parallelism() by default
• select! {} on solutions or new block templates using long_poll_id
• cancel and restart the solver if there's a new template
• rate-limit template changes

Related Changes:

• Fix a bug in the get_block_template RPC change detection
• make RPC instances cloneable and LongPollId copyable
• use the same generic constraints for the RPC struct and impls
• move some code around so it's easier to re-use
• add utility and convenience methods to existing types

Testing

I manually verified that:

• the mining threads are running, and there are approximately the configured number of threads
• they are using 100% CPU
• they are low priority (blue in htop)
• they produce valid blocks (according to Zebra's verifier)
• the miner runs overnight successfully
• the blocks are mined off the tip
• the blocks are accepted by other nodes
• the block nonces match the pattern used by Zebra, and it's not just the first thread successfully mining

Review

This PR is ready for review.

I'd like to focus on bugs and overall code structure in this review.

Reviewer Checklist

Check before approving the PR:

• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
  PR blockers can be dealt with in new tickets or PRs.

And check the PR Author checklist is complete.

Follow Up Work

• Get the equihash changes merged and released, and update that dependency
• Check we have at least one peer before starting mining (the existing sync to tip check partly covers this)
• Optional: Add a cached state test that mines a block on testnet?

[teor2345]

This seems to work on testnet, but it's currently causing orphaned chain forks due to some missing code:

• cancel and restart the solver if there's a new template

[teor2345]

This should be feature-complete. There aren't any new tests, but the existing tests have good coverage, and I tested it manually.

I am just writing the docs now.

[teor2345]

Now the docs should be complete as well.

[arya2]

This looks excellent, thank you.