Require two reviewers for complex or risky code changes

[teor2345]

Motivation

We'd like to require two reviewers for complex or risky code changes.

We could do that by adding a new Mergify workflow at the top of the workflow list for these labels:

• A-compatibility
• A-concurrency
• A-consensus
• I-consensus
• C-security
• I-crash
• I-destructive
• I-hang
• I-privacy
• I-unbounded-growth
• I-unsound

We'd also like two reviewers for unsafe code, we could:

1. auto-label PRs that change the paths for unsafe reviewers with a new A-unsafe label
2. Add that label to the Mergify list

Testing

We'll see if it works for the first consensus or compatibility PR.

[str4d]

For reference, here is the review policy we use for zcashd, which is designed around "levels of bug impact":

• PRs from ECC engineers that solely involve documentation or ancillary changes may be merged by the author without additional review. Equivalent PRs from community members may be reviewed and merged by a single ECC engineer.
  • A bug here can affect new users who aren’t familiar with Zcash, or people using ancillary non-zcashd tools.
• PRs that touch the build system or general codebase require at least one code review. Either the author or reviewer may at their discretion block the PR on additional review; this will be stated explicitly in the PR comments.
  • A bug here can affect users that rely on zcashd for viewing the Zcash chain, or people who build zcashd. A DoS or crash-inducing bug could affect all zcashd users.
• PRs that touch parts of the codebase which risk user funds (e.g. wallet serialization, transaction creation, bad key generation or storage, parsers) or user privacy (e.g. side channels) require at least two code reviews. At least one of them needs to be from an ECC engineer; if it’s a cryptography change, at least two of them need to be from ECC engineers.
  • A bug here can affect any user that relies on zcashd for managing their funds.
• PRs that alter the consensus rules (or that perform significant refactoring of consensus code whether or not they are intended to alter the rules) require at least three code reviews; at least two of them need to be from ECC engineers. Just the two reviews from ECC engineers are sufficient if the change is a backport from Bitcoin Core-maintained code.
  • A bug here can affect the entire Zcash ecosystem.

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[mpguerra]

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