serai/substrate/tendermint/machine

Tendermint

An implementation of the Tendermint state machine in Rust.

This is solely the state machine, intended to be mapped to any arbitrary system. It supports an arbitrary signature scheme, weighting, and block definition accordingly. It is not intended to work with the Cosmos SDK, solely to be an implementation of the academic protocol.

Caveats

• Only SCALE serialization is supported currently. Ideally, everything from SCALE to borsh to bincode would be supported. SCALE was chosen due to this being under Serai, which uses Substrate, which uses SCALE. Accordingly, when deciding which of the three (mutually incompatible) options to support...

• tokio is explicitly used for the asynchronous task which runs the Tendermint machine. Ideally, futures-rs would be used enabling any async runtime to be used.

• It is possible for add_block to be called on a block which failed (or never went through in the first place) validation. This is a break from the paper which is accepted here. This is for two reasons.

  1. Serai needing this functionality.
  2. If a block is committed which is invalid, either there's a malicious majority now defining consensus OR the local node is malicious by virtue of being faulty. Considering how either represents a fatal circumstance, except with regards to system like Serai which have their own logic for pseudo-valid blocks, it is accepted as a possible behavior with the caveat any consumers must be aware of it. No machine will vote nor precommit to a block it considers invalid, so for a network with an honest majority, this is a non-issue.

Paper

The paper describes the algorithm with pseudocode on page 6. This pseudocode is written as a series of conditions for advancement. This is extremely archaic, as its a fraction of the actually required code. This is due to its hand-waving away of data tracking, lack of comments (beyond the entire rest of the paper, of course), and lack of specification regarding faulty nodes.

While the "hand-waving" is both legitimate and expected, as it's not the paper's job to describe a full message processing loop nor efficient variable handling, it does leave behind ambiguities and annoyances, not to mention an overall structure which cannot be directly translated. This section is meant to be a description of it as used for translation.

The included pseudocode segments can be minimally described as follows:

01-09 Init
10-10 StartRound(0)
11-21 StartRound
22-27 Fresh proposal
28-33 Proposal building off a valid round with prevotes
34-35 2f+1 prevote -> schedule timeout prevote
36-43 First proposal with prevotes -> precommit Some
44-46 2f+1 nil prevote -> precommit nil
47-48 2f+1 precommit -> schedule timeout precommit
49-54 First proposal with precommits -> finalize
55-56 f+1 round > local round, jump
57-60 on timeout propose
61-64 on timeout prevote
65-67 on timeout precommit

The corresponding Rust code implementing these tasks are marked with their related line numbers.