What should Tendermint do when a precommit message is received without Extension

[sergio-mena]

Problem Definition

When a precommit messsge is received, VerifyVoteExtension is called even if the message does not have an extension or the extension has length 0, according to the current version of the ABCI++ spec

The question is whether is would make sense to skip the verification in the case of empty or non-existent extension. This probably depends on the use cases for vote extensions. Currently we don't see an obvious reason not skip the verification under such conditions.

Once a decision is reached, we need to adapt the implementation (should be quick). Also, the spec should be updated to clearly describe the approach described (probably in section "When does Tendermint use it")

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[sergio-mena]

The discussion during yesterday's ABCI++ bi-weekly went around the following three cases:
a) The App does not extend the vote (nil-byte array)
b) The App returns a 0-length extension (0-length byte array)
c) The App returns at least 1 byte as extension (the normal case)

@williambanfield rightly pointed out that, according to the latest vote extension design, if we allow a) as a possibility, an intermediate node in the gossip network could always remove a vote extension from the vote message. Therefore we must exclude a) as a possibility.

The second decision taken, which was the initial goal of this discussion, is that VerifyVoteExtension will also be called in the case the extension received has 0 length.

In conclusion:

• If the App does not have a vote extension to provide, it returns a 0-byte array in ResponseExtendVote
  • Then, Tendermint will construct and sign the CanonicalVoteExtension structure in memory, including the empty byte array.
  • The (possibly 0-length) vote extension, together with its signature MUST be part of the precommit message sent
• If Tendermint receives a precommit message without a vote extension (which can be the empty byte array) AND its signature, it discards the message
• If Tendermint receives a precommit message with the empty byte array as vote extension, together with its (valid) signature, it MUST call RequestVeryfyVoteExtension with the empty byte array.

Please feel free to correct any inaccuracy in the text above.

I will open a PR to address this in the spec.

[williambanfield]

@sergio-mena is there anything left to do for this issue?

[sergio-mena]

@sergio-mena is there anything left to do for this issue?

This should be closed by @thanethomson's PR #8141

[sergio-mena]

@thanethomson please make sure we reject a precommit message that does not contain the vote extension AND its signature

[sergio-mena]

Closed by #8141