refactor(privval): rename sign_extension to skip_sign_extension (come…

…tbft#2519)

because the former is not backwards compatible. I.e., if the remote
signer runs a new version for old chain and `sign_extension` field is
absent, then it will be false by default, BUT the expected behavior is
different. For this reason, the field is renamed to skip_sign_extension.
The remote signer might! skip signing extension bytes, but the default
behavior is to sign.

Thanks to @mzabaluev for spotting this.

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---

#### PR checklist

- [x] Tests written/updated
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

.changelog/unreleased/improvements/2496-privval-ext-signature.md

@@ -1,4 +1,4 @@
 - `[privval]` DO NOT require extension signature from privval if vote
-  extensions are disabled. Remote signers must ONLY sign the extension if
-  `sign_extension` flag in `SignVoteRequest` is true.
+  extensions are disabled. Remote signers can skip signing the extension if
+  `skip_sign_extension` flag in `SignVoteRequest` is true.
   [\#2496](https://github.com/cometbft/cometbft/pull/2496)

privval/signer_client.go

@@ -93,7 +93,7 @@ func (sc *SignerClient) GetPubKey() (crypto.PubKey, error) {
 
 // SignVote requests a remote signer to sign a vote.
 func (sc *SignerClient) SignVote(chainID string, vote *cmtproto.Vote, signExtension bool) error {
-	response, err := sc.endpoint.SendRequest(mustWrapMsg(&pvproto.SignVoteRequest{Vote: vote, ChainId: chainID, SignExtension: signExtension}))
+	response, err := sc.endpoint.SendRequest(mustWrapMsg(&pvproto.SignVoteRequest{Vote: vote, ChainId: chainID, SkipSignExtension: !signExtension}))
 	if err != nil {
 		return err
 	}

privval/signer_requestHandler.go

@@ -62,7 +62,7 @@ func DefaultValidationRequestHandler(
 
 		vote := r.SignVoteRequest.Vote
 
-		err = privVal.SignVote(chainID, vote, r.SignVoteRequest.SignExtension)
+		err = privVal.SignVote(chainID, vote, !r.SignVoteRequest.SkipSignExtension)
 		if err != nil {
 			res = mustWrapMsg(&pvproto.SignedVoteResponse{
 				Vote: cmtproto.Vote{}, Error: &pvproto.RemoteSignerError{Code: 0, Description: err.Error()},

proto/cometbft/privval/v1/types.proto

@@ -28,7 +28,7 @@ message PubKeyResponse {
 message SignVoteRequest {
   cometbft.types.v1.Vote vote           = 1;
   string                 chain_id       = 2;
-  bool                   sign_extension = 3;
+  bool                   skip_sign_extension = 3; // if true, the signer may skip signing the extension bytes.
 }
 
 // SignedVoteResponse is a response containing a signed vote or an error