fix(p2p,eth): remove per-peer dual-connection mechanism, close #2351 #2359

[gzliudan]

Proposed changes

XDPoSChain kept two independent RLPx/TCP connections per remote peer (a "primary" and a "pair"), a private divergence from upstream go-ethereum which allows only one connection per NodeID. This adds state-machine complexity (isPair, pairRw, PairPeer, ErrAddPairPeer, DiscPairPeerStop) without measured benefit, and the pair connection already carried both block-sync and BFT traffic, so it never isolated Vote/Timeout/SyncInfo from large BlockBodies/NodeData frames.

Revert to a single connection per NodeID (upstream semantics) and route all eth messages, including the XDPoS v2 BFT messages, over that single connection using the existing upstream FIFO write scheduling. No write-priority lane is added; RLPx framing, message ids, capability strings and the handshake are unchanged, so the change is rolling-upgrade safe against legacy pair-capable nodes.

Changes:

• p2p/server.go: postHandshakeChecks rejects any duplicate NodeID with DiscAlreadyConnected; addPeer no longer stores a second connection.
• p2p/peer.go: drop pairPeer field/methods and the DiscPairPeerStop cascade on Peer.run exit.
• p2p/peer_error.go: remove ErrAddPairPeer; keep the DiscPairPeerStop enum slot (deprecated) to preserve on-the-wire numbering.
• eth/peer.go: remove pairRw; all Send/Request helpers use p.rw; peerSet.Register reverts to one peer per id.
• eth/handler.go: drop ErrAddPairPeer special-casing so every peer registers with the downloader and syncs transactions.

fix #2351 and #2359

replace #2360

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• test: Adding missing tests or correcting existing tests

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• Consensus
• Account
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• Geth
• Smart Contract
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Checklist

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Tested on a private network from the genesis block and monitored the chain operating correctly for multiple epochs.
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A

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[Copilot]

Pull request overview

This PR removes XDPoSChain’s per-peer “dual connection” (primary + pair) RLPx mechanism and returns to upstream-style semantics of a single active connection per remote NodeID, routing all ETH/BFT traffic over the single negotiated session.

Changes:

• Enforce single-connection-per-NodeID in the p2p server by rejecting duplicates with DiscAlreadyConnected.
• Remove pair-peer lifecycle/state from p2p.Peer and delete the associated test.
• Simplify the ETH peer implementation to use only p.rw, and make peerSet.Register strictly reject duplicate IDs; handler always registers peers with the downloader.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
p2p/server.go	Rejects duplicate NodeID connections and stores only one peer per ID.
p2p/peer.go	Removes pair-peer state and the pair-disconnect cascade on shutdown.
p2p/peer_test.go	Deletes the pair-peer shutdown test (no longer applicable).
p2p/peer_error.go	Removes ErrAddPairPeer; keeps DiscPairPeerStop enum slot as deprecated for wire-number stability.
eth/peer.go	Removes pairRw routing and makes peer registration strictly one-per-id.
eth/peer_test.go	Adds a regression test that duplicate IDs are rejected by peerSet.Register.
eth/handler.go	Removes ErrAddPairPeer special-casing so every peer is registered with the downloader and tx sync.