refactor(p2p,eth): collapse pair-peer dual links into a single prioritized write path, close #2351 #2359

[gzliudan]

Proposed changes

This PR close issue #2359 and replace PR #2351

Replace the pair-peer dual-link write model with a single peer-owned write ingress backed by internal high/low priority queues.

Preserve BFT message priority across wrapper layers, keep synchronous write errors visible to callers, and make queued versus in-flight shutdown handling deterministic.

Update queue metrics and add regression coverage for priority preemption, starvation bounds, ping serialization, wrapper forwarding, and shutdown behavior.

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• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Changes that don't change source code or tests
• docs: Documentation only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that neither fixes a bug nor adds a feature
• revert: Revert something
• style: Changes that do not affect the meaning of the code
• test: Adding missing tests or correcting existing tests

Impacted Components

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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
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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 the XDPoS pair-peer dual-connection mechanism and replaces its BFT-latency goal with per-peer high/low priority write scheduling in the p2p message path.

Changes:

• Removes pair-peer state, duplicate-connection allowances, and eth pair writer fallback paths.
• Adds PriorityMsgWriter / SendPriority and routes Vote, Timeout, and SyncInfo messages through the high-priority lane.
• Updates dial/server tests and adds priority scheduler tests for preemption, starvation guard, and fallback behavior.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
p2p/server.go	Rejects duplicate NodeID connections and removes pair-peer tracking.
p2p/server_test.go	Updates server peer tracking tests for single-peer semantics.
p2p/peer.go	Adds high/low write request queues and priority arbitration.
p2p/peer_test.go	Removes pair-peer disconnect behavior test.
p2p/peer_priority_test.go	Adds tests for priority write scheduling behavior.
p2p/peer_error.go	Removes ErrAddPairPeer.
p2p/message.go	Adds SendPriority helper with fallback to normal writes.
p2p/dial.go	Restores duplicate peer dial rejection.
p2p/dial_test.go	Updates dial state expectations for one connection per NodeID.
eth/peer.go	Removes pairRw and uses priority sends for BFT messages.
eth/handler.go	Simplifies peer registration now that pair peers are removed.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.