fix(core/txpool): subscribe to head events in New and release on Init error #35048

[gzliudan]

Summary

Fixes lost chain head events in the txpool that left the pool resetting against a stale head, rejecting fresh transactions as ErrNonceTooHigh (this also caused sporadic TestResubmit/TestJournal flakes on CI).

Ref: ethereum#35048

Root cause

The pool subscribed to chain head events only after its loop goroutine started:

var (
    newHeadCh  = make(chan core.ChainHeadEvent)
    newHeadSub = p.chain.SubscribeChainHeadEvent(newHeadCh)
)

Because the subscription is established asynchronously, a head event emitted by InsertChain right after New could be missed. The pool then reset against a stale head, keeping pending nonces at genesis (nonce 0); transactions built on the new chain nonce were rejected with ErrNonceTooHigh.

Fix

1. Subscribe synchronously in New (matching upstream geth): newHeadCh/newHeadSub become TxPool fields and chain.SubscribeChainHeadEvent is called before any block can be inserted, so head events are never lost.
2. Release the subscription on Init failure: New subscribes before the fallible SubPool.Init calls; if Init fails, New returns without starting the loop, so the loop's deferred Unsubscribe never runs and the chain feed stays subscribed to an unconsumed, unbuffered channel — a later head publication can block until the chain is stopped. The error path now unsubscribes before returning.

Tests

• TestHeadEventDeliveredAfterNew: inserts blocks right after New and waits for the pool's pending nonce to advance to the new head — it times out if the head event is lost.
• TestNewUnsubscribesOnInitError: forces a SubPool.Init failure and asserts the head event subscription is released (fails if leaked).

Upstream reference

Aligns the head-event subscription with upstream geth commit b71f75091 (core, core/txpool, eth: move subscriptions to constructor, ethereum#35048; closes ethereum#20554). XDPoSChain was still on the pre-ethereum#35048 shape where the loop subscribed asynchronously inside its goroutine. The Init-failure release goes beyond upstream, fixing a leak also present in geth's current shape.

Verification

• core/txpool: full package passes;
• TestHeadEventDeliveredAfterNew and TestNewUnsubscribesOnInitError pass repeated runs under -cover and -race;
• core/txpool/locals: 100 consecutive runs under -cover all pass (the TestResubmit/TestJournal flakes were rooted in this head-event loss).

Compatibility

Internal txpool concurrency control only; no protocol, RPC, or state-transition changes.

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[Copilot]

Pull request overview

Moves txpool head-event subscription into construction to prevent missed events and stale nonce state.

Changes:

• Subscribes synchronously in txpool.New.
• Adds regression coverage for immediate block insertion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
core/txpool/txpool.go	Moves head subscription into the constructor.
core/txpool/txpool_head_event_test.go	Tests immediate head-event delivery and transaction acceptance.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.