fix(core/txpool): coordinate reset lifecycle and shutdown signaling #28837 (#2132)

Improve txpool loop synchronization around background resets.

This change:
- adds an explicit termination channel to signal pool shutdown
- tracks forced-reset intent and a waiter channel inside the reset loop
- ensures reset waiters are notified on completion or on pool termination
- allows an explicit sync request path to trigger an additional reset round when needed

Scope is limited to internal txpool concurrency control in core/txpool/txpool.go, with no protocol or RPC behavior change.

core/txpool/txpool.go

@@ -17,6 +17,7 @@
 package txpool
 
 import (
+	"errors"
 	"fmt"
 	"maps"
 	"math/big"
@@ -57,6 +58,9 @@ type TxPool struct {
 
 	subs event.SubscriptionScope // Subscription scope to unsubscribe all on shutdown
 	quit chan chan error         // Quit channel to tear down the head updater
+	term chan struct{}           // Termination channel to detect a closed pool
+
+	sync chan chan error // Testing / simulator channel to block until internal reset is done
 }
 
 // New creates a new transaction pool to gather, sort and filter inbound
@@ -70,6 +74,8 @@ func New(gasTip uint64, chain BlockChain, subpools []SubPool) (*TxPool, error) {
 	pool := &TxPool{
 		subpools: subpools,
 		quit:     make(chan chan error),
+		term:     make(chan struct{}),
+		sync:     make(chan chan error),
 	}
 	reserver := NewReservationTracker()
 	for i, subpool := range subpools {
@@ -113,6 +119,9 @@ func (p *TxPool) Close() error {
 // outside blockchain events as well as for various reporting and transaction
 // eviction events.
 func (p *TxPool) loop(head *types.Header, chain BlockChain) {
+	// Close the termination marker when the pool stops
+	defer close(p.term)
+
 	// Subscribe to chain head events to trigger subpool resets
 	var (
 		newHeadCh  = make(chan core.ChainHeadEvent)
@@ -129,13 +138,26 @@ func (p *TxPool) loop(head *types.Header, chain BlockChain) {
 	var (
 		resetBusy = make(chan struct{}, 1) // Allow 1 reset to run concurrently
 		resetDone = make(chan *types.Header)
+
+		resetForced bool       // Whether a forced reset was requested, only used in simulator mode
+		resetWaiter chan error // Channel waiting on a forced reset, only used in simulator mode
 	)
+	// Notify the live reset waiter without blocking if the txpool is closed.
+	defer func() {
+		if resetWaiter != nil {
+			select {
+			case resetWaiter <- errors.New("pool already terminated"):
+			default:
+			}
+			resetWaiter = nil
+		}
+	}()
 	var errc chan error
 	for errc == nil {
 		// Something interesting might have happened, run a reset if there is
 		// one needed but none is running. The resetter will run on its own
 		// goroutine to allow chain head events to be consumed contiguously.
-		if newHead != oldHead {
+		if newHead != oldHead || resetForced {
 			// Try to inject a busy marker and start a reset if successful
 			select {
 			case resetBusy <- struct{}{}:
@@ -147,8 +169,17 @@ func (p *TxPool) loop(head *types.Header, chain BlockChain) {
 					resetDone <- newHead
 				}(oldHead, newHead)
 
+				// If the reset operation was explicitly requested, consider it
+				// being fulfilled and drop the request marker. If it was not,
+				// this is a noop.
+				resetForced = false
+
 			default:
-				// Reset already running, wait until it finishes
+				// Reset already running, wait until it finishes.
+				//
+				// Note, this will not drop any forced reset request. If a forced
+				// reset was requested, but we were busy, then when the currently
+				// running reset finishes, a new one will be spun up.
 			}
 		}
 		// Wait for the next chain head event or a previous reset finish
@@ -162,8 +193,37 @@ func (p *TxPool) loop(head *types.Header, chain BlockChain) {
 			oldHead = head
 			<-resetBusy
 
+			// If someone is waiting for a reset to finish, notify them, unless
+			// the forced op is still pending. In that case, wait another round
+			// of resets.
+			if resetWaiter != nil && !resetForced {
+				select {
+				case resetWaiter <- nil:
+					// notification delivered
+				default:
+					// no active listener; avoid blocking the event loop
+				}
+				resetWaiter = nil
+			}
+
 		case errc = <-p.quit:
 			// Termination requested, break out on the next loop round
+
+		case syncc := <-p.sync:
+			// Transaction pool is running inside a simulator, and we are about
+			// to create a new block. Request a forced sync operation to ensure
+			// that any running reset operation finishes to make block imports
+			// deterministic. On top of that, run a new reset operation to make
+			// transaction insertions deterministic instead of being stuck in a
+			// queue waiting for a reset.
+			if resetWaiter != nil {
+				// A previous sync waiter is still pending; notify it to avoid
+				// leaking a goroutine waiting on the old channel.
+				resetWaiter <- errors.New("sync request superseded by a new request")
+				resetWaiter = nil
+			}
+			resetForced = true
+			resetWaiter = syncc
 		}
 	}
 	// Notify the closer of termination (no error possible for now)
@@ -375,3 +435,20 @@ func (pool *TxPool) IsSigner(addr common.Address) bool {
 	}
 	return false
 }
+
+// Sync is a helper method for unit tests or simulator runs where the chain events
+// are arriving in quick succession, without any time in between them to run the
+// internal background reset operations. This method will run an explicit reset
+// operation to ensure the pool stabilises, thus avoiding flakey behavior.
+//
+// Note, do not use this in production / live code. In live code, the pool is
+// meant to reset on a separate thread to avoid DoS vectors.
+func (p *TxPool) Sync() error {
+	sync := make(chan error)
+	select {
+	case p.sync <- sync:
+		return <-sync
+	case <-p.term:
+		return errors.New("pool already terminated")
+	}
+}