From f7d81d07336145861214229ccbe8452cb810fb92 Mon Sep 17 00:00:00 2001
From: Oleksandr Lystopad <oleksandr.lystopad@erigon.tech>
Date: Mon, 17 Aug 2026 10:12:53 +0200
Subject: [PATCH] txnprovider/txpool: release the pool lock when a caller stops
 waiting for a block

best takes the pool lock and then waits for the block it was asked to build on top of. A
caller that goes away in the meantime returned from inside that loop without releasing it, so
the lock stayed held and every later pool operation blocked, including the block updates that
would have let the wait finish. Nothing recovers from that on its own.

Reachable today only at shutdown, because the only caller that cancels is the one shutting the
node down. It stops being shutdown-only as soon as anything cancels a live build, which is
what discarding an evicted payload builder does.
---
 txnprovider/txpool/pool.go                |  2 +
 txnprovider/txpool/pool_best_lock_test.go | 46 +++++++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 txnprovider/txpool/pool_best_lock_test.go

diff --git a/txnprovider/txpool/pool.go b/txnprovider/txpool/pool.go
index 5b3b64a3676..b7fba981023 100644
--- a/txnprovider/txpool/pool.go
+++ b/txnprovider/txpool/pool.go
@@ -729,6 +729,8 @@ func (p *TxPool) best(ctx context.Context, n int, txns *TxnsRlp, onTopOf uint64,
 	for last := p.lastSeenBlock.Load(); last < onTopOf; last = p.lastSeenBlock.Load() {
 		select {
 		case <-ctx.Done():
+			// Leaving with the lock held would stop every later pool operation, not just this one.
+			p.lock.Unlock()
 			return false, 0, ctx.Err()
 		default:
 			// continue
diff --git a/txnprovider/txpool/pool_best_lock_test.go b/txnprovider/txpool/pool_best_lock_test.go
new file mode 100644
index 00000000000..b71ea6bce1e
--- /dev/null
+++ b/txnprovider/txpool/pool_best_lock_test.go
@@ -0,0 +1,46 @@
+// Copyright 2026 The Erigon Authors
+// This file is part of Erigon.
+//
+// Erigon is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Erigon is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with Erigon. If not, see <http://www.gnu.org/licenses/>.
+
+package txpool
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	mapset "github.com/deckarep/golang-set/v2"
+	"github.com/stretchr/testify/require"
+
+	mdgas "github.com/erigontech/erigon/execution/protocol/mdgas"
+)
+
+func TestBestReleasesTheLockWhenTheCallerGivesUpWaitingForABlock(t *testing.T) {
+	lock := &sync.Mutex{}
+	p := &TxPool{lock: lock, lastSeenCond: sync.NewCond(lock)}
+
+	// The requested block has not been seen, so the wait loop is entered, and the caller is already
+	// gone by the time it checks.
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	_, _, err := p.best(ctx, 1, &TxnsRlp{}, 1, mdgas.FullMdGas{}, mapset.NewSet[[32]byte](), 0)
+	require.ErrorIs(t, err, context.Canceled)
+
+	// Returning with the lock still held blocks every later pool operation, including the block
+	// updates that would have let this caller through, so nothing recovers on its own.
+	require.True(t, p.lock.TryLock(), "best returned holding the pool lock")
+	p.lock.Unlock()
+}