gopls/internal/util/persistent: add concurrency test

It didn't find any problems.

Updates golang/go#72931

Change-Id: Idb65548480af1fd6777dffdcc0e6c6e89b5a06f5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/659015
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/internal/util/persistent/map.go

@@ -203,6 +203,8 @@ func (pm *Map[K, V]) SetAll(other *Map[K, V]) {
 // Set updates the value associated with the specified key.
 // If release is non-nil, it will be called with entry's key and value once the
 // key is no longer contained in the map or any clone.
+//
+// TODO(adonovan): fix release, which has the wrong type.
 func (pm *Map[K, V]) Set(key K, value V, release func(key, value any)) {
 	first := pm.root
 	second := newNodeWithRef(key, value, release)

gopls/internal/util/persistent/race_test.go

@@ -0,0 +1,66 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build race
+
+package persistent
+
+import (
+	"context"
+	"maps"
+	"testing"
+	"time"
+
+	"golang.org/x/sync/errgroup"
+)
+
+// TestConcurrency exercises concurrent map access.
+// It doesn't assert anything, but it runs under the race detector.
+func TestConcurrency(t *testing.T) {
+	ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
+	defer cancel()
+	var orig Map[int, int] // maps subset of [0-10] to itself (values aren't interesting)
+	for i := range 10 {
+		orig.Set(i, i, func(k, v any) { /* just for good measure*/ })
+	}
+	g, ctx := errgroup.WithContext(ctx)
+	const N = 10 // concurrency level
+	g.SetLimit(N)
+	for range N {
+		g.Go(func() error {
+			// Each thread has its own clone of the original,
+			// sharing internal structures. Each map is accessed
+			// only by a single thread; the shared data is immutable.
+			m := orig.Clone()
+
+			// Run until the timeout.
+			for ctx.Err() == nil {
+				for i := range 1000 {
+					key := i % 10
+
+					switch {
+					case i%2 == 0:
+						_, _ = m.Get(key)
+					case i%11 == 0:
+						m.Set(key, key, func(key, value any) {})
+					case i%13 == 0:
+						_ = maps.Collect(m.All())
+					case i%17 == 0:
+						_ = m.Delete(key)
+					case i%19 == 0:
+						_ = m.Keys()
+					case i%31 == 0:
+						_ = m.String()
+					case i%23 == 0:
+						_ = m.Clone()
+					}
+					// Don't call m.Clear(), as it would
+					// disentangle the various maps from each other.
+				}
+			}
+			return nil
+		})
+	}
+	g.Wait() // no errors
+}