syncs: add LockFunc, LockValue, LockValues, and Mutex

The first 3 functions are helpers for running functions
under the protection of a lock.

The Mutex type is a wrapper over sync.Mutex with a Do method
that runs a function under the protection of a lock.

Updates #11038
Updates #cleanup

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/lock_example_test.go

@@ -0,0 +1,37 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syncs_test
+
+import (
+	"encoding/hex"
+	"log"
+	"sync"
+
+	"tailscale.com/syncs"
+)
+
+func ExampleLockFunc() {
+	var nodesMu sync.Mutex
+	var nodes []string
+	syncs.LockFunc(&nodesMu, func() { nodes = append(nodes, "node123") })
+}
+
+func ExampleLockValue() {
+	var nodesMu sync.Mutex
+	var nodes []string
+	n := syncs.LockValue(&nodesMu, func() int { return len(nodes) })
+	log.Printf("there are %d nodes", n)
+}
+
+func ExampleLockValues() {
+	var bufferMu sync.Mutex
+	var buffer string
+	b, err := syncs.LockValues(&bufferMu, func() ([]byte, error) {
+		return hex.DecodeString(buffer)
+	})
+	if err != nil {
+		log.Fatalf("Decode error: %v", err)
+	}
+	log.Printf("decoded %d bytes", len(b))
+}

syncs/syncs.go

@@ -304,3 +304,39 @@ func (wg *WaitGroup) Go(f func()) {
 		f()
 	}()
 }
+
+// TODO(https://go.dev/issue/63941): LockFunc, LockValue, and LockValues
+// are helper functions proposed upstream. Their naming and signature
+// are based on the existing OnceFunc, OnceValue, and OnceValues
+// helper functions already in the [sync] package.
+
+// LockFunc runs f while holding the lock.
+func LockFunc(lock sync.Locker, f func()) {
+	lock.Lock()
+	defer lock.Unlock()
+	f()
+}
+
+// LockValue runs f while holding the lock and returns the argument.
+func LockValue[T any](lock sync.Locker, f func() T) T {
+	lock.Lock()
+	defer lock.Unlock()
+	return f()
+}
+
+// LockValues runs f while holding the lock and returns the arguments.
+func LockValues[T1, T2 any](lock sync.Locker, f func() (T1, T2)) (T1, T2) {
+	lock.Lock()
+	defer lock.Unlock()
+	return f()
+}
+
+// Mutex is identical to [sync.Mutex], but with additional methods.
+type Mutex struct{ sync.Mutex }
+
+// Do runs f while holding the lock.
+func (m *Mutex) Do(f func()) {
+	m.Lock()
+	defer m.Unlock()
+	f()
+}