syncs: add Mutex/RWMutex alias/wrappers for future mutex debugging

Updates #17852

Change-Id: I477340fb8e40686870e981ade11cd61597c34a20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

syncs/locked.go

@@ -8,7 +8,7 @@ import (
 )
 
 // AssertLocked panics if m is not locked.
-func AssertLocked(m *sync.Mutex) {
+func AssertLocked(m *Mutex) {
 	if m.TryLock() {
 		m.Unlock()
 		panic("mutex is not locked")
@@ -16,7 +16,7 @@ func AssertLocked(m *sync.Mutex) {
 }
 
 // AssertRLocked panics if rw is not locked for reading or writing.
-func AssertRLocked(rw *sync.RWMutex) {
+func AssertRLocked(rw *RWMutex) {
 	if rw.TryLock() {
 		rw.Unlock()
 		panic("mutex is not locked")

syncs/mutex.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_mutex_debug
+
+package syncs
+
+import "sync"
+
+// Mutex is an alias for sync.Mutex.
+//
+// It's only not a sync.Mutex when built with the ts_mutex_debug build tag.
+type Mutex = sync.Mutex
+
+// RWMutex is an alias for sync.RWMutex.
+//
+// It's only not a sync.RWMutex when built with the ts_mutex_debug build tag.
+type RWMutex = sync.RWMutex

syncs/mutex_debug.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_mutex_debug
+
+package syncs
+
+import "sync"
+
+type Mutex struct {
+	sync.Mutex
+}
+
+type RWMutex struct {
+	sync.RWMutex
+}
+
+// TODO(bradfitz): actually track stuff when in debug mode.

syncs/shardedint_test.go

@@ -1,13 +1,14 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-package syncs
+package syncs_test
 
 import (
 	"expvar"
 	"sync"
 	"testing"
 
+	. "tailscale.com/syncs"
 	"tailscale.com/tstest"
 )