syncs: add AssertLocked

This allows us to check lock invariants.

It was proposed upstream and rejected in:
https://github.com/golang/go/issues/1366

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

syncs/locked.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.13,!go1.16
+
+// This file makes assumptions about the inner workings of sync.Mutex and sync.RWMutex.
+// This includes not just their memory layout but their invariants and functionality.
+// To prevent accidents, it is limited to a known good subset of Go versions.
+
+package syncs
+
+import (
+	"sync"
+	"sync/atomic"
+	"unsafe"
+)
+
+const (
+	mutexLocked = 1
+
+	// sync.Mutex field offsets
+	stateOffset = 0
+
+	// sync.RWMutext field offsets
+	mutexOffset       = 0
+	readerCountOffset = 16
+)
+
+// add returns a pointer with value p + off.
+func add(p unsafe.Pointer, off uintptr) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(p) + off)
+}
+
+// AssertLocked panics if m is not locked.
+func AssertLocked(m *sync.Mutex) {
+	p := add(unsafe.Pointer(m), stateOffset)
+	if atomic.LoadInt32((*int32)(p))&mutexLocked == 0 {
+		panic("mutex is not locked")
+	}
+}
+
+// AssertRLocked panics if rw is not locked for reading or writing.
+func AssertRLocked(rw *sync.RWMutex) {
+	p := add(unsafe.Pointer(rw), readerCountOffset)
+	if atomic.LoadInt32((*int32)(p)) != 0 {
+		// There are readers present or writers pending, so someone has a read lock.
+		return
+	}
+	// No readers.
+	AssertWLocked(rw)
+}
+
+// AssertWLocked panics if rw is not locked for writing.
+func AssertWLocked(rw *sync.RWMutex) {
+	m := (*sync.Mutex)(add(unsafe.Pointer(rw), mutexOffset))
+	AssertLocked(m)
+}