move implementation to internal/ directory (#828)

internal/util/chainlock/chainlock.go

@@ -0,0 +1,46 @@
+// package chainlock implements a mutex whose Lock and Unlock
+// methods return the lock itself, to enable chaining.
+//
+// Intended Usage
+//
+//	  defer s.lock().unlock()
+//	  // drop lock while waiting for wait group
+//	  func() {
+//		     defer a.l.Unlock().Lock()
+//		     fssesDone.Wait()
+//		 }()
+package chainlock
+
+import "sync"
+
+type L struct {
+	mtx sync.Mutex
+}
+
+func New() *L {
+	return &L{}
+}
+
+func (l *L) Lock() *L {
+	l.mtx.Lock()
+	return l
+}
+
+func (l *L) Unlock() *L {
+	l.mtx.Unlock()
+	return l
+}
+
+func (l *L) NewCond() *sync.Cond {
+	return sync.NewCond(&l.mtx)
+}
+
+func (l *L) DropWhile(f func()) {
+	defer l.Unlock().Lock()
+	f()
+}
+
+func (l *L) HoldWhile(f func()) {
+	defer l.Lock().Unlock()
+	f()
+}