replication: refactor driving logic (no more explicit state machine)

util/chainlock/chainlock.go

@@ -0,0 +1,42 @@
+// 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()
+}