Use atomic.Value instead of lock for ConnPool.lastDialError

This makes the reading and writing of lastDialError from the pool
faster, as atomic.Value is much more lightweight than the mutex.

Note that using error in atomic.Value directly could cause panics,
because errors could have inconsistent types. Thus wrap them with a
simple struct.

internal/pool/pool.go

@@ -60,13 +60,16 @@ type Options struct {
 	IdleCheckFrequency time.Duration
 }
 
+type lastDialErrorWrap struct {
+	err error
+}
+
 type ConnPool struct {
 	opt *Options
 
 	dialErrorsNum uint32 // atomic
 
-	lastDialErrorMu sync.RWMutex
-	lastDialError   error
+	lastDialError atomic.Value
 
 	queue chan struct{}
 
@@ -204,16 +207,15 @@ func (p *ConnPool) tryDial() {
 }
 
 func (p *ConnPool) setLastDialError(err error) {
-	p.lastDialErrorMu.Lock()
-	p.lastDialError = err
-	p.lastDialErrorMu.Unlock()
+	p.lastDialError.Store(&lastDialErrorWrap{err: err})
 }
 
 func (p *ConnPool) getLastDialError() error {
-	p.lastDialErrorMu.RLock()
-	err := p.lastDialError
-	p.lastDialErrorMu.RUnlock()
-	return err
+	err, _ := p.lastDialError.Load().(*lastDialErrorWrap)
+	if err != nil {
+		return err.err
+	}
+	return nil
 }
 
 // Get returns existed connection from the pool or creates a new one.