Use net.ErrClosed

Go 1.16 has introduced net.ErrClosed, which should be returned/wrapped when an
I/O call is performed on a network connection which has already been closed.
This is useful to avoid cluttering logs with messages like "failed to close
WebSocket: already wrote close".

Closes: coder#286

close_notjs.go

@@ -5,7 +5,6 @@ package websocket
 import (
 	"context"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"log"
 	"time"
@@ -48,15 +47,13 @@ func (c *Conn) closeHandshake(code StatusCode, reason string) (err error) {
 	return nil
 }
 
-var errAlreadyWroteClose = errors.New("already wrote close")
-
 func (c *Conn) writeClose(code StatusCode, reason string) error {
 	c.closeMu.Lock()
 	wroteClose := c.wroteClose
 	c.wroteClose = true
 	c.closeMu.Unlock()
 	if wroteClose {
-		return errAlreadyWroteClose
+		return errClosed
 	}
 
 	ce := CloseError{

write.go

@@ -7,7 +7,6 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"io"
 	"time"
@@ -53,14 +52,14 @@ type msgWriter struct {
 
 func (mw *msgWriter) Write(p []byte) (int, error) {
 	if mw.closed {
-		return 0, errors.New("cannot use closed writer")
+		return 0, errClosed
 	}
 	return mw.mw.Write(p)
 }
 
 func (mw *msgWriter) Close() error {
 	if mw.closed {
-		return errors.New("cannot use closed writer")
+		return errClosed
 	}
 	mw.closed = true
 	return mw.mw.Close()