net: remove unused nil check

This is part 1 of a series of proposals to fix issue 4369.

In resolving issue 3507 it was decided not to nil out the inner conn.fd field to avoid a race. This implies the checks for fd == nil inside incref/decref are never true.

Removing this logic removes one source of errClosing error values, which affects issue 4373 and moves towards bradfitz's request that fd.accept() return io.EOF when closed concurrently.

Update #4369.
Update #4373.

R=mikioh.mikioh, bradfitz, dvyukov, rsc
CC=golang-dev
https://golang.org/cl/6852057

src/pkg/net/fd_unix.go

@@ -353,9 +353,6 @@ func (fd *netFD) connect(ra syscall.Sockaddr) error {
 // If closing==true, pollserver must be locked; mark the fd as closing.
 // Returns an error if the fd cannot be used.
 func (fd *netFD) incref(closing bool) error {
-	if fd == nil {
-		return errClosing
-	}
 	fd.sysmu.Lock()
 	if fd.closing {
 		fd.sysmu.Unlock()
@@ -372,9 +369,6 @@ func (fd *netFD) incref(closing bool) error {
 // Remove a reference to this FD and close if we've been asked to do so (and
 // there are no references left.
 func (fd *netFD) decref() {
-	if fd == nil {
-		return
-	}
 	fd.sysmu.Lock()
 	fd.sysref--
 	if fd.closing && fd.sysref == 0 && fd.sysfile != nil {

src/pkg/net/unixsock_posix.go

@@ -313,9 +313,7 @@ func (l *UnixListener) Close() error {
 	if l.path[0] != '@' {
 		syscall.Unlink(l.path)
 	}
-	err := l.fd.Close()
-	l.fd = nil
-	return err
+	return l.fd.Close()
 }
 
 // Addr returns the listener's network address.