Transparently handle EINTR during Read

This has been observed in particular on MacOS, in this case just retry
the call without throwing the error back to the user.

Related to:
https://github.com/arduino/arduino-cli/issues/504
https://github.com/arduino/arduino-pro-ide/issues/157

serial_unix.go

@@ -15,8 +15,8 @@ import (
 	"sync"
 	"unsafe"
 
-	"golang.org/x/sys/unix"
 	"go.bug.st/serial/unixutils"
+	"golang.org/x/sys/unix"
 )
 
 type unixPort struct {
@@ -51,7 +51,7 @@ func (port *unixPort) Close() error {
 	return nil
 }
 
-func (port *unixPort) Read(p []byte) (n int, err error) {
+func (port *unixPort) Read(p []byte) (int, error) {
 	port.closeLock.RLock()
 	defer port.closeLock.RUnlock()
 	if !port.opened {
@@ -59,14 +59,23 @@ func (port *unixPort) Read(p []byte) (n int, err error) {
 	}
 
 	fds := unixutils.NewFDSet(port.handle, port.closeSignal.ReadFD())
+	for {
 		res, err := unixutils.Select(fds, nil, fds, -1)
+		if err == unix.EINTR {
+			continue
+		}
 		if err != nil {
 			return 0, err
 		}
 		if res.IsReadable(port.closeSignal.ReadFD()) {
 			return 0, &PortError{code: PortClosed}
 		}
-	return unix.Read(port.handle, p)
+		n, err := unix.Read(port.handle, p)
+		if err == unix.EINTR {
+			continue
+		}
+		return n, err
+	}
 }
 
 func (port *unixPort) Write(p []byte) (n int, err error) {