Replaced queryFileType with IsPipe

This is the only functionality we actually use. Mocking a boolean is
easier as we don't leak Windows' internals.

Author: EduardGomezEscandell

exec.go

@@ -367,8 +367,8 @@ func (c *Cmd) readerDescriptor(r io.Reader) (f *os.File, err error) {
 	}
 
 	if f, ok := r.(*os.File); ok {
-		ft, err := queryFileType(f)
-		if err == nil && ft == fileTypePipe {
+		isPipe, err := IsPipe(f)
+		if err == nil && isPipe {
 			// It's a pipe: no need to create our own pipe.
 			return f, nil
 		}
@@ -410,8 +410,8 @@ func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) {
 	}
 
 	if f, ok := w.(*os.File); ok {
-		ft, err := queryFileType(f)
-		if err == nil && ft == fileTypePipe {
+		isPipe, err := IsPipe(f)
+		if err == nil && isPipe {
 			// It's a pipe: no need to create our own pipe.
 			return f, nil
 		}

win32_linux.go

@@ -10,8 +10,10 @@ import (
 	"github.com/ubuntu/decorate"
 )
 
-func queryFileType(f *os.File) (fileType, error) {
-	return 0, errors.New("not implemented")
+// IsPipe checks if a file's descriptor is a pipe vs. any other type of object.
+// This implementation will always fail on Linux.
+func IsPipe(f *os.File) (bool, error) {
+	return false, errors.New("not implemented")
 }
 
 func wslConfigureDistribution(distributionName string, defaultUID uint32, wslDistributionFlags wslFlags) (err error) {

win32_windows.go

@@ -28,15 +28,23 @@ var (
 )
 
 // Windows' typedefs.
-type wBOOL = int // Windows' BOOL
-type char = byte // Windows' CHAR (which is the same as C's char)
+type wBOOL = int            // Windows' BOOL
+type char = byte            // Windows' CHAR (which is the same as C's char)
+const fileTypePipe = 0x0003 // Windows' FILE_TYPE_PIPE
 
-func queryFileType(f *os.File) (fileType, error) {
+// IsPipe checks if a file's descriptor is a pipe vs. any other type of object.
+func IsPipe(f *os.File) (bool, error) {
 	n, err := windows.GetFileType(windows.Handle(f.Fd()))
 	if err != nil {
-		return fileTypeUnknown, err
+		return false, err
 	}
-	return fileType(n), nil
+
+	var isPipe bool
+	if n == fileTypePipe {
+		isPipe = true
+	}
+
+	return isPipe, nil
 }
 
 // wslLaunch replaces os.StartProcess with WSL commands.