Add IsCygwinTerminal to check cygwin/msys2 terminal

Thanks to @k-takata

Close #8

_example/example.go

@@ -2,13 +2,16 @@ package main
 
 import (
 	"fmt"
-	"github.com/mattn/go-isatty"
 	"os"
+
+	"github.com/mattn/go-isatty"
 )
 
 func main() {
 	if isatty.IsTerminal(os.Stdout.Fd()) {
 		fmt.Println("Is Terminal")
+	} else if isatty.IsCygwinTerminal(os.Stdout.Fd()) {
+		fmt.Println("Is Cygwin/MSYS2 Terminal")
 	} else {
 		fmt.Println("Is Not Terminal")
 	}

isatty_not_windows.go

@@ -0,0 +1,9 @@
+// +build !windows appengine
+
+package isatty
+
+// IsCygwinTerminal() return true if the file descriptor is a cygwin or msys2
+// terminal. This is also always false on this environment.
+func IsCygwinTerminal(fd uintptr) bool {
+	return false
+}

isatty_windows.go

@@ -4,12 +4,30 @@
 package isatty
 
 import (
+	"strings"
 	"syscall"
+	"unicode/utf16"
 	"unsafe"
 )
 
-var kernel32 = syscall.NewLazyDLL("kernel32.dll")
-var procGetConsoleMode = kernel32.NewProc("GetConsoleMode")
+const (
+	fileNameInfo uintptr = 2
+	fileTypePipe         = 3
+)
+
+var (
+	kernel32                         = syscall.NewLazyDLL("kernel32.dll")
+	procGetConsoleMode               = kernel32.NewProc("GetConsoleMode")
+	procGetFileInformationByHandleEx = kernel32.NewProc("GetFileInformationByHandleEx")
+	procGetFileType                  = kernel32.NewProc("GetFileType")
+)
+
+func init() {
+	// Check if GetFileInformationByHandleEx is available.
+	if procGetFileInformationByHandleEx.Find() != nil {
+		procGetFileInformationByHandleEx = nil
+	}
+}
 
 // IsTerminal return true if the file descriptor is terminal.
 func IsTerminal(fd uintptr) bool {
@@ -17,3 +35,37 @@ func IsTerminal(fd uintptr) bool {
 	r, _, e := syscall.Syscall(procGetConsoleMode.Addr(), 2, fd, uintptr(unsafe.Pointer(&st)), 0)
 	return r != 0 && e == 0
 }
+
+// IsCygwinTerminal() return true if the file descriptor is a cygwin or msys2
+// terminal. This is also always false on this environment.
+func IsCygwinTerminal(fd uintptr) bool {
+	if procGetFileInformationByHandleEx == nil {
+		return false
+	}
+
+	// Cygwin/msys's pty is a pipe.
+	ft, _, e := syscall.Syscall(procGetFileType.Addr(), 1, fd, 0, 0)
+	if ft != fileTypePipe || e != 0 {
+		return false
+	}
+
+	var buf [2 + syscall.MAX_PATH]uint16
+	r, _, e := syscall.Syscall6(procGetFileInformationByHandleEx.Addr(),
+		4, fd, fileNameInfo, uintptr(unsafe.Pointer(&buf)),
+		uintptr(len(buf)*2), 0, 0)
+	if r == 0 || e != 0 {
+		return false
+	}
+
+	l := *(*uint32)(unsafe.Pointer(&buf))
+	name := string(utf16.Decode(buf[2 : 2+l/2]))
+	if !strings.HasPrefix(name, `\msys-`) && !strings.HasPrefix(name, `\cygwin-`) {
+		return false
+	}
+
+	token := strings.Split(name, "-")
+	if len(token) < 3 || !strings.HasPrefix(token[2], "pty") {
+		return false
+	}
+	return true
+}