os, syscall: fix errno in Seek on windows

Current implementation use EPIPE as an error for Seek on pipes.
According to http://pubs.opengroup.org/onlinepubs/009695399/functions/lseek.html,
it should use ESPIPE instead.

Fixes #20066

Change-Id: I24c3b95be946bc19a287d6b10f447b034a9a1283
Reviewed-on: https://go-review.googlesource.com/41311
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/os_test.go

@@ -1286,6 +1286,32 @@ func TestSeek(t *testing.T) {
 	}
 }
 
+func TestSeekError(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9", "nacl":
+		t.Skipf("skipping test on %v", runtime.GOOS)
+	}
+
+	r, w, err := Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = r.Seek(0, 0)
+	if err == nil {
+		t.Fatal("Seek on pipe should fail")
+	}
+	if perr, ok := err.(*PathError); !ok || perr.Err != syscall.ESPIPE {
+		t.Errorf("Seek returned error %v, want &PathError{Err: syscall.ESPIPE}", err)
+	}
+	_, err = w.Seek(0, 0)
+	if err == nil {
+		t.Fatal("Seek on pipe should fail")
+	}
+	if perr, ok := err.(*PathError); !ok || perr.Err != syscall.ESPIPE {
+		t.Errorf("Seek returned error %v, want &PathError{Err: syscall.ESPIPE}", err)
+	}
+}
+
 type openErrorTest struct {
 	path  string
 	mode  int

src/syscall/syscall_windows.go

@@ -348,7 +348,7 @@ func Seek(fd Handle, offset int64, whence int) (newoffset int64, err error) {
 	// use GetFileType to check pipe, pipe can't do seek
 	ft, _ := GetFileType(fd)
 	if ft == FILE_TYPE_PIPE {
-		return 0, EPIPE
+		return 0, ESPIPE
 	}
 	rlo, e := SetFilePointer(fd, lo, &hi, w)
 	if e != nil {