server: use os.IsNotExist to map sshFxNoSuchFile

Always use os.IsNotExist to identify any OS specific error types that
represent missing files or directories.  This resolves an issue on
Windows where some system errors (ENOTDIR) were not being identified as
'not found' errors and mapped to sshFxNoSuchFile.

fixes pkg#381

server.go

@@ -594,7 +594,9 @@ func statusFromError(p ider, err error) sshFxpStatusPacket {
 		ret.StatusError.Code = translateErrno(e)
 	case *os.PathError:
 		debug("statusFromError,pathError: error is %T %#v", e.Err, e.Err)
-		if errno, ok := e.Err.(syscall.Errno); ok {
+		if os.IsNotExist(err) {
+			ret.StatusError.Code = sshFxNoSuchFile
+		} else if errno, ok := e.Err.(syscall.Errno); ok {
 			ret.StatusError.Code = translateErrno(errno)
 		}
 	case fxerr:

server_test.go

@@ -344,3 +344,18 @@ func TestOpenStatRace(t *testing.T) {
 	os.Remove(tmppath)
 	checkServerAllocator(t, server)
 }
+
+// Ensure that proper error codes are returned for non existent files, such
+// that they are mapped back to a 'not exists' error on the client side.
+func TestStatNonExistent(t *testing.T) {
+	client, server := clientServerPair(t)
+	defer client.Close()
+	defer server.Close()
+
+	for _, file := range []string{"/doesnotexist", "/doesnotexist/a/b"} {
+		_, err := client.Stat(file)
+		if err == nil || !os.IsNotExist(err) {
+			t.Errorf("expected 'does not exist' err for file %q.  got: %v", file, err)
+		}
+	}
+}