base: Introduce .write_x() for unavailable data

Introduce a method that can be used when the stub needs to send back
data that is unavailable to the GDB client. In that case, the GDB
protocol expects literal `'x'` characters to be sent (one per nibble).

Note: no functional change intended.

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>

src/protocol/response_writer.rs

@@ -212,6 +212,14 @@ impl<'a, C: Connection + 'a> ResponseWriter<'a, C> {
         Ok(())
     }
 
+    /// Marks data of 'size' bytes as unavailable.
+    pub fn write_x(&mut self, size: usize) -> Result<(), Error<C::Error>> {
+        for _ in 0..(size * 2) {
+            self.write(b'x')?;
+        }
+        Ok(())
+    }
+
     /// Write a number as a big-endian hex string using the most compact
     /// representation possible (i.e: trimming leading zeros).
     pub fn write_num<D: BeBytes + PrimInt>(&mut self, digit: D) -> Result<(), Error<C::Error>> {

src/stub/core_impl/base.rs

@@ -202,7 +202,7 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
                 regs.gdb_serialize(|val| {
                     let res = match val {
                         Some(b) => res.write_hex_buf(&[b]),
-                        None => res.write_str("xx"),
+                        None => res.write_x(std::mem::size_of::<u8>()),
                     };
                     if let Err(e) = res {
                         err = Err(e);