fix(pdu): set COMPRESSION_USED on the FastPath update header when compressed (#1382)

Author: glamberson

crates/ironrdp-pdu/src/basic_output/fast_path/mod.rs

@@ -141,11 +141,15 @@ impl Encode for FastPathUpdatePdu<'_> {
         let mut header = 0u8;
         header.set_bits(0..4, self.update_code.as_u8());
         header.set_bits(4..6, self.fragmentation.as_u8());
+        if self.compression_flags.is_some() {
+            // The COMPRESSION_USED bit must be set on the header byte before it
+            // is written, so the decoder knows a compression flags byte follows.
+            header.set_bits(6..8, Compression::COMPRESSION_USED.bits());
+        }
 
         dst.write_u8(header);
 
         if self.compression_flags.is_some() {
-            header.set_bits(6..8, Compression::COMPRESSION_USED.bits());
             let compression_flags_with_type =
                 self.compression_flags.map(|f| f.bits()).unwrap_or(0) | self.compression_type.map_or(0, |f| f.as_u8());
             dst.write_u8(compression_flags_with_type);

crates/ironrdp-pdu/src/basic_output/fast_path/tests.rs

@@ -196,3 +196,23 @@ fn palette_decode_with_code_returns_palette_variant() {
         other => panic!("Expected Palette variant, got: {other:?}"),
     }
 }
+
+#[test]
+fn compressed_update_round_trips() {
+    // The encoder must set the COMPRESSION_USED bit on the update header when
+    // compression flags are present, otherwise the decoder does not consume the
+    // trailing compression flags byte and misreads the data length.
+    let data = [0xAAu8; 8];
+    let pdu = FastPathUpdatePdu {
+        fragmentation: Fragmentation::Single,
+        update_code: UpdateCode::SurfaceCommands,
+        compression_flags: Some(CompressionFlags::COMPRESSED),
+        compression_type: Some(CompressionType::K64),
+        data: &data,
+    };
+
+    let mut buffer = vec![0u8; pdu.size()];
+    encode(&pdu, buffer.as_mut_slice()).unwrap();
+
+    assert_eq!(pdu, decode::<FastPathUpdatePdu<'_>>(&buffer).unwrap());
+}