CMF Watch Pro: Reassemble chunks larger than mtu

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/service/devices/cmfwatchpro/CmfCharacteristic.java

@@ -59,6 +59,7 @@ public class CmfCharacteristic {
     private int mtu = 247;
 
     private final Map<CmfCommand, ChunkBuffer> chunkBuffers = new HashMap<>();
+    private final ByteArrayOutputStream packetBuffer = new ByteArrayOutputStream();
 
     public CmfCharacteristic(final BluetoothGattCharacteristic bluetoothGattCharacteristic,
                              final Handler handler) {
@@ -197,15 +198,30 @@ public class CmfCharacteristic {
     }
 
     public void onCharacteristicChanged(final byte[] value) {
-        final ByteBuffer buf = ByteBuffer.wrap(value).order(ByteOrder.BIG_ENDIAN);
+        final ByteBuffer buf;
+        if (packetBuffer.size() > 0) {
+            // We're already in the middle of a packet
+            packetBuffer.write(value, 0, value.length);
+            buf = ByteBuffer.wrap(packetBuffer.toByteArray()).order(ByteOrder.BIG_ENDIAN);
+        } else {
+            buf = ByteBuffer.wrap(value).order(ByteOrder.BIG_ENDIAN);
+        }
 
         final byte header = buf.get();
         if (header != PAYLOAD_HEADER) {
             LOG.error("Unexpected first byte {}", String.format("0x%02x", header));
+            packetBuffer.reset();
             return;
         }
 
         final int payloadLength = buf.getShort();
+        if (payloadLength > buf.remaining()) {
+            LOG.debug("Got partial payload - waiting for more bytes");
+            // First chunk
+            packetBuffer.write(value, 0, value.length);
+            return;
+        }
+
         final int cmd1 = buf.getShort() & 0xFFFF;
         final int chunkCount = buf.getShort();
         final int chunkIndex = buf.getShort();
@@ -247,6 +263,8 @@ public class CmfCharacteristic {
                     chunkBuffers.remove(cmd);
                 }
                 return;
+            } finally {
+                packetBuffer.reset();
             }
         } else {
             payload = new byte[0];