Handle malformed exception response PDU in ModbusTcpClient (#123)

Strict interpretation of the spec would dictate that the response PDU is malformed if it's not exactly 2 bytes, but Modbus implementations in the wild are notoriously crap, so this approach that looks for at least 2 bytes might be a little safer and friendlier.

Author: kevinherron

modbus/src/main/java/com/digitalpetri/modbus/client/ModbusTcpClient.java

@@ -7,6 +7,7 @@
 import com.digitalpetri.modbus.exceptions.ModbusExecutionException;
 import com.digitalpetri.modbus.exceptions.ModbusResponseException;
 import com.digitalpetri.modbus.exceptions.ModbusTimeoutException;
+import com.digitalpetri.modbus.internal.util.Hex;
 import com.digitalpetri.modbus.pdu.ModbusPdu;
 import com.digitalpetri.modbus.pdu.ModbusRequestPdu;
 import com.digitalpetri.modbus.pdu.ModbusResponsePdu;
@@ -181,17 +182,19 @@ private void onFrameReceived(ModbusTcpFrame frame) {
       int functionCode = buffer.get(buffer.position()) & 0xFF;
 
       if (functionCode == promise.functionCode) {
-        try {
-          promise.future.complete(buffer);
-        } catch (Exception e) {
-          promise.future.completeExceptionally(e);
-        }
+        promise.future.complete(buffer);
       } else if (functionCode == promise.functionCode + 0x80) {
-        buffer.get(); // skip FC byte
-        int exceptionCode = buffer.get() & 0xFF;
-
-        promise.future.completeExceptionally(
-            new ModbusResponseException(promise.functionCode, exceptionCode));
+        if (buffer.remaining() >= 2) {
+          buffer.get(); // skip FC byte
+          int exceptionCode = buffer.get() & 0xFF;
+
+          promise.future.completeExceptionally(
+              new ModbusResponseException(promise.functionCode, exceptionCode));
+        } else {
+          promise.future.completeExceptionally(
+              new ModbusException(
+                  "malformed exception response PDU: %s".formatted(Hex.format(buffer))));
+        }
       } else {
         promise.future.completeExceptionally(
             new ModbusException("unexpected function code: 0x%02X".formatted(functionCode)));

modbus/src/test/java/com/digitalpetri/modbus/client/ModbusTcpClientTest.java

@@ -38,6 +38,30 @@ void emptyResponsePdu() {
     assertEquals("empty response PDU", cause.getMessage());
   }
 
+  /**
+   * Tests handling of a malformed exception response PDU containing only the function code | 0x80
+   * and missing the required exception code byte.
+   */
+  @Test
+  void malformedExceptionResponsePdu() {
+    var transport = new TestTransport();
+    var client = ModbusTcpClient.create(transport);
+
+    // Send a request with function code 0x04
+    CompletionStage<byte[]> cs = client.sendRawAsync(1, new byte[] {0x04, 0x03, 0x00, 0x00, 0x01});
+
+    // Receive a malformed exception response: only 1 byte (0x84), no exception code
+    transport.frameReceiver.accept(
+        new ModbusTcpFrame(
+            new MbapHeader(0, 1, 1, 1), ByteBuffer.wrap(new byte[] {(byte) 0x84})));
+
+    ExecutionException ex =
+        assertThrows(ExecutionException.class, () -> cs.toCompletableFuture().get());
+
+    ModbusException cause = (ModbusException) ex.getCause();
+    assertEquals("malformed exception response PDU: 84", cause.getMessage());
+  }
+
   private static class TestTransport implements ModbusTcpClientTransport {
 
     boolean connected = false;