coredump in `TryCopyLastError`

[hnwyllmm]

Describe the bug, including details regarding any error messages, version, and platform.

If Consumer throws exception, TryCopyLastError would copy exception message to C buffer. TryCopyLastError assume the string(byte[]) returned by lastError is '\0' terminated and test the length of the string by strlen which result in invalid memory access. In actually, lastError is a UTF8 encoded buffer(byte[]).

The code of TryCopyLastError:

  jobject error_data =
      env->GetObjectField(private_data->j_private_data_, kPrivateDataLastErrorField);
  ...
  auto arr = reinterpret_cast<jbyteArray>(error_data);
  jbyte* error_bytes = env->GetByteArrayElements(arr, nullptr);
  if (!error_bytes) {
    private_data->last_error_.clear();
    return;
  }

  char* error_str = reinterpret_cast<char*>(error_bytes);
  private_data->last_error_ = std::string(error_str, std::strlen(error_str));

The code below shows how lastError be setted:

private int setLastError(Throwable err) {
      // Do not let exceptions propagate up to JNI
      try {
        StringWriter buf = new StringWriter();
        PrintWriter writer = new PrintWriter(buf);
        err.printStackTrace(writer);
        lastError = buf.toString().getBytes(StandardCharsets.UTF_8);
      } catch (Throwable e) {
        // Bail out of setting the error message - we'll still return an error code
        lastError = null;
      }
      return 5; // = EIO
    }

[hnwyllmm]

I'd like to fix this issue.

[lidavidm]

PRs are welcome!