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!