src/main/java/software/amazon/awssdk/crt/eventstream/ClientConnection.java [67:99]:
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            if (errorCode == 0) {
                messageFlush.complete(null);
            } else {
                messageFlush.completeExceptionally(new CrtRuntimeException(errorCode));
            }
        });

        return messageFlush;
    }

    /**
     * Sends a protocol message on the connection. Callback will be invoked upon the message flushing to the underlying
     * transport
     *
     * @param headers List of event-stream headers. Can be null.
     * @param payload Payload to send for the message. Can be null.
     * @param messageType Message type for the rpc message.
     * @param messageFlags Union of message flags from MessageFlags.getByteValue()
     * @param callback will be invoked upon the message flushing to the underlying transport
     */
    public void sendProtocolMessage(final List<Header> headers, final byte[] payload,
                                    final MessageType messageType, int messageFlags, MessageFlushCallback callback) {
        if (isNull()) {
            throw new IllegalStateException("close() has already been called on this object.");
        }

        byte[] headersBuf = headers != null ? Header.marshallHeadersForJNI(headers) : null;

        int result = sendProtocolMessage(getNativeHandle(), headersBuf, payload, messageType.getEnumValue(), messageFlags, callback);

        if (result != 0) {
            int errorCode = CRT.awsLastError();
            throw new CrtRuntimeException(errorCode);
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src/main/java/software/amazon/awssdk/crt/eventstream/ServerConnection.java [67:98]:
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                if (errorCode == 0) {
                    messageFlush.complete(null);
                } else {
                    messageFlush.completeExceptionally(new CrtRuntimeException(errorCode));
                }
            }
        });

        return messageFlush;
    }

    /**
     * Sends a protocol message on the connection. Returns a completable future for synchronizing on the message
     * flushing to the underlying transport.
     * @param headers List of event-stream headers. Can be null.
     * @param payload Payload to send for the message. Can be null.
     * @param messageType Message type for the rpc message.
     * @param messageFlags Union of message flags from MessageFlags.getByteValue()
     * @param callback invoked upon the message flushing to the underlying transport.
     */
    public void sendProtocolMessage(final List<Header> headers, final byte[] payload,
                                    final MessageType messageType, int messageFlags, MessageFlushCallback callback) {
        if (isNull()) {
            throw new IllegalStateException("close() has already been called on this object.");
        }
        byte[] headersBuf = headers != null ? Header.marshallHeadersForJNI(headers) : null;

        int result = sendProtocolMessage(getNativeHandle(), headersBuf, payload, messageType.getEnumValue(), messageFlags, callback);

        if (result != 0) {
            int errorCode = CRT.awsLastError();
            throw new CrtRuntimeException(errorCode);
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