PriceProvider: Add volatile to cachedResult field

[alvasw]

The get() and put() calls can happen on different threads. The volatile keyword makes sure that all updates to cachedResult are visible to all threads.

Relates to #33.

[ghost]

I disagree with the assertion "can happen on different threads" since a provider is run on a single TimerTask thread.

Regardless, this change seems benign.

ACK.

[alvasw]

Every ExchangeRateProvider starts its own Timer running on its own background thread. When many clients call the REST API at the same time, several threads from the server thread pool handle the requests concurrently. Without the volatile keyword these threads could return a stale state because the last updates are not visible to them yet.

Timer / Background Thread Creation:

bisq-pricenode/src/main/java/bisq/price/PriceProvider.java

Lines 69 to 81 in 6073a39

	timer.scheduleAtFixedRate(new TimerTask() {
	@Override
	public void run() {
	try {
	refresh();
	} catch (Throwable t) {
	// we only log scheduled calls to refresh that fail to ensure that
	// the application does *not* halt, assuming the failure is temporary
	// and on the side of the upstream price provider, eg. BitcoinAverage
	log.warn("refresh failed", t);
	}
	}
	}, refreshInterval.toMillis(), refreshInterval.toMillis());

[ghost]

ACK

[gabernard]

utACK