Method CAS

kv/memberlist/memberlist_client.go:1295–1343

CAS implements Compare-And-Set/Swap operation. CAS expects that value returned by 'f' function implements Mergeable interface. If it doesn't, CAS fails immediately. This method combines Compare-And-Swap with Merge: it calls 'f' function to get a new state, and then merges this new state into curre

(ctx context.Context, key string, codec codec.Codec, f func(in interface{}) (out interface{}, retry bool, err error))

// detect removals. If Merge doesn't result in any change (returns nil), then operation fails and is retried again.
// After too many failed retries, this method returns error.
func (m *KV) CAS(ctx context.Context, key string, codec codec.Codec, f func(in interface{}) (out interface{}, retry bool, err error)) error {
	var lastError error

outer:
	for retries := m.maxCasRetries; retries > 0; retries-- {
		m.casAttempts.Inc()

		if lastError == errNoChangeDetected {
			// We only get here, if 'f' reports some change, but Merge function reports no change. This can happen
			// with Ring's merge function, which depends on timestamps (and not the tokens) with 1-second resolution.
			// By waiting for one second, we hope that Merge will be able to detect change from 'f' function.

			select {
			case <-time.After(noChangeDetectedRetrySleep):
				// ok
			case <-ctx.Done():
				lastError = ctx.Err()
				break outer
			}
		}

		change, newver, retry, deleted, updated, err := m.trySingleCas(key, codec, f)
		if err != nil {
			level.Debug(m.logger).Log("msg", "CAS attempt failed", "err", err, "retry", retry)

			lastError = err
			if !retry {
				break
			}
			continue
		}

		if change != nil {
			m.casSuccesses.Inc()
			m.notifyWatchers(key)

			m.broadcastNewValue(key, change, newver, codec, true, deleted, updated)
		}

		return nil
	}
	if errors.Is(lastError, errVersionMismatch) {
		// Version mismatch err on CAS would have been retried up to the limit
		lastError = errors.Wrap(lastError, "too many retries")
	}

	m.casFailures.Inc()
	return fmt.Errorf("failed to CAS-update key %s: %v", key, lastError)
}

// returns change, error (or nil, if CAS succeeded), and whether to retry or not.
// returns errNoChangeDetected if merge failed to detect change in f's output.