cense->canReceiveProductUpdates() ) {
				return $result;
			}
		}

		$status = $license->getStatus();
		$messages = array(
			'no_license_yet' => "Please enter your license key to enable plugin updates.",
			'expired' => sprintf(
				'Your access to %s updates has expired. Please renew your license.',
				apply_filters('wslm_product_name-' . $this->slug, $this->slug)
			)
		);

		$result = new WP_Error(
			'wslm_update_not_available',
			isset($messages[$status]) ? $messages[$status] : 'Update not available. Please (re)enter your license key.',
			'[' . $status . ']'
		);

		//This bit is important. At least in WP 4.3, the return value will be lost or replaced with a generic
		//"download failed" error unless you also set it on the upgrader skin.
		$upgrader->skin->set_result($result);

		return $result;
	}

	public function autoActivateLicense() {
	    $doingCron = defined('DOING_CRON') && DOING_CRON;
		if ( !$this->currentUserCanManageLicense() && !$doingCron ) {
			return;
		}

		$license = $this->licenseManager->getLicense();
		if ( $license->isValid() ) {
			return;
		}

		$failureFlag = self::ACTIVATION_FAILURE_FLAG . $this->slug;
		$state = get_site_option($failureFlag, null);
		if ( !is_array($state) || !isset($state['failures']) ) {
			$state = array(
				'failures' => $state ? 1 : 0,
                'lastAttemptTime' => 0,
			);
        }

		if ( ($state['failures'] > $this->maxAutoActivationAttempts) ) {
			return;
		}

		$elapsedTime = time() - $state['lastAttemptTime'];
		$desiredDelay = $this->calculateLicenseActivationDelay($state['failures']);
		if ( $elapsedTime < $desiredDelay ) {
		    return;
        }

		$state['failures']++;
		$state['lastAttemptTime'] = time();
		update_site_option($failureFlag, $state);

		$result = null;
		$tokenHistory = $this->licenseManager->getTokenHistory();
		if ( !empty($this->keyConstant) && defined($this->keyConstant) ) {
			//Attempt to activate the license key that's defined in wp-config.php.
			$result = $this->licenseManager->licenseThisSite(constant($this->keyConstant));
		} else if ( !empty($tokenHistory) ) {
			//Check if there's a known token that matches the current site URL. Try to activate that token.
			$possibleToken = array_search($this->licenseManager->getSiteUrl(), array_reverse($tokenHistory, true));
			if ( !empty($possibleToken) ) {
				$result = $this->licenseManager->licenseThisSiteByToken($possibleToken);
			}
		}

		if ( is_wp_error($result) ) {
		    $productName = apply_filters('wslm_product_name-' . $this->slug, $this->slug);
		    if ( is_admin() && !$doingCron ) {
			    printf(
				    '<div class="error">
                        <p>
                            %1$s tried to automatically activate your license, but it didn\'t work.<br/>
                            Error: <code>%2$s [%3$s]</code>
                        </p>
                        <p>Please go to the <a href="%4$s">Plugins</a> page and enter your license key.</p>
                    </div>',
				    $productName,
				    $result->get_error_message(),
				    $result->get_error_code(),
				    is_multisite() ? network_admin_url('plugins.php') : admin_url('plugins.php')
			    );
		    }

			$plainError = sprintf(
				'%1$s failed to automatically activate a license. Error: %2$s [%3$s]',
				$productName,
				$result->get_error_message(),
				$result->get_error_code()
			);
		    error_log($plainError);

			//Try again later, but only if we have a license key.
			if (
				($state['failures'] < $this->maxAutoActivationAttempts)
				&& !empty($this->keyConstant)
				&& defined($this->keyConstant)
			) {
				wp_schedule_single_event(
					time() + $this->calculateLicenseActivationDelay($state['failures']) + 1,
					self::AUTO_LICENSE_CRON_ACTION . $this->slug,
                    array($state['failures'] + 1)
				);
			}
		} else if ( $result instanceof Wslm_ProductLicense ) {
			//Success! Don't output anything, just proceed as normal.
			$this->clearActivationFailureFlag();
		}
	}

	/**
     * Calculate the minimum delay after the Nth automatic license
     * activation attempt.
     *
	 * @param int $attempt
	 * @return float|int Delay in seconds.
	 */
	private function calculateLicenseActivationDelay($attempt) {
		if ( $attempt < 1 ) {
			return 0;
		}

	    $minDelay = 20;
	    $maxDelay = 600;
	    $growthLimit = 4; //Stop increasing the delay after this many attempts.

        $fraction = (min($attempt - 1, $growthLimit) / ($growthLimit));

		$desiredDelay = $minDelay + round(($maxDelay - $minDelay) * $fraction);
		if ( $desiredDelay < $minDelay ) {
		    $desiredDelay = $minDelay;
        }
		return $desiredDelay;
	}

	public function clearActivationFailureFlag() {
	    delete_site_option(self::ACTIVATION_FAILURE_FLAG . $this->slug);
	}
}