Allow subscriptions to decline to generate an invoice

Summary: This is a useful capability in Phacility for disabled/suspended instances.

Test Plan: Used `bin/phortune invoice` to invoice a disabled instance, saw it decline to invoice.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11837

src/applications/phortune/storage/PhortuneSubscription.php

@@ -207,6 +207,13 @@ final class PhortuneSubscription extends PhortuneDAO
       $end_epoch);
   }
 
+  public function shouldInvoiceForBillingPeriod($start_epoch, $end_epoch) {
+    return $this->getImplementation()->shouldInvoiceForBillingPeriod(
+      $this,
+      $start_epoch,
+      $end_epoch);
+  }
+
   public function getPurchaseName(
     PhortuneProduct $product,
     PhortunePurchase $purchase) {

src/applications/phortune/subscription/PhortuneSubscriptionImplementation.php

@@ -22,6 +22,13 @@ abstract class PhortuneSubscriptionImplementation {
     $start_epoch,
     $end_epoch);
 
+  public function shouldInvoiceForBillingPeriod(
+    PhortuneSubscription $subscription,
+    $start_epoch,
+    $end_epoch) {
+    return true;
+  }
+
   public function getCartName(
     PhortuneSubscription $subscription,
     PhortuneCart $cart) {

src/applications/phortune/worker/PhortuneSubscriptionWorker.php

@@ -8,6 +8,13 @@ final class PhortuneSubscriptionWorker extends PhabricatorWorker {
     $range = $this->getBillingPeriodRange($subscription);
     list($last_epoch, $next_epoch) = $range;
 
+    $should_invoice = $subscription->shouldInvoiceForBillingPeriod(
+      $last_epoch,
+      $next_epoch);
+    if (!$should_invoice) {
+      return;
+    }
+
     $account = $subscription->getAccount();
     $merchant = $subscription->getMerchant();
 
@@ -21,7 +28,6 @@ final class PhortuneSubscriptionWorker extends PhabricatorWorker {
     $cart_implementation = id(new PhortuneSubscriptionCart())
       ->setSubscription($subscription);
 
-
     // TODO: This isn't really ideal. It would be better to use an application
     // actor than the original author of the subscription. In particular, if
     // someone initiates a subscription, adds some other account managers, and