Provide precautionary handling for theoretical error scenario.

While testing payments I hit a bug where I tried to add a payment to a contribution with no financial items.

I never managed to replicate it again or determine how the payment came to be in that state but
it's been playing in my mind that people could get fatal errors if the financial_items don't exist
and dealing with those as regression reports will very tough. So my plan is
- for 5.20 add this extra routine to create it if it does not exist
- use this mechanism + more digging to figure out how legit an issue it is #15706
- in future releases 'get noisy' about having to create them if they don't exist
- eventually remove this routine

CRM/Financial/BAO/Payment.php

@@ -105,10 +105,18 @@ public static function create($params) {
       if ($value['allocation'] === (float) 0) {
         continue;
       }
+
+      if (!empty($ftIds[$value['price_field_value_id']])) {
+        $financialItemID = $ftIds[$value['price_field_value_id']];
+      }
+      else {
+        $financialItemID = self::getNewFinancialItemID($value, $params['trxn_date'], $contribution['contact_id'], $paymentTrxnParams['currency']);
+      }
+
       $eftParams = [
         'entity_table' => 'civicrm_financial_item',
         'financial_trxn_id' => $trxn->id,
-        'entity_id' => $ftIds[$value['price_field_value_id']],
+        'entity_id' => $financialItemID,
         'amount' => $value['allocation'],
       ];
 
@@ -413,6 +421,7 @@ private static function updateContributionStatus(int $contributionID, string $st
    * @param $params
    *
    * @return array
+   * @throws \CiviCRM_API3_Exception
    */
   protected static function getPayableLineItems($params): array {
     $lineItems = CRM_Price_BAO_LineItem::getLineItemsByContributionID($params['contribution_id']);
@@ -433,6 +442,8 @@ protected static function getPayableLineItems($params): array {
       $ratio = 0;
     }
     foreach ($lineItems as $lineItemID => $lineItem) {
+      // Ideally id would be set deeper but for now just add in here.
+      $lineItems[$lineItemID]['id'] = $lineItemID;
       $lineItems[$lineItemID]['paid'] = self::getAmountOfLineItemPaid($lineItemID);
       $lineItems[$lineItemID]['balance'] = $lineItem['subTotal'] - $lineItems[$lineItemID]['paid'];
 
@@ -503,4 +514,49 @@ protected static function reverseAllocationsFromPreviousPayment($params, $trxnID
     }
   }
 
+  /**
+   * Create a financial items & return the ID.
+   *
+   * Ideally this will never be called.
+   *
+   * However, I hit a scenario in testing where 'something' had  created a pending payment with
+   * no financial items and that would result in a fatal error without handling here. I failed
+   * to replicate & am not investigating via a new test methodology
+   * https://github.com/civicrm/civicrm-core/pull/15706
+   *
+   * After this is in I will do more digging & once I feel confident new instances are not being
+   * created I will add deprecation notices into this function with a view to removing.
+   *
+   * However, I think we want to add it in 5.20 as there is a risk of users experiencing an error
+   * if there is incorrect data & we need time to ensure that what I hit was not a 'thing.
+   * (it might be the demo site data is a bit flawed & that was the issue).
+   *
+   * @param array $lineItem
+   * @param string $trxn_date
+   * @param int $contactID
+   * @param string $currency
+   *
+   * @return int
+   *
+   * @throws \CiviCRM_API3_Exception
+   */
+  protected static function getNewFinancialItemID($lineItem, $trxn_date, $contactID, $currency): int {
+    $financialAccount = CRM_Financial_BAO_FinancialAccount::getFinancialAccountForFinancialTypeByRelationship(
+      $lineItem['financial_type_id'],
+      'Income Account Is'
+    );
+    $itemParams = [
+      'transaction_date' => $trxn_date,
+      'contact_id' => $contactID,
+      'currency' => $currency,
+      'amount' => $lineItem['line_total'],
+      'description' => $lineItem['label'],
+      'status_id' => 'Unpaid',
+      'financial_account_id' => $financialAccount,
+      'entity_table' => 'civicrm_line_item',
+      'entity_id' => $lineItem['id'],
+    ];
+    return (int) civicrm_api3('FinancialItem', 'create', $itemParams)['id'];
+  }
+
 }

tests/phpunit/api/v3/ContributionTest.php

@@ -2146,6 +2146,8 @@ public function testCheckTaxAmount($thousandSeparator) {
 
   /**
    * Test repeat contribution successfully creates line item.
+   *
+   * @throws \CRM_Core_Exception
    */
   public function testRepeatTransaction() {
     $originalContribution = $this->setUpRepeatTransaction($recurParams = [], 'single');

tests/phpunit/api/v3/PaymentTest.php

@@ -309,6 +309,29 @@ public function testCreatePaymentPendingOrderNoLineItems() {
     $this->validateAllPayments();
   }
 
+  /**
+   * Test that Payment.create does not fail if the line items are missing.
+   *
+   * In the original spec it was anticipated that financial items would not be created
+   * for pending contributions in some circumstances. We've backed away from this and
+   * I mostly could not find a way to do it through the UI. But I did seem to once &
+   * I want to be sure that if they ARE missing no fatal occurs so this tests
+   * that in an artificial way.
+   *
+   * @throws \CRM_Core_Exception
+   */
+  public function testAddPaymentMissingFinancialItems() {
+    $contribution = $this->callAPISuccess('Contribution', 'create', [
+      'total_amount' => 50,
+      'financial_type_id' => 'Donation',
+      'contact_id' => $this->individualCreate(),
+      'contribution_status_id' => 'Pending',
+    ]);
+    CRM_Core_DAO::executeQuery('DELETE FROM civicrm_financial_item');
+    $this->callAPISuccess('Payment', 'create', ['contribution_id' => $contribution['id'], 'payment_instrument_id' => 'Check', 'total_amount' => 5]);
+    $this->validateAllPayments();
+  }
+
   /**
    * Add participant with contribution
    *