Add basic function for updating a contribution (eg. the trxn_id) without touching other things

CRM/Core/Payment/MJWIPNTrait.php

@@ -248,11 +248,27 @@ trait CRM_Core_Payment_MJWIPNTrait {
       $paymentParams['contribution_id'] = $contribution['id'];
       $paymentParams['is_send_contribution_notification'] = $this->getSendEmailReceipt();
       $paymentParams['skipCleanMoney'] = TRUE;
-
-      $payment = civicrm_api3('Mjwpayment', 'create_payment', $paymentParams);
+      civicrm_api3('Mjwpayment', 'create_payment', $paymentParams);
     }
   }
 
+  /**
+   * @param array $params
+   *
+   * @throws \CiviCRM_API3_Exception
+   * @throws \Civi\Payment\Exception\PaymentProcessorException
+   */
+  private function updateContribution($params) {
+    $this->checkRequiredParams('updateContribution', ['contribution_id'], $params);
+    $params['id'] = $params['contribution_id'];
+    unset($params['contribution_id']);
+    $params['skipCleanMoney'] = TRUE;
+    $params['skipRecentView'] = TRUE;
+    $params['skipLineItem'] = TRUE;
+    $params['is_post_payment_create'] = TRUE;
+    civicrm_api3('Contribution', 'create', $params);
+  }
+
   /**
    * Complete a pending contribution and update associated entities (recur/membership)
    *