Add an action to link sessions to customer ids.

plugins/woocommerce/changelog/issue-44852-session-to-login-action

@@ -0,0 +1,4 @@
+Significance: minor
+Type: add
+
+Add the `woocommerce_guest_session_to_user_id` action hook that fires when a guest session is migrated to a customer ID

plugins/woocommerce/includes/class-wc-session-handler.php

@@ -1,4 +1,4 @@
 <?php
 /**
  * Handle data for the current customers session.
  * Implements the WC_Session abstract class.
@@ -108,6 +108,20 @@
 				$this->_dirty       = true;
 				$this->save_data( $guest_session_id );
 				$this->set_customer_session_cookie( true );
+
+				/**
+				 * Fires after a customer has logged in, and their guest session id has been
+				 * deleted with its data migrated to a customer id.
+				 *
+				 * This hook gives extensions the chance to connect the old session id to the
+				 * customer id, if the key is being used externally.
+				 *
+				 * @since 8.8.0
+				 *
+				 * @param string $guest_session_id The former session ID, as generated by `::generate_customer_id()`.
+				 * @param int    $customer_id      The Customer ID that the former session was converted to.
+				 */
+				do_action( 'woocommerce_guest_session_to_user_id', $guest_session_id, $this->_customer_id );
 			}
 
 			// Update session if its close to expiring.