From 5370bc8af68646634396a9d13202b39b6c3719b2 Mon Sep 17 00:00:00 2001
From: Dylan Wagstaff <dylan@silverstripe.com>
Date: Wed, 4 Jul 2018 14:43:01 +1200
Subject: [PATCH] FIX apply SubsiteID getVar to CMS Preview fetches

A preview link must be loaded on the same domain the CMS is loaded
through, which was previously causing issues when a page (identified
via URLSegment) did not exist on the subsite domain. By _always_
prepending the identifier to the preview link, this should never happen.
---
 docs/en/technical.md                |  2 +-
 javascript/LeftAndMain_Subsites.js  |  2 +-
 src/Extensions/SiteTreeSubsites.php | 28 +++++++++++++++++++++-------
 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/docs/en/technical.md b/docs/en/technical.md
index cb705f1..27bfbe4 100644
--- a/docs/en/technical.md
+++ b/docs/en/technical.md
@@ -28,5 +28,5 @@ This method is called when a pages are being copied between the main site or ano
 ### alternateAbsoluteLink
 This method modifies the absolute link to contain the valid subsite domain
 
-### alternatePreviewLink
+### updatePreviewLink
 This method modifies the preview link for the CMS.
diff --git a/javascript/LeftAndMain_Subsites.js b/javascript/LeftAndMain_Subsites.js
index b1541ba..8dd2b24 100644
--- a/javascript/LeftAndMain_Subsites.js
+++ b/javascript/LeftAndMain_Subsites.js
@@ -101,7 +101,7 @@
 
 			/**
 			 * Update links and forms with GET/POST SubsiteID param, so we remaing on the current subsite.
-			 * The initial link for the iframe comes from SiteTreeSubsites::alternatePreviewLink.
+			 * The initial link for the iframe comes from SiteTreeSubsites::updatePreviewLink.
 			 *
 			 * This is done so we can use the CMS domain for displaying previews so we prevent single-origin
 			 * violations and SSL cert problems that come up when iframing content from a different URL.
diff --git a/src/Extensions/SiteTreeSubsites.php b/src/Extensions/SiteTreeSubsites.php
index 8a7aef1..488fef6 100644
--- a/src/Extensions/SiteTreeSubsites.php
+++ b/src/Extensions/SiteTreeSubsites.php
@@ -382,17 +382,31 @@ class SiteTreeSubsites extends DataExtension
 
     /**
      * Use the CMS domain for iframed CMS previews to prevent single-origin violations
-     * and SSL cert problems.
-     * @param null $action
+     * and SSL cert problems. Always set SubsiteID to avoid errors because a page doesn't
+     * exist on the CMS domain.
+     *
+     * @param string &$link
+     * @param string|null $action
+     * @return string
+     */
+    public function updatePreviewLink(&$link, $action = null)
+    {
+        $url = Director::absoluteURL($this->owner->Link($action));
+        $link = HTTP::setGetVar('SubsiteID', $this->owner->SubsiteID, $url);
+        return $link;
+    }
+
+    /**
+     * This function is marked as deprecated for removal in 5.0.0 in silverstripe/cms
+     * so now simply passes execution to where the functionality exists for backwards compatiblity.
+     *
+     * @param string|null $action
      * @return string
      */
     public function alternatePreviewLink($action = null)
     {
-        $url = Director::absoluteURL($this->owner->Link());
-        if ($this->owner->SubsiteID) {
-            $url = HTTP::setGetVar('SubsiteID', $this->owner->SubsiteID, $url);
-        }
-        return $url;
+        $link = '';
+        return $this->updatePreviewLink($link, $action);
     }
 
     /**