From 9f7aaa8ee4cbd8c36c5cfb6e55e5366be4ff238c Mon Sep 17 00:00:00 2001
From: epriestley <git@epriestley.com>
Date: Tue, 1 Oct 2019 18:46:56 -0700
Subject: [PATCH] Fix an issue where any diff which could possibly be rendered
 as Jupyter decided to render as Jupyter

Summary:
See PHI1468. Engine selection for diffs is currently too aggressive in trying to find a shared engine and will fall back a shared engine with a very low score, causing all ".json" files to render as Jupyter files.

Only pick an engine as a difference engine by default if it's the highest-scoring engine for the new file.

Test Plan: Viewed ".json" files and ".ipynb" files in a revision. Before, both rendered as Jupyter. Now, the former rendered as JSON and the latter rendered as Jupyter.

Differential Revision: https://secure.phabricator.com/D20850
---
 .../parser/DifferentialChangesetParser.php        | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/applications/differential/parser/DifferentialChangesetParser.php b/src/applications/differential/parser/DifferentialChangesetParser.php
index a485a787cb..6c4ed3d14f 100644
--- a/src/applications/differential/parser/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/DifferentialChangesetParser.php
@@ -1718,7 +1718,8 @@ final class DifferentialChangesetParser extends Phobject {
       $viewer,
       $new_ref);
 
-    $shared_engines = array_intersect_key($old_engines, $new_engines);
+    $shared_engines = array_intersect_key($new_engines, $old_engines);
+    $default_engine = head_key($new_engines);
 
     foreach ($shared_engines as $key => $shared_engine) {
       if (!$shared_engine->canDiffDocuments($old_ref, $new_ref)) {
@@ -1734,7 +1735,17 @@ final class DifferentialChangesetParser extends Phobject {
         $document_engine = null;
       }
     } else {
-      $document_engine = head($shared_engines);
+      // If we aren't rendering with a specific engine, only use a default
+      // engine if the best engine for the new file is a shared engine which
+      // can diff files. If we're less picky (for example, by accepting any
+      // shared engine) we can end up with silly behavior (like ".json" files
+      // rendering as Jupyter documents).
+
+      if (isset($shared_engines[$default_engine])) {
+        $document_engine = $shared_engines[$default_engine];
+      } else {
+        $document_engine = null;
+      }
     }
 
     if ($document_engine) {