In JSON DocumentEngine, preserve the distinction between "{}" and "[]"

Summary:
Ref T13635. Currently, the JSON DocumentEngine uses "phutil_json_decode()", but this can confuse "{}" and "[]".

Be more careful about how the JSON value is decoded, to preserve the distinction.

Test Plan: {F8520479}

Maniphest Tasks: T13635

Differential Revision: https://secure.phabricator.com/D21605

src/__phutil_library_map__.php

···
     'PhabricatorDocumentEngineBlock' => 'applications/files/diff/PhabricatorDocumentEngineBlock.php',
     'PhabricatorDocumentEngineBlockDiff' => 'applications/files/diff/PhabricatorDocumentEngineBlockDiff.php',
     'PhabricatorDocumentEngineBlocks' => 'applications/files/diff/PhabricatorDocumentEngineBlocks.php',
+    'PhabricatorDocumentEngineParserException' => 'applications/files/document/exception/PhabricatorDocumentEngineParserException.php',
     'PhabricatorDocumentRef' => 'applications/files/document/PhabricatorDocumentRef.php',
     'PhabricatorDocumentRenderingEngine' => 'applications/files/document/render/PhabricatorDocumentRenderingEngine.php',
     'PhabricatorDoorkeeperApplication' => 'applications/doorkeeper/application/PhabricatorDoorkeeperApplication.php',
···
     'PhabricatorDocumentEngineBlock' => 'Phobject',
     'PhabricatorDocumentEngineBlockDiff' => 'Phobject',
     'PhabricatorDocumentEngineBlocks' => 'Phobject',
+    'PhabricatorDocumentEngineParserException' => 'Exception',
     'PhabricatorDocumentRef' => 'Phobject',
     'PhabricatorDocumentRenderingEngine' => 'Phobject',
     'PhabricatorDoorkeeperApplication' => 'PhabricatorApplication',

src/applications/files/document/PhabricatorJSONDocumentEngine.php

···
     try {
       $data = phutil_json_decode($raw_data);
 
+      // See T13635. "phutil_json_decode()" always turns JSON into a PHP array,
+      // and we lose the distinction between "{}" and "[]". This distinction is
+      // important when rendering a document.
+      $data = json_decode($raw_data, false);
+      if (!$data) {
+        throw new PhabricatorDocumentEngineParserException(
+          pht(
+            'Failed to "json_decode(...)" JSON document after successfully '.
+            'decoding it with "phutil_json_decode(...).'));
+      }
+
       if (preg_match('/^\s*\[/', $raw_data)) {
         $content = id(new PhutilJSON())->encodeAsList($data);
       } else {
···
       $message = $this->newMessage(
         pht(
           'This document is not valid JSON: %s',
+          $ex->getMessage()));
+
+      $content = $raw_data;
+    } catch (PhabricatorDocumentEngineParserException $ex) {
+      $message = $this->newMessage(
+        pht(
+          'Unable to parse this document as JSON: %s',
           $ex->getMessage()));
 
       $content = $raw_data;

src/applications/files/document/exception/PhabricatorDocumentEngineParserException.php

···
+<?php
+
+final class PhabricatorDocumentEngineParserException
+  extends Exception {}