PHPLIB-95: Massage findAndModify null results before 3.0

Earlier server versions incorrectly return an empty document instead of null. Thankfully, we can detect this edge case given the command result and input options.

Author: jmikola

src/Collection.php

@@ -534,7 +534,7 @@ public function findOneAndReplace(array $filter, array $replacement, array $opti
 
         $doc = current($this->_runCommand($this->dbname, $cmd)->toArray());
         if ($doc["ok"]) {
-            return is_object($doc["value"]) ? (array) $doc["value"] : $doc["value"];
+            return $this->_massageFindAndModifyResult($doc, $options);
         }
 
         throw $this->_generateCommandException($doc);
@@ -572,7 +572,7 @@ public function findOneAndUpdate(array $filter, array $update, array $options =
 
         $doc = current($this->_runCommand($this->dbname, $cmd)->toArray());
         if ($doc["ok"]) {
-            return is_object($doc["value"]) ? (array) $doc["value"] : $doc["value"];
+            return $this->_massageFindAndModifyResult($doc, $options);
         }
 
         throw $this->_generateCommandException($doc);
@@ -1179,6 +1179,36 @@ final protected function _massageFindAndModifyOptions($options, $update = array(
         return $ret;
     }
 
+    /**
+     * Internal helper for massaging the findAndModify result.
+     *
+     * @internal
+     * @param array $result
+     * @param array $options
+     * @return array|null
+     */
+    final protected function _massageFindAndModifyResult(array $result, array $options)
+    {
+        if ($result['value'] === null) {
+            return null;
+        }
+
+        /* Prior to 3.0, findAndModify returns an empty document instead of null
+         * when an upsert is performed and the pre-modified document was
+         * requested.
+         */
+        if ($options['upsert'] && ! $options['new'] &&
+            isset($result['lastErrorObject']->updatedExisting) &&
+            ! $result['lastErrorObject']->updatedExisting) {
+
+            return null;
+        }
+
+        return is_object($result["value"])
+            ? (array) $result['value']
+            : $result['value'];
+    }
+
     /**
      * Constructs the Query Wire Protocol field 'flags' based on $options
      * provided to other helpers