Replace serialization approach with recursive comp

Signed-off-by: Josh <josh.t.richards@gmail.com>

lib/private/Files/Cache/Scanner.php

@@ -222,15 +222,7 @@ public function scanFile($file, $reuseExisting = 0, $parentId = -1, $cacheData =
 
 						// Only update metadata that has changed
 						// i.e. get all the values in $data that are not present in the cache already
-						// NOTE: we serialize then unserialize here because array_diff_assoc() doesn't 
-						// support multidimensional arrays on its own (and otherwise internally casts any 
-						// embedded array elements to attempt to compare them - not only generating warnings 
-						// like "Array to string conversion" but also, as a resut, overlooking real differences)
-						$newData = array_diff_assoc(
-							array_map('serialize', $data), 
-							array_map('serialize', $cacheData->getData())
-							);
-						$newData = array_map('unserialize', $newData);
+						$newData = $this->array_diff_assoc_multi($data, $cacheData->getData());
 
 						// make it known to the caller that etag has been changed and needs propagation
 						if (isset($newData['etag'])) {
@@ -378,6 +370,64 @@ public function scan($path, $recursive = self::SCAN_RECURSIVE, $reuse = -1, $loc
 		return $data;
 	}
 
+	/**
+   	 * Compares $array1 against $array2 and returns all the values in $array1 that are not in $array2
+	 * Note this is a one-way check - i.e. we don't care about things that are in $array2 that aren't in $array1
+	 *
+	 * Supports multi-dimensional arrays
+  	 * Also checks keys/indexes
+	 * Comparisons are strict just like array_diff_assoc
+	 * Order of keys/values does not matter
+	 *
+  	 * @param array $array1
+	 * @param array|? $array2
+	 * @return array with the differences between $array1 and $array1
+  	 * @throws \InvalidArgumentException if $array1 isn't an actual array 
+   	 *
+	 */
+	protected function array_diff_assoc_multi($array1, $array2) {
+		if (!is_array($array1)) {
+			// this ain't gonna work
+			throw new \InvalidArgumentException('$array1 must be an array!');
+		}
+
+		if (!is_array($array2)) {
+			// everything is different
+			return $array1;
+		}
+
+    	$result = [];
+
+    	foreach ($array1 as $key => $value) {
+
+			// if $array2 doesn't have the same key, that's a result
+			if (!array_key_exists($key, $array2)) {
+        		$result[$key] = $value;
+				continue;
+			}
+
+			// if $array2's value for the same key is different, that's a result
+			if ($array2[$key] !== $value && !is_array($value)) {
+        		$result[$key] = $value;
+				continue;
+			}
+
+			if (is_array($value)) {
+    			$nestedDiff = $this->array_diff_assoc_multi($value, $array2[$key]);
+		    	if (!empty($nestedDiff)) {
+		        	$result[$key] = $nestedDiff;
+		        	continue;
+		   		}
+			} else { // not an array
+            	if ($array2[$key] != $value) {
+                	$result[$key] = $value;
+                	continue;
+            	}
+			}
+    	}
+    	return $result;
+	}
+
 	/**
 	 * Get the children currently in the cache
 	 *