MDL-50932 cache: fix bug in prescanned file store

Thanks to Mark Nielsen for providing a patch

cache/stores/file/lib.php

@@ -341,8 +341,8 @@ public function get($key) {
             $maxtime = cache::now() - $ttl;
         }
         $readfile = false;
-        if ($this->prescan && array_key_exists($key, $this->keys)) {
-            if (!$ttl || $this->keys[$filename] >= $maxtime && file_exists($file)) {
+        if ($this->prescan && array_key_exists($filename, $this->keys)) {
+            if ((!$ttl || $this->keys[$filename] >= $maxtime) && file_exists($file)) {
                 $readfile = true;
             } else {
                 $this->delete($key);

cache/stores/file/tests/file_test.php

@@ -44,4 +44,34 @@ class cachestore_file_test extends cachestore_tests {
     protected function get_class_name() {
         return 'cachestore_file';
     }
+
+    /**
+     * Testing cachestore_file::get with prescan enabled and with
+     * deleting the cache between the prescan and the call to get.
+     *
+     * The deleting of cache simulates some other process purging
+     * the cache.
+     */
+    public function test_cache_get_with_prescan_and_purge() {
+        global $CFG;
+
+        $definition = cache_definition::load_adhoc(cache_store::MODE_REQUEST, 'cachestore_file', 'phpunit_test');
+        $name = 'File test';
+
+        $path = make_cache_directory('cachestore_file_test');
+        $cache = new cachestore_file($name, array('path' => $path, 'prescan' => true));
+        $cache->initialise($definition);
+
+        $cache->set('testing', 'value');
+
+        $path  = make_cache_directory('cachestore_file_test');
+        $cache = new cachestore_file($name, array('path' => $path, 'prescan' => true));
+        $cache->initialise($definition);
+
+        // Let's pretend that some other process purged caches.
+        remove_dir($CFG->cachedir.'/cachestore_file_test', true);
+        make_cache_directory('cachestore_file_test');
+
+        $cache->get('testing');
+    }
 }