bug symfony#21211 Classloader tmpname (lyrixx)

This PR was merged into the 2.7 branch.

Discussion
----------

Classloader tmpname

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In dev env:

from:

> Notice: tempnam(): file created in the system's temporary directory

to:

> Failed to write cache file "/var/www/html/var/cache/dev/classes.php".

Commits
-------

3c887da [ClassLoader] Throw an exception if the cache is not writeable

Author: fabpot

src/Symfony/Component/ClassLoader/ClassCollectionLoader.php

@@ -275,7 +275,13 @@ private static function compressCode($code)
      */
     private static function writeCacheFile($file, $content)
     {
-        $tmpFile = tempnam(dirname($file), basename($file));
+        $dir = dirname($file);
+        if (!is_writable($dir)) {
+            throw new \RuntimeException(sprintf('Cache directory "%s" is not writable.', $dir));
+        }
+
+        $tmpFile = tempnam($dir, basename($file));
+
         if (false !== @file_put_contents($tmpFile, $content) && @rename($tmpFile, $file)) {
             @chmod($file, 0666 & ~umask());