bug symfony#16312 [HttpKernel] clearstatcache() so the Cache sees when a .lck file has been released (mpdude)

This PR was squashed before being merged into the 2.3 branch (closes symfony#16312).

Discussion
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[HttpKernel] clearstatcache() so the Cache sees when a .lck file has been released

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#15813
| License       | MIT
| Doc PR        | n/a

I've been trying to debug symfony#15813 and modified the Store in a way to keep unique request IDs in the .lck file. That way, I was hoping to find out which request is blocking and/or if the request is actually still running.

It turned out that `is_file()` would claim that a lock file still exists, but a subsequent attempt to read the information from that file returned "file not found" errors.

So, my assumption is that the `is_file()` result is based on the fstat cache and wrong once a process has seen the lock file.

@jakzal said in symfony#15813 (comment) that `unlink()`ing the lock file should clear the statcache, but I doubt this is true across PHP processes.

Commits
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982710f [HttpKernel] clearstatcache() so the Cache sees when a .lck file has been released

Author: fabpot

src/Symfony/Component/HttpKernel/HttpCache/Store.php

@@ -110,7 +110,10 @@ public function unlock(Request $request)
 
     public function isLocked(Request $request)
     {
-        return is_file($this->getPath($this->getCacheKey($request).'.lck'));
+        $path = $this->getPath($this->getCacheKey($request).'.lck');
+        clearstatcache(true, $path);
+
+        return is_file($path);
     }
 
     /**