bug #39298 [Cache] Fixed incorrect usage of UNLINK with PHPRedis with Redis < 4.0 (wickex)

This PR was merged into the 5.2 branch.

Discussion
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[Cache] Fixed incorrect usage of UNLINK with PHPRedis with Redis < 4.0

| Q             | A
| ------------- | ---
| Branch?       | 5.2
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #39280
| License       | MIT

Currently, deleting cache keys is broken for users using PhpRedis with Redis-server < 4.0.0. The current implementation expects PhpRedis to throw an exception if the 'unlink'-function is used but unavailable, after which it's supposed to switch to using the 'del'-function. Using the 'unlink'-function on lower Redis-server versions seems to not throw an exception, but instead it appears to silently fail.

This pull request changes this behavior and checks the Redis-server version instead. If the version is 4.0 or higher, it uses the unlink function. If not, it uses the del-function.

Also see https://redis.io/commands/unlink > "Available since 4.0.0".

(Footnote: this is one of my first times contributing to an open-source project and my first time contributing to Symfony. I've tried following the guidelines, but please let me know if I missed anything. I'm unsure how I would go about unit testing this specific bugfix due to it being dependent on the Redis version, so I omitted it. Please let me know if a unit test is indeed required for this and if so, let me know if you have any suggestions on how to go about that.)

Commits
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9363f3b973 [Cache] Fixed incorrect usage of UNLINK with PHPRedis with Redis < 4.0

Author: nicolas-grekas

Session/Storage/Handler/RedisSessionHandler.php

@@ -93,15 +93,15 @@ protected function doDestroy(string $sessionId): bool
 
         if ($unlink) {
             try {
-                $this->redis->unlink($this->prefix.$sessionId);
-
-                return true;
+                $unlink = false !== $this->redis->unlink($this->prefix.$sessionId);
             } catch (\Throwable $e) {
                 $unlink = false;
             }
         }
 
-        $this->redis->del($this->prefix.$sessionId);
+        if (!$unlink) {
+            $this->redis->del($this->prefix.$sessionId);
+        }
 
         return true;
     }