minor #35079 [Serializer] add array cache in front of serializer cach…

…e (bastnic)

This PR was merged into the 5.1-dev branch.

Discussion
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[Serializer] add array cache in front of serializer cache

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | performance
| New feature?  | ...
| Deprecations? | no
| Tickets       | #35041
| License       | MIT
| Doc PR        |

This is another implementation of #35041, after #35046.

The DI solution is cleaner, but 10ms slower on my scenario (13 vs 3) and this data is not supposed to change at all (even in the original method there is a local cache).

Diff from master (without warmup):

![image](https://user-images.githubusercontent.com/84887/71313592-2cb5be00-243b-11ea-8983-b30abb7d0698.png)

Diff form master (with warmup):

![image](https://user-images.githubusercontent.com/84887/71313633-8e762800-243b-11ea-8044-003c810d3c0b.png)

Diff from #35046:

![image](https://user-images.githubusercontent.com/84887/71313668-312ea680-243c-11ea-9768-8531f4f33f3c.png)

Ping @nicolas-grekas

Commits
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6ee306b [Cache] add array cache in front of serializer cache

src/Symfony/Component/Serializer/Mapping/Factory/CacheClassMetadataFactory.php

@@ -32,6 +32,8 @@ class CacheClassMetadataFactory implements ClassMetadataFactoryInterface
      */
     private $cacheItemPool;
 
+    private $loadedClasses = [];
+
     public function __construct(ClassMetadataFactoryInterface $decorated, CacheItemPoolInterface $cacheItemPool)
     {
         $this->decorated = $decorated;
@@ -44,18 +46,23 @@ public function __construct(ClassMetadataFactoryInterface $decorated, CacheItemP
     public function getMetadataFor($value)
     {
         $class = $this->getClass($value);
+
+        if (isset($this->loadedClasses[$class])) {
+            return $this->loadedClasses[$class];
+        }
+
         // Key cannot contain backslashes according to PSR-6
         $key = strtr($class, '\\', '_');
 
         $item = $this->cacheItemPool->getItem($key);
         if ($item->isHit()) {
-            return $item->get();
+            return $this->loadedClasses[$class] = $item->get();
         }
 
         $metadata = $this->decorated->getMetadataFor($value);
         $this->cacheItemPool->save($item->set($metadata));
 
-        return $metadata;
+        return $this->loadedClasses[$class] = $metadata;
     }
 
     /**