Add a safeguard against multiple objects competing for the same ident…

…ity map entry

While trying to understand #3037, I found that it may happen that we have more entries in `\Doctrine\ORM\UnitOfWork::$entityIdentifiers` than in `\Doctrine\ORM\UnitOfWork::$identityMap`.

The former is a mapping from `spl_object_id` values to ID hashes, the latter an array first of entity class names and then from ID hash to entity object instances.

(Basically, "ID hash" is a concatenation of all field values making up the `@Id` for a given entity.)

This means that at some point, we must have _different_ objects representing the same entity.

I don't think it makes sense to overwrite an entity in the identity map, since that may leave `\Doctrine\ORM\UnitOfWork::$entityIdentifiers` in an inconsistent state.

If it makes sense at all to _replace_ an entity, that should happen through dedicated management methods to first detach the old entity before persisting, merging or otherwise adding the new one. This way we could make sure the internal structures remain consistent.

lib/Doctrine/ORM/UnitOfWork.php

@@ -1569,6 +1569,14 @@ public function addToIdentityMap($entity)
         $className     = $classMetadata->rootEntityName;
 
         if (isset($this->identityMap[$className][$idHash])) {
+            if ($this->identityMap[$className][$idHash] !== $entity) {
+                throw new RuntimeException(sprintf(
+                    'While adding an entity of class %s with an ID hash of "%s" to the identity map, another object was already present for the same ID. This exception is a safeguard against an internal inconsistency - IDs should uniquely map to entity object instances. This problem may occur if you create proxy instances directly using the ProxyFactory, instead of using \Doctrine\ORM\EntityManagerInterface::getReference(). Otherwise, it might be an ORM-internal inconsistency, please report it.',
+                    $className,
+                    $idHash
+                ));
+            }
+
             return false;
         }
 

tests/Doctrine/Tests/ORM/Functional/ProxiesLikeEntitiesTest.php

@@ -95,7 +95,7 @@ public function testEntityWithIdentifier(): void
      */
     public function testProxyAsDqlParameterPersist(): void
     {
-        $proxy     = $this->_em->getProxyFactory()->getProxy(CmsUser::class, ['id' => $this->user->getId()]);
+        $proxy     = $this->_em->getReference(CmsUser::class, ['id' => $this->user->getId()]);
         $proxy->id = $this->user->getId();
         $result    = $this
             ->_em

tests/Doctrine/Tests/ORM/Functional/Ticket/DDC1238Test.php

@@ -56,7 +56,7 @@ public function testIssueProxyClear(): void
 
         $user2 = $this->_em->getReference(DDC1238User::class, $userId);
 
-        $user->__load();
+        //$user->__load();
 
         self::assertIsInt($user->getId(), 'Even if a proxy is detached, it should still have an identifier');
 

tests/Doctrine/Tests/ORM/UnitOfWorkTest.php

@@ -108,6 +108,8 @@ protected function setUp(): void
         $driverConnection = $this->createMock(Driver\Connection::class);
         $driverConnection->method('prepare')
             ->willReturn($driverStatement);
+        $driverConnection->method('lastInsertId')
+            ->willReturnOnConsecutiveCalls(1, 2, 3, 4, 5, 6);
 
         $driver = $this->createMock(Driver::class);
         $driver->method('getDatabasePlatform')