FIX DataObject singleton creation (#8516)

Ensure DataObject instances are aware they are singletons so functions like populateDefaults() can be skipped. (fixes #4878)

Correctly applies https://github.com/silverstripe/silverstripe-framework/pull/7850 to the 4.x line
This has already been fixed in 3.x

src/Core/Injector/Injector.php

@@ -13,6 +13,7 @@ use SilverStripe\Core\ClassInfo;
 use SilverStripe\Core\Config\Config;
 use SilverStripe\Core\Environment;
 use SilverStripe\Dev\Deprecation;
+use SilverStripe\ORM\DataObject;
 
 /**
  * A simple injection manager that manages creating objects and injecting
@@ -581,6 +582,14 @@ class Injector implements ContainerInterface
             $constructorParams = $spec['constructor'];
         }
 
+        // If we're dealing with a DataObject singleton without specific constructor params, pass through Singleton
+        // flag as second argument
+        if ((!$type || $type !== self::PROTOTYPE)
+            && empty($constructorParams)
+            && is_subclass_of($class, DataObject::class)) {
+            $constructorParams = array(null, true);
+        }
+
         $factory = isset($spec['factory']) ? $this->get($spec['factory']) : $this->getObjectCreator();
         $object = $factory->create($class, $constructorParams);
 

tests/php/ORM/DataObjectTest.php

@@ -66,6 +66,51 @@ class DataObjectTest extends SapphireTest
         );
     }
 
+    /**
+     * @dataProvider provideSingletons
+     */
+    public function testSingleton($inst, $defaultValue, $altDefaultValue)
+    {
+        $inst = $inst();
+        // Test that populateDefaults() isn't called on singletons
+        // which can lead to SQL errors during build, and endless loops
+        if ($defaultValue) {
+            $this->assertEquals($defaultValue, $inst->MyFieldWithDefault);
+        } else {
+            $this->assertEmpty($inst->MyFieldWithDefault);
+        }
+
+        if ($altDefaultValue) {
+            $this->assertEquals($altDefaultValue, $inst->MyFieldWithAltDefault);
+        } else {
+            $this->assertEmpty($inst->MyFieldWithAltDefault);
+        }
+    }
+
+    public function provideSingletons()
+    {
+        // because PHPUnit evalutes test providers *before* setUp methods
+        // any extensions added in the setUp methods won't be available
+        // we must return closures to generate the arguments at run time
+        return array(
+            'create() static method' => array(function () {
+                return DataObjectTest\Fixture::create();
+            }, 'Default Value', 'Default Value'),
+            'New object creation' => array(function () {
+                return new DataObjectTest\Fixture();
+            }, 'Default Value', 'Default Value'),
+            'singleton() function' => array(function () {
+                return singleton(DataObjectTest\Fixture::class);
+            }, null, null),
+            'singleton() static method' => array(function () {
+                return DataObjectTest\Fixture::singleton();
+            }, null, null),
+            'Manual constructor args' => array(function () {
+                return new DataObjectTest\Fixture(null, true);
+            }, null, null),
+        );
+    }
+
     public function testDb()
     {
         $schema = DataObject::getSchema();