Content translation metadata is only being saved when using the content translation form

As far as I have seen, there is no way of knowing the source of the translation if it not explicitly given, as content translation does in the form.
So I guess this is a won't fix, and needs to be set in the calling code.

@facine reported a problem when trying to create a translation with image fields via API:

use Drupal\node\Entity\Node;
use Drupal\file\Entity\File;
$images = [
  'sites/default/files/article/my_image_001.jpg',
  'sites/default/files/article/my_image_002.jpg',
  'sites/default/files/article/my_image_003.jpg',
];
$files = [];
foreach ($images as $image) {
  $file = File::create([
    'uid' => 1,
    'uri' => 'public://' . $image,
    'status' => 1,
  ]);
  $file->save();
  $files[] = $file->id();
}
$node = Node::create([
  'type' => 'article',
  'langcode' => 'en',
  'created' => REQUEST_TIME,
  'changed' => REQUEST_TIME,
  'uid' => 1,
  'title' => 'My title',
  'body' => [
    'summary' => '',
    'value' => '<p>My body</p>',
    'format' => 'full_html',
  ],
]);
foreach ($files as $image) {
  $node->field_images[] = [
    'target_id' => $image,
    'alt' => 'My image alt',
    'title' => 'My image title',
  ];
}
$node->save();
\Drupal::service('path.alias_storage')->save('/node/' . $node->id(), '/my/node/path', 'en');
$node_es = $node->addTranslation('es');
$node_es->title = 'Mi título';
$node_es->body->value = '<p>Mi cuerpo</p>';
$node_es->body->format = 'full_html';
/**
 * This foreach give me this error:
Drupal\Core\Entity\EntityStorageException: Invalid translation language (und) specified. in Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 757 of core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).
Drupal\content_translation\FieldTranslationSynchronizer->synchronizeFields(Object, 'es', 'und')
content_translation_entity_presave(Object)
call_user_func_array('content_translation_entity_presave', Array)
Drupal\Core\Extension\ModuleHandler->invokeAll('entity_presave', Array)
Drupal\Core\Entity\EntityStorageBase->invokeHook('presave', Object)
Drupal\Core\Entity\ContentEntityStorageBase->invokeHook('presave', Object)
Drupal\Core\Entity\EntityStorageBase->doPreSave(Object)
Drupal\Core\Entity\ContentEntityStorageBase->doPreSave(Object)
Drupal\Core\Entity\EntityStorageBase->save(Object)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->save(Object)
Drupal\Core\Entity\Entity->save()
eval()
Drupal\devel\Form\ExecutePHP->submitForm(Array, Object)
call_user_func_array(Array, Array)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object)
Drupal\Core\Form\FormBuilder->processForm('devel_execute_form', Array, Object)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)
 */
foreach ($files as $image) {
  $node_es->field_images[] = [
    'target_id' => $image,
    'alt' => 'Alt de mi imagen',
    'title' => 'Título de mi imagen',
  ];
}
$node_es->save();
\Drupal::service('path.alias_storage')->save("/node/" . $node->id(),  '/mi/ruta', 'es');

As said before, it should be responsability of the calling code to set the source before saving (which fixes the problem he was having):

$node_es->set('content_translation_source', 'en');

However, I'm wondering if instead of failing, we should ignore it if not set and do something like:

diff --git a/core/modules/content_translation/content_translation.module b/core/modules/content_translation/content_translation.module
index ff1b34d..dfed390 100644
--- a/core/modules/content_translation/content_translation.module
+++ b/core/modules/content_translation/content_translation.module
@@ -406,7 +406,9 @@ function content_translation_entity_presave(EntityInterface $entity) {
     /** @var \Drupal\content_translation\ContentTranslationManagerInterface $manager */
     $manager = \Drupal::service('content_translation.manager');
     $source_langcode = !$entity->original->hasTranslation($langcode) ? $manager->getTranslationMetadata($entity)->getSource() : NULL;
-    \Drupal::service('content_translation.synchronizer')->synchronizeFields($entity, $langcode, $source_langcode);
+    if ($source_langcode !== LanguageInterface::LANGCODE_NOT_SPECIFIED) {
+      \Drupal::service('content_translation.synchronizer')->synchronizeFields($entity, $langcode, $source_langcode);
+    }
   }
 }

Thoughts?

What was confusing to me is that the behavior translating with the API without content_translation was different than translating with the API with content_translation enabled. But yes, the difference is that we don't take file sync into account in the former case.
Changing the source language would be an API change, so I will close this issue for now. It will help people searching for the error message though, thanks! :)

I happen to find this issue myself (glad I knew where to look).

      $translation->set('content_translation_source', $entity->getUntranslated()->language()->getId());

before save() fixed it, but I'm more convinced now that we should default to the source language.

Wondering if this is acceptable for a minor release or should wait for 8.1.x.

I doubt this is a major.

@mohit_rocks Not sure why I posted that here, but IMHO it's unrelated to the OP (hard to know years later). When I created this issue my goals were more aligned with what #11 is attempting.