Error with hash when importing referenced entities in multiple feeds

Problem/Motivation

I am importing .csv data that includes references to another entity. I am using the following setup and no tampers
“Reference by: Title
Autocreate entities: No”

Here is error I am getting on import when I have an entity with the title that does not exist

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /batch?id=3846&op=do_nojs&op=do
StatusText: error
ResponseText: The website encountered an unexpected error. Try again later.Error: Attempt to assign property "hash" on null in Drupal\feeds\Feeds\Target\EntityReference->setTarget() (line 127 of modules/contrib/feeds/src/Feeds/Target/EntityReference.php). Drupal\feeds\Feeds\Processor\EntityProcessorBase->map() (Line: 302)
Drupal\feeds\Feeds\Processor\EntityProcessorBase->process() (Line: 128)
Drupal\feeds\EventSubscriber\LazySubscriber->Drupal\feeds\EventSubscriber\{closure}()
call_user_func() (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 41)
Drupal\feeds\FeedsExecutable->dispatchEvent() (Line: 267)
Drupal\feeds\FeedsExecutable->doProcess() (Line: 111)
Drupal\feeds\FeedsExecutable->processItem() (Line: 297)
_batch_process() (Line: 139)
_batch_do() (Line: 95)
_batch_page() (Line: 52)
Drupal\system\Controller\BatchController->batchPage()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 637)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)

This happens only when:

• More than one feed is used to update the same content.
• Mapping to Feeds Item is defined earlier than mapping to Entity Reference.
• The source provides a value for the entity reference field, but Feeds cannot find an existing entity for it.

It is caused by a bug in the FeedsItem mapping target. What the target currently does when it receives a value to import (for example a 'guid' value), it writes that to the first row of the feeds_item field and it destroys all other rows on the feeds_item field. Not only this may set a 'guid' value on the wrong row, the feeds_item field may also loose a reference to the current feed.

This results into the fatal error later on the EntityReference mapping target. The current code expects that the feeds_item field contains a reference to the current feed, but if that value was destroyed earlier by the FeedsItem mapping target then this is not the case and it results into the reported error.

Steps to reproduce

• Create two feeds that will update the same content.
• For the feed type that the second feed uses, add mappings to the feeds_item field and mapping to an entity reference field. Add mapping to the feeds_item field first, so that it occurs before the mapping to the entity reference field.
• For the entity reference value in the second feed, use a value for which no entity can be found.
• Import the first feed.
• Import the second feed.

Proposed resolution

When setting a value for the feeds_item field, the FeedsItem mapping target should look for the row that contains a reference to the current feed and update that row. It should not assume that the first row should be updated.

Remaining tasks