Add hooks to act on a new revision being created

+++ b/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
@@ -237,6 +237,8 @@ public function createRevision(RevisionableInterface $entity, $default = TRUE, $
+    $original_keep_untranslatable_fields = $keep_untranslatable_fields;

@@ -307,6 +309,11 @@ public function createRevision(RevisionableInterface $entity, $default = TRUE, $
+    $arguments = [$new_revision, $entity, $original_keep_untranslatable_fields];
+    $this->moduleHandler()->invokeAll($this->entityTypeId . '_revision_create', $arguments);
+    $this->moduleHandler()->invokeAll('entity_revision_create', $arguments);

+++ b/core/lib/Drupal/Core/Entity/entity.api.php
@@ -909,6 +909,54 @@ function hook_ENTITY_TYPE_create(\Drupal\Core\Entity\EntityInterface $entity) {
+ *   default revision (FALSE) when generating a merged revision. If no value was
+ *   explicitly specified (NULL), a default value of TRUE should be assumed if
+ *   the provided entity is the default translation and untranslatable fields
+ *   should only affect the default translation, FALSE otherwise.
...
+ *   default revision (FALSE) when generating a merged revision. If no value was
+ *   explicitly specified (NULL), a default value of TRUE should be assumed if
+ *   the provided entity is the default translation and untranslatable fields
+ *   should only affect the default translation, FALSE otherwise.

The value of $keep_untranslatable_fields might change in the function body before the hooks are executed. We provide a documentation of the hook, which is explaining this, but this makes the whole pretty complex for developers. We could instead skip documenting that NULL should be assumed as TRUE and instead of calling the hooks with value of NULL we call them with value of TRUE if we've changed the value in the function body before calling the hooks. What do you think about this?

Well then probably we have to pass both values to the hook - the original one and the newly computed one, which has been used by the method.

@hchonov:

Tentatively marking this RTBC, feel free to unmark if you still have concerns.

I think we need a change record to announce the new hooks.

@alexpott:

An all-encompassing CR for the parent meta issue is available at https://www.drupal.org/node/2975280, I guess we should publish it once the meta itself is fixed, right?

@plach thanks for adding this issue to the CR :) https://www.drupal.org/node/2975280/revisions/view/10983402/10990108

Yep, that was missing :)

1. +++ b/core/lib/Drupal/Core/Entity/TranslatableRevisionableStorageInterface.php
   @@ -20,7 +20,9 @@
       * @param bool|null $keep_untranslatable_fields
       *   (optional) Whether untranslatable field values should be kept or copied
   -   *   from the default revision when generating a merged revision.
   +   *   from the default revision when generating a merged revision. Defaults to
   +   *   TRUE if the provided entity is the default translation and untranslatable
   +   *   fields should only affect the default translation, FALSE otherwise.
   

   This description doesn't explain the NULL state.

2. +++ b/core/lib/Drupal/Core/Entity/entity.api.php
   @@ -909,6 +909,54 @@ function hook_ENTITY_TYPE_create(\Drupal\Core\Entity\EntityInterface $entity) {
   + *   Whether untranslatable field values were kept (TRUE) or copied from the
   + *   default revision (FALSE) when generating a merged revision. If no value was
   + *   explicitly specified (NULL), a default value of TRUE should be assumed if
   + *   the provided entity is the default translation and untranslatable fields
   + *   should only affect the default translation, FALSE otherwise.
   

   Is there anyway we can remove the NULL state and set it to either TRUE or FALSE according to the logic specified in this comment?

3. I'm leaving this at RTBC because I'm not sure what is possible but the docs confuse me on first reading. I have not looked at in-depth to work out what they are describing.

It sounds like this is a blocker for contrib, so adding that tag.

I created https://www.drupal.org/node/2985957 as a change record with a copy of the part of https://www.drupal.org/node/2975280 that is relevant for this issue. Ideally all issues would be committed and that big CR published, but if there is no CR for this and the rest cannot make it, then we are not allowed to commit this one either. So let's make that possible at least :) We make this CR a redirect later or replace its body with a link to the bigger CR once it becomes obsolete.

Also giving all the credits possible here. Also carried over credits from #2960253: [meta] Allow Paragraphs widget/field and similar use cases to to be considered translatable.

Thanks for the explanation @berdir, makes sense to me.

Committed 1e8a4de and pushed to 8.7.x and merged to 8.6.x. Thanks!

@Gábor Hojtsy:

Thanks for committing this, a separate CR makes sense for now, I just wanted to point out that the only other core ticket mentioned in the parent CR is #2826021: FieldItemList::equals is sufficient from the storage perspective but not for code checking for changes, which has stalled but seemed very close at a certain point, so we might still be able to fix core parts before alpha.

To me it seems like this hook is not always fired when creating new revisions, I made a new issue about it here #3031405: New hook_entity_revision_create() is not always fiered when new revisions are created