"The host entity cannot be changed" - error saving non-current version

Problem/Motivation

I have a node, with revisioning enabled.
It has a paragraphs field.
That paragraphs field has a field collection field.
In some circumstances when saving the exception:

The host entity cannot be changed.

Is thrown by field collection.

This appears to be because field collection looks to see if the host entity is the default revision, which it is, because the paragraph has been saved. This then forces the field collection item to think that it is also the default revision, but it isn't. So fetching the host details fails.

Similar issues have been noted in

• #2382089: Logic issue with fetchHostDetails() causes false "the host entity cannot be changed" error in updateHostEntity()
• #2658882: "the host entity cannot be changed" error with VBO
• #2550061: Creating new draft of node sets 'archived' flag of field collection to FALSE and 'default_revision' to TRUE
• #2352047: Creating or updating non-standard revisions leads to data loss
• #2514800: fetchHostDetails() (and thus 'host_entity' property) don't work when both host and FC use revisions

Proposed resolution

Add part of the patch give in #2658882-27: "the host entity cannot be changed" error with VBO.

Remaining tasks

Review! Write tests.

User interface changes

None

API changes

None

Data model changes

None