Doctrine eager loading should handle cycle / circular references

[teohhanhui]

I don't remember why it was not done. Is there some reason why it's not possible?

We already use visited tracking when creating the subresource operations:

core/src/Operation/Factory/SubresourceOperationFactory.php

Lines 98 to 100 in c20b86c

	if (isset($visited[$visiting])) {
	continue;
	}

Related: #876, #1910

[carlobeltrame]

I have had a look at this issue, and I have opened a PR #4377 to improve the situation a little.

Visited tracking does not work for the EagerLoadingExtension. Consider the following case:
A book may be written by one or more authors, and an author may have one or more books, so we have a ManyToMany relation. Now when eager loading this relation, we start at one book and add a join for its authors. The Author entity has a relation back to Book, but we can't ignore this just because we have visited Book before. The author may have more books, and these books may have more authors, and so on. For this reason, it's no use to keep track of the entities or relations we have visited.

One thing we can do is: With ToOne relations, we can avoid going back to the parent. So assume a book can now only have one author. We join from Book to Author, and from the Author to all their Books. Now these books all have the same author that we already have in the query, so there is no need to add another join. I implemented this in #4377.