[release/9.0] Avoid infinite recursion on identifying shadow FKs

[AndriySvyryd]

Fixes #34875

Description
When a user configures a foreign key property without specifying the type or whether it's required EF creates a shadow nullable property by default.

1. If the entity type in question is owned or is the join entity type in a many-to-many relationship, EF will configure the FK properties as the primary key (i.e. an identifying FK).
2. This will mark the properties as required and trigger a convention to change their types to be non-nullable.
3. This triggers a convention to check again whether the new property types correspond to their nullability, but the condition relies only on the requiredness of the FKs defined on the properties, so it decided that that should be nullable instead.
4. After the properties are made nullable again the key is reapplied on them, leading to step 2

EF throws a custom exception instead of a StackOverflow because the convention implementation unwraps the recursive calls.

Customer impact
For affected models, an exception is thrown whenever the context is used. There are several workarounds for the reported scenario, but identifying shadow FKs are pretty common, so this can potentially affect a significant portion of users.

How found
Customer reported on 8.0.10

Regression
Yes, from 8.0.8. This issue is old, but it was made easier to hit by #34388

Testing
Test added.

Risk
Low.

[SamMonoRT]

approved via email