Invitation to comment and decide
In the BE forms it's possible to mount another mount point from a mount point form.
Neither the BE page tree nor the FE menus create mount point parameters that can deal with a recursive mount point.
To get a smooth user experience instead of broken root line exceptions, it should be clarified and communicated how to deal with recursive mount points.
 Prohibiting recursive mount points.
 Disabling recursive mount points by configuration
 Targeting the support of recursive mount points in all parts of TYPO3.
Are there use cases at all?
I discover only one.
Given directories/pages A, B, C, D, E, etc. mount a page P that is a mount point itself. At point P shared prepared subdirectories can be exchanged in a single point. M replaces N. Without recursive mount point the replacement with N is necessary in all of A, B, C, D, E, ...
Do you find more use cases? Are they worth to support recursive mount points? If not, how are they prohibited with useful feedback for the user?
1. Prohibiting recursive mount points.
The policy is to not support recursive mount points. A clear understandable exception is thrown. The BE forms shall inform the user in a suitable way, that recursive mount points are not supported.
2. Disabling recursive mount points by configuration
Like case 1, when disabled (the default).
When enabled, it's possible to create recursive mount points in the BE. It's up to the developer, to care that recursive mount points work in the FE, i.e. by creation of an extension.
3. Targeting the support of recursive mount points in all parts of TYPO3
All parts of TYPO3 shall be prepared to support recursive mount points:
- Root line loader
- FE Menus
- BE links
Pro and Cons
- The uses cases are that rare, that all support shall be dropped.
- Clear and easy.
- It's against the spirit of TYPO3 to disable features.
- There may be installations having a private implementation of it.
- Disabled behaviour is what most users want.
- Existing installations don't break when enabled..
- It's overhead to set up a configuration for such a rare feature.
- There are already to many configurations.
- It's a cool feature.
- It's in the spirit of TYPO3 to support such enterprise features.
- It's not that difficult to do.
- We love technical perfection.
- Nobody really needs it.
- Will never get done.
Remarks and notes
I basically proved that the resolution of recursive mount points is doable.
The URL of a recursive mount point requires multiple pairs of adjacent mount point parameters.
Here page 66 is mounted on 55, 55 on 44 and 44 on 33. If all mount point records set the overlay setting, the content of page 66 is propagated up to page 33.
It would be doable to create this kind of parameters in page tree and menu URLs.
(If a page is mounted from multiple pages the preview button in the page tree can only create one of the possible mount point combinations. The combinations determine the breadcrumb in the FE, if multiple alternative paths to the root page are given.)
The current resolution of the mount point only supports a single MP pair, likely causing the issues with recursive records in the BE page tree.
Topic Initiator: Elmar Hinz