Layout chaining should check for recursion

[fhemberger]

Describe the bug
I ran into a loop during build because I was carelessly using search & replace in my code. 😉

To Reproduce

1. Create a layout page.njk with the frontend matter

   ---
   layout: page.njk
   ---

2. Create a page using this layout

3. Run npx eleventy

Eleventy seemed to be happy to oblige and I just noticed my mistake because the build seemed to run forever. (I didn't wait for a memory issue to occur.)

Expected behavior
Either:

• Eleventy should detect accidental layout recursion and stop the build immediately with an error message.

Or:

• Eleventy should issue a performance warning after the n-th nested layout (n=10?) in general
• Eleventy should stop the build with an error message after the n-th nested layout (n=?)

Environment:

• OS and Version: macOS 10.15.4
• Eleventy Version: 0.10.0

[Ryuno-Ki]

I find the OR variant easier to realise technical (less bookkeeping). There are examples in the wild for it, e.g. preventing fork bombs.

[fhemberger]

Hmm, I'd rather say the first variant is easier to implement:

Have an array layouts, and whenever you process frontend matter with layout: ..., check if the layout is already in the layouts array stop immediately, otherwise store the layout name in the array.

And if you produce a loop within the template using the template language's include mechanisms, this should be handled by the template language parser itself.

[zachleat]

This was fixed with #2076 and is available on 2.0