[Twisted-Python] Strange recursion error with twisted.web

[Glyph]

> On Mar 9, 2021, at 4:54 AM, Peter Westlake <peter.westlake at pobox.com> wrote:
> 
> I'm getting a "maximum recursion depth exceeded" error that appears to be coming from flatten(). The odd thing is that it only happens sometimes. The HTML that's being flattened does have a few Deferreds in it. Those come from function calls, which cache the results, which might explain why I only see the error on the first visit to the page (as far as I can tell). 
> 
> The system recursion limit is the standard 1000. My HTML is only nested a few tags deep, two orders of magnitude short of that. Is there anything about the way flatten() works that might cause this behaviour?

flatten() can definitely result in some deep recursive stacks, particularly in combination with synchronous Deferreds which have their own accumulating stack costs. I'd be interested to see a minimal reproducer for this though, I'm sure we could do a lot better.

-g