Proxying setup delivering wrong cache entry in some edge cases

[benzimmer]

We've been using Nginx as a caching proxy for quite a while in different
scenarios now. Since a few weeks and especially in the last couple of days
we continue to encounter a strange behaviour in one of our scenarios leading
to wrong content being delivered.

In that case we use Nginx as a caching proxy for a bunch of subdomains on a
kind of multitenancy application. We established the setup 4 months ago and
never had any problems until recently.

For example, a request to https://test.example.org/bla/fasel would deliver
the content for https://foo.example.org/bla/fasel. So basically it delivers
content for the wrong subdomain. Those occasions are very, very rare and
totally random in regards to the subdomain from which the content gets
delivered.

We currently use openresty 1.13.6.1.

Our config is quite large, so I will put it into a gist if that's OK:
https://gist.github.com/benzimmer/a4ee7b43ae4ade24a570301dfd0c12c2

This seems to be working fine for the most part, but every now and then we
see the described behaviour without being able to consistently reproduce
it.

If anyone has any clue why this might be happening, we'd be very grateful.
If you need any additional information, please feel free to ask away!

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,281606,281606#msg-281606