[Bug 237568] Panic when closing socket descriptor?

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237568

            Bug ID: 237568
           Summary: Panic when closing socket descriptor?
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Keywords: crash, panic
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: rlwestlund at gmail.com

Created attachment 204020
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=204020&action=edit
Stack trace from kernel panic

Our setup is like this: FreeBSD 12.0-RELEASE-p3 running deployments of a web
service in multiple jails, each has nginx as an upstream to the nginx running
on the host, which is a reverse proxy (I think that's the right term?). The
in-jail nginx's are themselves partially reverse proxies to a Go program;
static files are served by nginx itself but dynamic content comes from the Go
server.

Problem: Occasionally, loading the web page crashes the entire host.

There doesn't seem to be any reliable way to reproduce it. We've tried turning
HTTP 2 on and off; it happens either way, but we haven't found any combination
of circumstances that causes it to happen more than occasionally (I'd estimate
something like 1 in 20 page reloads on average). Many times we've made a change
and thought we fixed it after >50 successful reloads but found that it does
still crash occasionally.

I'll attach the stack trace from the crash.


Kernel: GENERIC
Architecture: amd64
nginx version: 1.14.2_13,2

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