bug#65858: mumi crashes

[Arun Isaac]

Hi Chris and Maxim,

>> I think this is kind of expected. If NGinx hits the proxy_read_timeout
>> it'll return a 504 to the client and close the connection to Mumi I
>> think. I think what you're seeing here is Mumi trying to respond to a
>> request from NGinx that NGinx has closed.

Thanks for the tip. This indeed makes a lot of sense!

But, I don't understand why a proxy read timeout happens. The requests
in question seem to be very simple requests that shoud be processed
quickly. Should we increase nginx's proxy_read_timeout?

> If it's expected, we should handle it and produce a useful warning
> instead of crashing, right?

A couple of weeks ago, I put in a mechanism to better catch and report
errors.

https://git.savannah.gnu.org/cgit/guix/mumi.git/commit/?id=7a71c12a1f7f033ae93f26fb09387f6c04c143ba

I'm not sure this is working correctly, though. I have used `guard` in the
commit above. Does `guard` catch all exceptions correctly, or should I
instead use `catch`?

Also, I believe the logs are written in parallel by fibers. This may
mangle up the logs a bit, but I'm not sure if we're actually see
requests rapid enough for this to happen.

Regards,
Arun