bug#38411: HTTP pipelining of narinfo requests broken for https://ci.guix.gnu.org

[Ludovic Courtès]

Starting from a couple of days ago (it seems; roughly around the same
time berlin hit ENOSPC), people have been experiencing issues during the
“updating list of substitutes” phase from https://ci.guix.gnu.org, where
they’d get an ugly backtrace when they’re at 80% or so.

Here’s a small reproducer:

(use-modules (guix scripts substitute)
             (srfi srfi-1)
             (srfi srfi-26)
             (web uri)
             (web request)
             (web response)
             (rnrs io ports))

(define http-multiple-get
  (@@ (guix scripts substitute) http-multiple-get))

(define %base-url
  "https://berlin.guix.gnu.org";)

(define %request-count
  ;; Number of requests to send.  Starts failing at 85 (that is, we don't
  ;; receive the 85th response).
  200)

(http-multiple-get (string->uri %base-url)
                   (lambda (request response port result)
                     (let ((len (or (response-content-length response)
                                    0)))
                       (pk 'resp (length result)
                           (uri-path (request-uri request)))
                       (get-bytevector-n port len)
                       (cons result result)))
                   '()
                   (unfold (cut >= <> %request-count)
                           (lambda (n)
                             (build-request
                              (string->uri
                               (string-append
                                %base-url
                                "/"
                                (string-pad (number->string n) 32 #\a)
                                ".narinfo"))
                              #:method 'GET
                              #:headers '((User-Agent . "GNU Guile"))))
                           1+
                           0)
                   #:verify-certificate? #f)

You’ll see that it hangs waiting for a response as soon as you pipeline
85 requests or more.

Note that:

  1. https://bayfront.guix.gnu.org doesn’t have that problem;

  2. http://ci.guix.gnu.org doesn’t have that problem;

  3. when you send 85 requests, it hangs waiting for the 85th response;
     but when you send 200 requests, it hangs waiting for the 160th
     response; so it seems it’s not just a matter of TLS record size.

I suspected something having to do with TLS record size limits, but item
#3 above may invalidate that hypothesis.

The weird thing is that we haven’t change the config of berlin in recent
days.  Berlin runs nginx 1.17.5 on openssl 1.1.1d, while bayfront runs
nginx 1.17.0 on openssl 1.0.2p.

I very much welcome any ideas you may have!

Thanks,
Ludo’.