When trying to make more than one HTTPS request to the same host via
`url-retrieve-synchronously' behind my work proxy, only the first request
succeeds.  Depending on the host, subsequent requests result in 400/404/302
and perhaps a couple other response codes.  There's some minimal code below
that can be run with emacs -Q, but I don't know how to replicate the proxy.

It seems that commit 84613dae5c3 is at least partially responsible for
this.  It's a small change to `url-https-proxy-after-change-function' that
binds `url-http-proxy' around the call to `url-http-create-request'.
Without that binding, I see no errors.  I tested this both in 26.2.90 which
does not include that commit, and the revision in this report by simply
redefining the function to the older version.

This happens both when `url-proxy-services' is set manually and when it is
set from environment variables.

I'm not sure if this is a problem with Emacs, the proxy I'm behind, or if
there is some configuration I should be aware of that already addresses
this.  Please let me know if there's any other useful information I can
provide.

Sample code and the corresponding url-http buffer headers follow.  The
specific URLs don't seem to matter as long they are HTTPS.  The optional
arguments also don't seem to make a difference.

- David

Code:
(url-retrieve-synchronously "https://xkcd.com/1/")
(url-retrieve-synchronously "https://xkcd.com/2/")

First request (success):
HTTP/1.1 200 OK
Server: nginx
Content-Type: text/html; charset=UTF-8
Last-Modified: Thu, 15 Aug 2019 11:24:39 GMT
Expires: Thu, 15 Aug 2019 21:22:11 GMT
Cache-Control: max-age=300
Content-Length: 3012
Accept-Ranges: bytes
Date: Fri, 16 Aug 2019 01:32:01 GMT
Via: 1.1 varnish
Age: 142
Connection: keep-alive
X-Served-By: cache-mdw17348-MDW
X-Cache: HIT
X-Cache-Hits: 1
X-Timer: S1565919121.298692,VS0,VE0
Vary: Accept-Encoding

Second request (failuer):
HTTP/1.1 400 Bad Request
Server: nginx
Content-Type: text/html; charset=UTF-8
Content-Length: 166
Accept-Ranges: bytes
Date: Fri, 16 Aug 2019 01:32:01 GMT
Via: 1.1 varnish
Connection: keep-alive
X-Served-By: cache-mdw17348-MDW
X-Cache: MISS
X-Cache-Hits: 0
X-Timer: S1565919121.349321,VS0,VE26


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu)
 of 2019-08-07 built on slartibartfast
Repository revision: d5622eb6fff94714c5d5a64c98c5e02bc1be478c
Repository branch: master
Windowing system distributor 'Fedora Project', version 11.0.12005000
System Description: Fedora 30 (Workstation Edition)

Recent messages:
Making completion list...
Quit
Note: file is write protected
View mode: type C-h for help, h for commands, q to quit.
previous-line: Beginning of buffer [2 times]
Mark saved where search started
Note: file is write protected
View mode: type C-h for help, C-h m for commands, M-x View-quit to quit.
Quit [3 times]
Mark saved where search started
Quit
Configured using:
 'configure --with-imagemagick --with-modules --with-x-toolkit=no'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB
NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT
LIBOTF XFT ZLIB OLDXMENU X11 XDBE XIM MODULES THREADS PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Info

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired dired-loaddefs format-spec
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs text-property-search time-date mm-decode mm-bodies
mm-encode mailabbrev gmm-utils mailheader sendmail goto-addr thingatpt
mule-util jka-compr info noutline outline easy-mmode view help-mode
easymenu misearch multi-isearch mail-utils gnutls network-stream
url-cache url-http url-auth mail-parse rfc2231 rfc2047 rfc2045 mm-util
ietf-drums mail-prsvr url-gw nsm rmc puny url url-proxy url-privacy
url-expand url-methods url-history url-cookie url-domsuf url-util
url-parse auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache json subr-x map seq byte-opt gv bytecomp byte-compile
cconv cl-loaddefs cl-lib url-vars mailcap tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify lcms2 dynamic-setting
system-font-setting font-render-setting x multi-tty make-network-process
emacs)

Memory information:
((conses 16 76232 12727)
 (symbols 48 8198 1)
 (strings 32 28196 1967)
 (string-bytes 1 800056)
 (vectors 16 13427)
 (vector-slots 8 162016 12678)
 (floats 8 43 388)
 (intervals 56 1044 0)
 (buffers 992 19))