bug#43772: 27.1; (describe-variable) breaks eshell

[daniel]

When using eshell-mode, calling `(describe-variable)` on a variable that
starts with "eshell" causes eshell-watch-for-password-prompt to no
longer function correctly.  In the attached dribble file, I first show
the correct behavior by opening eshell and entering `sudo ls`; eshell
correctly prompts me for a password three times.  I then run
`(describe-variable 'eshell-after-prompt-hook)` and then enter `sudo
ls`.  This time, eshell displays the incorrect behavior: it prints the
string "password" but does not accept input as a password or print any
output from the command.  I selected `eshell-after-prompt-hook` becuase
it is one of the first variables starting with "eshell", but the same
behavior occurs with other variables, including user-defined ones.

The information below, generated with report-emacs-bug, is for my
laptop, where I observed the above bug on Emacs 27.1.  I also observed
the same bug on my desktop on Emacs 26.3 (build 2, x86_64-pc-linux-gnu),
so the issue isn't specific to the Void musl build.

I hope this information is helpful.  Thank you all for the wonderful
work you do on Emacs, and please let me know if I can provide any other
information.

Best regards,
Daniel 

In GNU Emacs 27.1 (build 1, x86_64-unknown-linux-musl)
 of 2020-08-12 built on vm1-a-mci-us
System Description: void

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list... [3 times]

Configured using:
 'configure --without-x --without-dbus --prefix=/usr --sysconfdir=/etc
 --sbindir=/usr/bin --bindir=/usr/bin --mandir=/usr/share/man
 --infodir=/usr/share/info --localstatedir=/var
 --with-file-notification=inotify --with-modules --with-jpeg --with-tiff
 --with-gif --with-png --with-xpm --with-rsvg --without-imagemagick
 --with-xml2 --with-gnutls --with-sound --with-m17n-flt --with-json
 --without-harfbuzz --without-cairo --with-libgmp
 --host=x86_64-unknown-linux-musl --build=x86_64-unknown-linux-musl
 'CFLAGS=-fno-PIE -mtune=generic -O2 -pipe -g' 'CPPFLAGS= '
 'LDFLAGS=-no-pie -Wl,--as-needed ''

Configured features:
SOUND NOTIFY INOTIFY ACL GNUTLS LIBXML2 ZLIB XIM MODULES THREADS JSON
PDUMPER LCMS2 GMP

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow regexp-opt sort mail-extr emacsbug message rmc puny dired
dired-loaddefs format-spec rfc822 mml easymenu mml-sec password-cache
epa derived epg epg-config gnus-util rmail tool-bar rmail-loaddefs
text-property-search seq mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils time-date subr-x cl-loaddefs
cl-lib term/xterm xterm byte-opt gv bytecomp byte-compile cconv tooltip
eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
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select mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
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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 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 inotify lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 46396 8814)
 (symbols 48 6012 1)
 (strings 32 15639 1721)
 (string-bytes 1 506946)
 (vectors 16 8020)
 (vector-slots 8 86910 7560)
 (floats 8 23 290)
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 (buffers 1000 12))