bug#25549: 25.1; eshell grep gives inconsistent output

[me]

When using grep on a large file in eshell, the output is sometimes
  inconsistent. For example, when grepping for a string in my
server's apache2 access log (~5 MB) I got anywhere from around 200 to
around 1300 results, just running the same grep command over and over
(the file itself was not changing significantly).

The bug cannot be consistently reproduced, but has happened in multiple
situations and others have reportedly reproduced it per discussion on
reddit
(https://www.reddit.com/r/emacs/comments/5qbd70/trampeshellgrep_returning_wildly_different/).


In GNU Emacs 25.1.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.22.5)
 of 2016-12-23 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11901000
System Description:     Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe
 -fstack-protector-strong' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

Major mode: EShell

Minor modes in effect:
  magit-auto-revert-mode: t
  global-git-commit-mode: t
  diff-auto-refine-mode: t
  global-flycheck-mode: t
  pyvenv-mode: t
  helm-mode: t
  shell-dirtrack-mode: t
  async-bytecomp-package-mode: t
  delete-selection-mode: t
  cua-mode: t
  electric-pair-mode: t
  pdf-occur-global-minor-mode: t
  show-paren-mode: t
  tabbar-mwheel-mode: t
  tabbar-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Git finished [2 times]
C-c p is undefined
Quit
[C-t] show common commands, [?] describe events, [C-h i] show manual
Running git push -v gl master:refs/heads/master [2 times]
Git finished
[C-t] show common commands, [?] describe events, [C-h i] show manual
Running git push -v production master:refs/heads/master [2 times]
Git finished [2 times]
Making completion list...

Load-path shadows:
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/home/alanm/.emacs.d/elpa/company-20170112.2005/company-clang hides 
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/home/alanm/.emacs.d/elpa/autopair-20160304.437/autopair hides 
~/.emacs.d/elisp/autopair
/home/alanm/.emacs.d/elpa/company-20170112.2005/company-css hides 
~/.emacs.d/elisp/company-css
/home/alanm/.emacs.d/elpa/cmake-mode-20160928.505/cmake-mode hides 
/usr/share/emacs/site-lisp/cmake-mode
~/.emacs.d/elisp/rst hides /usr/share/emacs/25.1/lisp/textmodes/rst
/home/alanm/.emacs.d/elpa/css-mode-1.0/css-mode hides 
/usr/share/emacs/25.1/lisp/textmodes/css-mode
/home/alanm/.emacs.d/elpa/scala-mode-20170118.558/ob-scala hides 
/usr/share/emacs/25.1/lisp/org/ob-scala
/home/alanm/.emacs.d/elpa/soap-client-3.1.1/soap-inspect hides 
/usr/share/emacs/25.1/lisp/net/soap-inspect
/home/alanm/.emacs.d/elpa/soap-client-3.1.1/soap-client hides 
/usr/share/emacs/25.1/lisp/net/soap-client
/home/alanm/.emacs.d/elpa/ntlm-2.1.0/ntlm hides 
/usr/share/emacs/25.1/lisp/net/ntlm
/home/alanm/.emacs.d/elpa/seq-20151121.1017/seq hides 
/usr/share/emacs/25.1/lisp/emacs-lisp/seq
/home/alanm/.emacs.d/elpa/tabulated-list-20120406.1351/tabulated-list hides 
/usr/share/emacs/25.1/lisp/emacs-lisp/tabulated-list

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custom widget hashtable-print-readable backquote dbusbind inotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
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Memory information:
((conses 16 722368 86999)
 (symbols 48 63599 2)
 (miscs 40 866 1138)
 (strings 32 197238 16662)
 (string-bytes 1 5612576)
 (vectors 16 99923)
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 (buffers 976 53))