bug#33252: 27.0.50; Recursive password prompt takes over

[Juri Linkov]

A bug in a shell script caused an infinitive stream of the same output lines:

Password: sh: 1: read: Illegal option -s

Password: sh: 1: read: Illegal option -s

Password: sh: 1: read: Illegal option -s

Password: sh: 1: read: Illegal option -s

Password: sh: 1: read: Illegal option -s

...

Currently Emacs handles such shell output very poorly and fails
with the following backtrace:

Debugger entered--Lisp error: (error "Variable binding depth exceeds 
max-specpdl-size")
  read-string("(In buffer *shell*) Password: sh: 1: read: Ill..." nil t nil)
  read-passwd("(In buffer *shell*) Password: sh: 1: read: Ill...")
  comint-send-invisible("Password: sh: 1: read: Illegal option 
-s\nPassword:...")
  comint-watch-for-password-prompt("Password: sh: 1: read: Illegal option 
-s\n\nPassword...")
  run-hook-with-args(comint-watch-for-password-prompt "Password: sh: 1: read: 
Illegal option -s\n\nPassword...")
  comint-output-filter(#<process shell<1>> "Password: sh: 1: read: Illegal 
option -s\n\nPassword...")
  ...
  read-string("(In buffer *shell*) sh: 1: read: Illegal optio..." nil t nil)
  read-passwd("(In buffer *shell*) sh: 1: read: Illegal optio...")
  comint-send-invisible("sh: 1: read: Illegal option -s\nPassword: sh: 1: 
re...")
  comint-watch-for-password-prompt("sh: 1: read: Illegal option -s\n\nPassword: 
sh: 1: r...")
  run-hook-with-args(comint-watch-for-password-prompt "sh: 1: read: Illegal 
option -s\n\nPassword: sh: 1: r...")
  comint-output-filter(#<process shell<1>> "sh: 1: read: Illegal option 
-s\n\nPassword: sh: 1: r...")
  ...
  read-string("(In buffer *shell*) sh: 1: read: Illegal optio..." nil t nil)
  read-passwd("(In buffer *shell*) sh: 1: read: Illegal optio...")
  comint-send-invisible("sh: 1: read: Illegal option -s\nPassword: sh: 1: 
re...")
  comint-watch-for-password-prompt("sh: 1: read: Illegal option -s\n\nPassword: 
sh: 1: r...")
  run-hook-with-args(comint-watch-for-password-prompt "sh: 1: read: Illegal 
option -s\n\nPassword: sh: 1: r...")
  comint-output-filter(#<process shell<1>> "sh: 1: read: Illegal option 
-s\n\nPassword: sh: 1: r...")
  ...

A reproducible short test case:

0. emacs -Q

1. M-&
   while true; do echo -n "Password: "; sleep 1; done
   RET

After 42 seconds Emacs session becomes completely unusable, and control
can be regained only after killing the shell script from outside Emacs.