builtin echo command redirection misbehaves in detached scripts when terminal is closed

[Pierre-Philippe Coupard]

Configuration Information [Automatically generated, do not change]:
Machine: i486
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i486' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i486-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_CONFIG_H   -I.  -I../bash -I../bash/include 
-I../bash/lib   -g -O2
uname output: Linux akula 2.6.17.7 #4 Sat Sep 8 23:10:46 CEST 2007 i686 
GNU/Linux
Machine Type: i486-pc-linux-gnu

Bash Version: 3.1
Patch Level: 17
Release Status: release

Description:
       With Unix-98 ptys, the builtin echo command gets executed
       even when writing to stdout or redirecting to stderr fails, and
       the output gets written to the wrong file descriptor if any other
       redirection is used in the script.

Repeat-By:
       For example, with the following script:

       while [ 1 ];do
         echo Test1
         echo Test2 >> file.txt
         sleep 1
       done

       As expected, when this script is run in the background (&), the 
console
       slowly fills with "Test1" lines, and the file.txt file slowly 
fills with
       "Test2" lines.

       Now exit the shell leaving the script running (don't simply 
close the
       xterm, that'd kill the script. Type "exit"). Since the terminal has
       closed, stdout is closed, so "echo Test1" should fail. It doesn't,
       instead it writes "Test1" lines into whatever open file 
descriptor it
       can find. In this case, file.txt starts filling up with

       Test2
       Test1
       Test2
       Test1
       ...

       This does not happen with BSD-style ptys, because apparently 
when the
       terminal is closed, the tty seen by the detached bash script stays
       intact, and whatever is written to the now-closed terminal is simply
       discarded by the kernel, so the script keeps seeing open stdout and
       stderr file descriptors. In the case of Unix-98 ptys, this bug 
happens
       because the tty file descriptors the bash script uses are really
       closed

       This also does not happen with an external echo command: with 
/bin/echo,
       the redirection fails and the command is not executed, as expected.