lastpipe doesn't lastpipe when stdin (fd 0) is closed

[Tomas Janousek]

Hi,

bash-5.1 (and judging from git blame, all versions that ever supported 
lastpipe, so 4.2+) fails to honor lastpipe when run without fd 0:

    $ bash -O lastpipe -c 'echo x | read x; echo x=$x'
    x=x

    $ bash -O lastpipe -c 'echo x | read x; echo x=$x' <&-
    x=

    $ bash -O lastpipe -c 'exec </dev/null; echo x | read x; echo x=$x' <&-
    x=x

The issue seems to be that execute_cmd.c:execute_pipeline insists on 
saving/restoring stdin which obviously fails when there's no stdin open 
at all:

    lstdin = move_to_high_fd (0, 1, -1);
    if (lstdin > 0)
        /* lastpipe in effect here */

(I got bitten by this because acpid runs action scripts with closed 
stdin, so lastpipe doesn't work in those while the same script invoked 
manually works. I wish bash gave me a warning instead of silently 
falling back to non-lastpipe behaviour.)

--
Tomáš Janoušek, a.k.a. Pivník, a.k.a. Liskni_si, https://work.lisk.in/