Re: Bad leaks file fd to child processes

[Dale R. Worley]

Alexey via Bug reports for the GNU Bourne Again SHell <bug-bash@gnu.org>
> Same behavior was in bash 4.4 (as well as now in bash 5.2):
>
> # echo $BASH_VERSION
> 4.4.0(1)-release
> # exec 66</etc/hosts
> # pvs
> File descriptor 66 (/etc/hosts) leaked on pvs invocation. Parent PID 
> 1057606: ./bash
>
> But we use the fact tat bash doesn't close FD for example to preliminary 
> open log file for utility that we will `exec' later.
> Unfortunately bash doesn't provide any fcntl() mechanism to control FD 
> flags.
> It'll be great to have  ability to manage CLOEXEC or any other FD flags 
> from the script.

Interesting!  I had misunderstood the complaint, which is that the fd
leaks *from* bash *to* the program being executed (pvs in the example).

One aspect is that is the specified behavior of bash.

It's fairly straightforward to prevent a particular fd from leaking to a
single command by explicitly closing it in a redirection on the command:

    # echo $BASH_VERSION
    5.1.0(1)-release
    # exec 66</dev/null
    # pvs
    File descriptor 66 (/dev/null) leaked on pvs invocation. Parent PID 673839: 
bash
      PV         VG          Fmt  Attr PSize    PFree   
      /dev/sda3  Hobgoblin01 lvm2 a--  <300.00g  <40.00g
      /dev/sda5  Hobgoblin01 lvm2 a--  <630.02g <580.02g
      /dev/sdb3  Hobgoblin01 lvm2 a--    <1.82t    1.77t
    # pvs 66<&-
      PV         VG          Fmt  Attr PSize    PFree   
      /dev/sda3  Hobgoblin01 lvm2 a--  <300.00g  <40.00g
      /dev/sda5  Hobgoblin01 lvm2 a--  <630.02g <580.02g
      /dev/sdb3  Hobgoblin01 lvm2 a--    <1.82t    1.77t
    #

A relatively natural way to handle this is to have a shell array
variable "fd_close_on_exec" whose values are connected to the fd's
close-on-exec value.  Then you could do

    $ exec 66</dev/null
    $ fd_close_on_exec[66]=1
    $

Dale