Re: fuse

[Greg Wooledge]

On Thu, Apr 06, 2023 at 03:13:53PM +0200, lacsaP Patatetom wrote:
> 8<-- /tmp/master --------
> #!/usr/bin/bash
> device=$( /tmp/sub /dev/sda1 ) || exit 1
> echo "$device"
> 8<--------------------------
> 
> 8<-- /tmp/sub ------------
> #/bin/bash
> exec 10>&1 && exec >/dev/null
> mkdir -p /sub
> echo "command that writes to /dev/fd/1 : don't want this data/text"
> echo "another command that writes to /dev/fd/1 : don't want this data/text"
> [ "$1" ] && [ -b "$1" ] && mount "$1" /sub || exit 1
> exec >&10 10>&-
> echo "$1 mounted" >&2
> echo "$1"
> 8<-------------------------
> 
> the `exec` command lines are supposed to capture/delete/clear anything
> that might be sent to /dev/fd/1 before the final `echo "$1"`.
> 
> if I run `/tmp/master` with the `exec` command lines present,
> `/dev/sda1 mounted` is displayed, but the master script does not get
> control.
> 
> on the other hand, if I comment out the `exec` command lines, the
> master script does get control but displays things I don't want.

Yeah, OK.  You're opening file descriptor 10, which points to the
same place as FD 1, which is the pipe that gets read by the command
substitution in the "master" script.

This FD is inherited by mount, and then, I presume, by the long-running
mount child process.  (You could verify that by looking in /proc/$pid/fd
where $pid is the PID of that mount child, but for now let's assume
it's true.)

Since the mount child is still running, and still has FD 10 open (which
points to the pipe), the pipe never reaches an End Of File state, and
the process substitution continues to wait for more input from that pipe.
The process substitution will "block" until that pipe reaches EOF, by
closing all of the FDs that point to it.

The obvious fix here would be to stop doing that exec 10>&1 dance, and
simply redirect the output of the two commands in between mkdir and mount.
You may also need to redirect the output of mount -- I'm not sure about
that.

If you don't want to do that, then the second most obvious fix would
be to move the "exec 1>&10 10>&-" line up above mount, and once again,
you may also need to redirect mount's output.

The third most obvious fix would be to close FD 10 when you invoke mount,
by adding a 10>&- redirection on the mount command.  And, yes, you may
also need to redirect stdout.