Re: [PATCH] t5570: fix forwarding of git-daemon messages via cat

> The shell function that starts git-daemon wants to read the first line of
> the daemon's stderr to ensure that it started correctly. Subsequent daemon
> errors should be redirected to fd 4 (which is the terminal in verbose mode
> or /dev/null in quiet mode). To that end the shell script used 'read' to
> get the first line of output, and then 'cat &' to forward everything else
> in a background process.
> 
> The problem is, that 'cat >&4 &' does not produce any output because the
> shell redirects a background process's stdin to /dev/null. To have this
> command invocation do anything useful, we have to redirect its stdin
> explicitly (which overrides the /dev/null redirection).
> 
> The shell function connects the daemon's stderr to its consumers via a
> FIFO. We cannot just do this:
> 
>    read line <git_daemon_output
>    cat <git_daemon_output >&4 &
> 
> because after the first redirection the pipe is closed and the daemon
> could receive SIGPIPE if it writes at the wrong moment. Therefore, we open
> the readable end of the FIFO only once on fd 7 in the shell and dup from
> there to the stdin of the two consumers.
> 
> Signed-off-by: Johannes Sixt <j6t@kdbg.org>