t5570: fix forwarding of git-daemon messages via cat

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>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>