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Implement filterfd().

	We can push a filter onto an fd.

--- a/lib/std/spork.myr

+++ b/lib/std/spork.myr

@@ -8,6 +8,7 @@

 pkg std =

 	const spork	: (cmd : byte[:][:]	-> result((pid, fd, fd), errno))

 	const espork	: (cmd : byte[:][:]	-> result((pid, fd, fd, fd), errno))

+	const filterfd	: (fd : fd, cmd  : byte[:][:] -> result(pid, errno))

 ;;

 const spork = {cmd

@@ -65,6 +66,26 @@

 	;;

 }

+const filterfd = {fd, cmd

+	var outfds : fd[2]

+	var err

+

+	err = pipe(&outfds)

+	if err != Enone

+		-> `Err err

+	;;

+

+	match sporkfd(cmd, [fd, -1], outfds, [-1, 2])

+	| `Ok pid:

+		dup2(outfds[0], fd)

+		close(outfds[0]);

+		close(outfds[1]);

+		-> `Ok pid

+	| `Err e:

+		-> `Err e

+	;;

+}

+

 const sporkfd = {cmd, infds, outfds, errfds

 	var pid, err

@@ -71,6 +92,13 @@

 	pid = fork()

 	/* error  */

 	if pid < 0

+		/* we don't want to leak the pipe fds on error */

+		close(infds[0]);

+		close(infds[1]);

+		close(outfds[0]);

+		close(outfds[1]);

+		close(errfds[0]);

+		close(errfds[1]);

 		-> `Err (pid : errno)

 	/* child */

 	elif pid == 0