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Add sporkdir variants.

	This allows running a command in a directory without
	modifying the global state of the program. This is
	useful in multithreaded programs that launch subcommands.

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

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

@@ -10,6 +10,8 @@

 	const run	: (cmd : byte[:][:]	-> waitstatus)

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

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

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

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

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

 ;;

@@ -28,8 +30,15 @@

 	;;

 }

-

 const spork = {cmd

+	-> sporkdir(cmd, "")

+}

+

+const espork = {cmd

+	-> esporkdir(cmd, "")

+}

+

+const sporkdir = {cmd, dir

 	var infds : fd[2], outfds : fd[2]

 	var err

@@ -46,7 +55,7 @@

 		goto sporkerr

 	;;

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

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

 	| `Ok pid:

 		/* close unused fd ends */

 		close(infds[0]);

@@ -64,7 +73,7 @@

 	-> `Err err

 }

-const espork = {cmd

+const esporkdir = {cmd, dir

 	var infds : fd[2], outfds : fd[2], errfds : fd[2]

 	var err

@@ -86,7 +95,7 @@

 		goto sporkerr

 	;;

-	match sporkfd(cmd, infds, outfds, errfds)

+	match sporkfd(cmd, dir, infds, outfds, errfds)

 	| `Ok pid:

 		/* close unused fd ends */

 		close(infds[0]);

@@ -117,7 +126,7 @@

 		-> `Err err

 	;;

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

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

 	| `Ok pid:

 		dup2(outfds[0], fd)

 		close(outfds[0]);

@@ -128,7 +137,7 @@

 	;;

 }

-const sporkfd = {cmd, infds, outfds, errfds

+const sporkfd = {cmd, dir, infds, outfds, errfds

 	var pid

 	pid = fork()

@@ -181,6 +190,9 @@

 		close(outfds[0])

 		close(errfds[0])

+		if dir.len != 0 && !chdir(dir)

+			std.die("could not chdir")

+		;;

 		execvp(cmd[0], cmd)

 		/* if fork succeeds, we never return */

 		suicide()