implement Tftp; include "sys.m";	sys: Sys; include "tftp.m"; Maxretry: con 5;	# retries per blockMaxblock: con 512;	# protocol's usual maximum data block sizeTftphdrlen: con 4;Read, Write, Data, Ack, Error: con 1+iota;	# tftp opcode progress: int; put2(buf: array of byte, o: int, val: int){	buf[o] = byte (val >> 8);	buf[o+1] = byte val;} get2(buf: array of byte, o: int): int{	return (int buf[o] << 8) | int buf[o+1];} kill(pid: int){	fd := sys->open("#p/" + string pid + "/ctl", sys->OWRITE);	if(fd == nil)		return; 	msg := array of byte "kill";	sys->write(fd, msg, len msg);} timeoutproc(c: chan of int, howlong: int){	c <-= sys->pctl(0, nil);	sys->sleep(howlong);	c <-= 1;} tpid := -1; timeoutcancel(){	if(tpid >= 0) {		kill(tpid);		tpid = -1;	}} timeoutstart(howlong: int): chan of int{	timeoutcancel();	tc := chan of int;	spawn timeoutproc(tc, howlong);	tpid = <-tc;	return tc;} init(p: int){	sys = load Sys Sys->PATH;	progress = p;} reader(pidc: chan of int, fd: ref Sys->FD, bc: chan of array of byte){	pid := sys->pctl(0, nil);	pidc <-= pid;	buf := array[Tftphdrlen + Maxblock] of byte;	for(;;){		n := sys->read(fd, buf, len buf);		bc <-= buf[0 : n];	}} receive(host: string, filename: string, fd: ref Sys->FD): string{	rbuf: array of byte;		(ok, conn) := sys->dial("udp!" + host + "!69", nil);	if(ok < 0) 		return sys->sprint("can't dial %s: %r", host);	buf := array[Tftphdrlen + Maxblock] of byte;	i := 0;	put2(buf, i, Read);	i += 2;	a := array of byte filename;	buf[i:] = a;	i += len a;	buf[i++] = byte 0;	mode := array of byte "binary";	buf[i:] = mode;	i += len mode;	buf[i++] = byte 0;	pidc := chan of int;	bc := chan of array of byte;	spawn reader(pidc, conn.dfd, bc);	tftppid := <-pidc;	lastblock := 0;	for(;;) {	  Retry:		for(count := 0;; count++) {			if(count >= Maxretry){				kill(tftppid);				return sys->sprint("tftp timeout");			} 			# (re)send request/ack			if(sys->write(conn.dfd, buf, i) < 0) {				kill(tftppid);				return sys->sprint( "error writing %s/data: %r", conn.dir);			}				# wait for next block			mtc := timeoutstart(3000);			for(;;){				alt {				<-mtc =>					if(progress)						sys->print("T");					continue Retry;				rbuf = <-bc =>					if(len rbuf < Tftphdrlen)						break;					op := get2(rbuf, 0);					case op {					Data =>						block := get2(rbuf, 2);						if(block == lastblock + 1) {							timeoutcancel();							break Retry;						}else if(progress)							sys->print("S");					Error =>						timeoutcancel();						kill(tftppid);						return sys->sprint("server error %d: %s", get2(rbuf, 2), string rbuf[4:]);					* =>						timeoutcancel();						kill(tftppid);						return sys->sprint("phase error op=%d", op);					}				}			}		}		n := len rbuf;		# copy the data somewhere		if(sys->write(fd, rbuf[Tftphdrlen:], n - Tftphdrlen) < 0) {			kill(tftppid);			return sys->sprint("writing destination: %r");		}		lastblock++;		if(progress && lastblock % 25 == 0)			sys->print(".");		if(n < Maxblock + Tftphdrlen) {			if(progress)				sys->print("\n");			break;		} 		# send an ack		put2(buf, 0, Ack);		put2(buf, 2, lastblock);	}	kill(tftppid);	return nil;}