Re: lua, connect, alarm, setjmp/longjmp mix like a bad hangover

["Guido Sohne" <guido@...>]

On 7/14/06, David Jones <drj@pobox.com> wrote:
> Generally it's unsafe to longjmp out of a signal handler.  Pretty
> much the only thing you can do in a signal handler portably and
> safely is to set a variable of type sig_atomic_t and return.  Of
> course, your platform may provide more guarantees (I doubt it).

I thought so too. I've ditched the alarm/longjmp approach and am using
setsockopt to tell the socket to use given receive and send timeouts,
and also telling the socket not to linger ...

> What's your C platform?

uClibc/uClinux running on a MC68VZ328 (Motorola 68000 variant,
Dragonball @ 33 Mhz)

> Your problem is general and thorny.  In the ideal world everything
> that might block (network, files, locks, threads, user input, alarms,
> etc) should be select(2)able, but sometimes you just can't get at the
> underlying fd.

I'd rather avoid select if I can. Code is thorny enough as it is :-)

> You may be able to make your select non-blocking before trying connect.

I tried this. It appears to work.

network.lua:

function net.ping(host, timeout)
	local ret, err = socket.ping(host, timeout)
	print('ret: ' .. tostring(ret) .. ' err: ' .. tostring(err))
end

net.ping('10.0.0.100', 3)
net.ping('www.google.com', 3)
net.ping('64.233.161.99', 10)

gives me

# lua ne*
ret: true err: nil
ret: nil err: bad IP address: Success
ret: nil err: ping write incomplete: Network is unreachable

using the below implementation of ping (adapted from busybox, adding
linger and timeouts)

#define DEFDATALEN      56
#define	MAXIPLEN	60
#define	MAXICMPLEN	76

static int pPing(lua_State *L)
{
	struct in_addr hostaddr;
	struct hostent *h;
	struct sockaddr_in pingaddr;
	struct icmp *pkt;
	struct timeval timeout;
	struct linger lingrr;
	int pingsock, c, secs, ret;
	const char *host;
	char packet[DEFDATALEN + MAXIPLEN + MAXICMPLEN];

	host = lua_tostring(L, 1);
	secs = (int) lua_tonumber(L, 2);

	if ((pingsock = socket(AF_INET, SOCK_RAW, 1)) < 0) 	/* 1 == ICMP */
		goto notreachable;

	memset(&pingaddr, 0, sizeof(struct sockaddr_in));
	pingaddr.sin_family = AF_INET;	
	
	if (!(h = gethostbyname(host)))
		goto notreachable;
	
	memcpy(&(pingaddr.sin_addr), h->h_addr, sizeof(pingaddr.sin_addr));

	pkt = (struct icmp *) packet;
	memset(pkt, 0, sizeof(packet));
	pkt->icmp_type = ICMP_ECHO;
	pkt->icmp_cksum = in_cksum((unsigned short *) pkt, sizeof(packet));

	/* set timeout to user supplied value */
	timeout.tv_sec = secs;
	timeout.tv_usec = 0;

	/* set socket to linger for user supplied time */
	lingrr.l_onoff = 1;
	lingrr.l_linger = secs;
	
   if (setsockopt (pingsock, SOL_SOCKET, SO_LINGER, (char *)&lingrr,
sizeof lingrr) < 0)
		goto notreachable;
	if (setsockopt (pingsock, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout,
sizeof timeout) < 0)
		goto notreachable;
	if (setsockopt (pingsock, SOL_SOCKET, SO_SNDTIMEO, (char *)&timeout,
sizeof timeout) < 0)
		goto notreachable;

	c = sendto(pingsock, packet, sizeof(packet), 0,
			   (struct sockaddr *) &pingaddr, sizeof(struct sockaddr_in));

	if (c < 0 || c != sizeof(packet)) {
		goto notreachable;
	}

	/* listen for replies */
	while (1) {
		struct sockaddr_in from;
		socklen_t fromlen = sizeof(from);

		if ((c = recvfrom(pingsock, packet, sizeof(packet), 0,
						  (struct sockaddr *) &from, (socklen_t *)&fromlen)) < 0) {
			if (errno == EINTR)
				continue;
			if (errno == ETIMEDOUT)
				goto notreachable;
			continue;
		}
		if (c >= 76) {			/* ip + icmp */
			struct iphdr *iphdr = (struct iphdr *) packet;

			pkt = (struct icmp *) (packet + (iphdr->ihl << 2));	/* skip ip hdr */
			if (pkt->icmp_type == ICMP_ECHOREPLY)
				break;
		}
	}
	
	close(pingsock);
	lua_pushboolean(L, 1);
	return 1;
	
notreachable:
	ret = pusherror(L, NULL);
	close(pingsock);
	return ret;
}

static int in_cksum(unsigned short *buf, int sz)
{
	int nleft = sz;
	int sum = 0;
	unsigned short *w = buf;
	unsigned short ans = 0;

	while (nleft > 1) {
		sum += *w++;
		nleft -= 2;
	}

	if (nleft == 1) {
		*(unsigned char *) (&ans) = *(unsigned char *) w;
		sum += ans;
	}

	sum = (sum >> 16) + (sum & 0xFFFF);
	sum += (sum >> 16);
	ans = ~sum;
	return (ans);
}

Hope this helps someone later :-)

-- G.