[GRASS-dev] native WinGRASS and attribute data deadlock, next try

[Glynn Clements]

Moritz Lennert wrote:

> > My next suggestion is to re-write xdr_stdio to use read() and write()
> > instead of fread() and fwrite(), e.g.:
> >
> > -	if (fread((caddr_t)lp, sizeof(long), 1, (FILE *)xdrs->x_private) != 1)
> > +	if (read(fileno((FILE *)xdrs->x_private), (caddr_t)lp, sizeof(long)) !=
> > sizeof(long))
> 
> That seems to work !

Now that's strange. I was expecting it to provide more information on
the errors, not to work.

On Unix, read/write can return short counts on pipes. If a process
tries to write 4 bytes to a pipe, and there's only enough space for 2
bytes, write() will return 2 rather than waiting for the pipe to
drain. Similarly if you try to read more data than is available in the
pipe.

If you want to block until the requested amount of data has been read
or written, you need to either use readn/writen (which are
non-standard), or implement them yourself, e.g.:

ssize_t readn(int fd, void *buf, size_t count)
{
	ssize_t total = 0;

	while (total < count)
	{
		ssize_t n = read(fd, (char *) buf + total, count - total)
		if (n < 0)
			return n;
		if (n == 0)
			break;
		total += n;
	}

	return total;
}

ssize_t writen(int fd, const void *buf, size_t count)
{
	ssize_t total = 0;

	while (total < count)
	{
		ssize_t n = write(fd, (const char *) buf + total, count - total)
		if (n < 0)
			return n;
		if (n == 0)
			break;
		total += n;
	}

	return total;
}

The above assumes that non-blocking I/O hasn't been enabled for the
descriptor. If it has, you need to either disable it for the duration
of the function or use select() or poll() to wait until the descriptor
is ready.

> I'll post new binaries as soon as compilation is over, so that others can
> test as well.

If you want to test it, I'd suggest reducing the size of the pipes in
lib/db/dbmi_client/start.c. A larger pipe won't prevent errors, but it
may make them less likely to show up.

-- 
Glynn Clements <glynn at gclements.plus.com>