[Qemu-devel] Re: [PATCH] Use sigwait instead of sigwaitinfo.

[Tristan Gingold]

On Feb 18, 2011, at 4:34 PM, Jan Kiszka wrote:
> 
> This and the above handling of sigwait return codes changes the error
> handling strategy.

Did it ?  I don't think so.

> So far we silently skipped errors, now we silently
> terminate the compatfd thread. I think none of both approaches is good.

I think that both silently terminate the compatfd.  The previous code is:

  do {
        siginfo_t siginfo;

        err = sigwaitinfo(&info->mask, &siginfo);
        if (err == -1 && errno == EINTR) {
            err = 0;
            continue;
        }

        if (err > 0) {
            char buffer[128];
            size_t offset = 0;

            memcpy(buffer, &err, sizeof(err));
            while (offset < sizeof(buffer)) {
                ssize_t len;

                len = write(info->fd, buffer + offset,
                            sizeof(buffer) - offset);
                if (len == -1 && errno == EINTR)
                    continue;

                if (len <= 0) {
                    err = -1;
                    break;
                }

                offset += len;
            }
        }
    } while (err >= 0);

So in case of any error, err is set to a negative value which exits the thread.

> Failing sigwait is likely a reason to bail out, but loudly, writing some
> error message to the console and triggering a shutdown of qemu.

I agree with that.

> An overflow of the compatfd pipe to the main thread may be due to some
> very unfortunate overload scenario. Not sure if that qualifies for a
> thread termination (definitely not for a silent one).

What do you mean by overflow ?  Unless I am wrong, the fd is not non-blocking, 
write will block.

> Error handling should probably be adjusted independently of this darwin
> build fix.

I agree too.

Tristan.