Re: [Libguestfs] [nbdkit] Windows errno handling

On Mon, Aug 17, 2020 at 07:36:00PM +0100, Richard W.M. Jones wrote: > The Windows port of nbdkit > (https://github.com/rwmjones/nbdkit/tree/2020-windows-mingw) now works > to some extent. However errno handling doesn't work. The way that > Winsock handles errors is incompatible with the way we expect to work > errno in several ways. The long story is here: > > https://docs.microsoft.com/en-us/windows/win32/winsock/error-codes-errno-... > https://docs.microsoft.com/en-us/windows/win32/winsock/windows-sockets-er... >

> > This is very invasive for existing code. There are ~60 places in the > existing code which seem to assign to errno, but some of these either > set errno = 0 or are preserving errno (item (3) above), and so we'd > probably want to handle those a bit differently. > > Printing of Winsock codes through perror() or %m actually works

> (surprisingly). However it does _not_ seem to work if we try to > translate the codes to errno E* values. I need to look at exactly > what's going on here. > > Number (5) is actually fairly easy to deal with because there's only > one place where we handle the errno returned by plugins > (server/plugins.c:get_error). I think we'd probably want > errno_is_preserved to mean "WSAGetLastError" or "GetLastError" > contains something of interest. > > Thoughts? > > Also I really need to look at how some other portable libraries like > curl and gnutls are handling this. Maybe they've already come up with > something. Take a look at what libvirt has done. We follow a simplified version of what GNULIB does, by defining custom wrapper functions around all winsock APIs we need that set errno, and then use a macro to transparently replace calls to use our wrappers: https://gitlab.com/libvirt/libvirt/-/blob/master/src/util/virsocket.h https://gitlab.com/libvirt/libvirt/-/blob/master/src/util/virsocket.c You should be able to just lift those two files straight into your git repo as they have no deps on other libvirt infra.

QEMU follows a similar kind of approach too, but its impl is harder to lift out. Note, we never use %m in libvirt so don't need to solve that particular problem. QEMU never uses %m either except in the few files which are 100% linux only and will never need to be portable. Regards, Daniel