Re: [Libguestfs] [nbdkit] Windows errno handling

On Mon, Aug 17, 2020 at 01:56:10PM -0500, Eric Blake wrote: > On 8/17/20 1:46 PM, Daniel P. Berrangé wrote: > > 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 > > That _is_ a surprise. But if it makes like easier, we'll take it. > > > > (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. > > Except that libvirt is GPLv2+, and nbdkit is not, so we'd need to relicense > (if all those lines are due to you, we may be safe; but if they derive too > much from gnulib, we're stuck). I wrote it all from scratch, but I was referencing the gnulib code, so can't claim its completely clean room, and the set_errno method was directly lifted from gnulib code. If nothing else you could do a similar kind of approach though. I felt it was preferrable to wrap the socket APIs in one place, instead of adding extra magic in every caller, so the cruft is isolated.