[Libguestfs] [nbdkit] Windows errno handling

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... The shorter story is: - Windows functions like send, recv, etc. do not set errno. - In fact it's not even possible for the concept of a shared global variable to exist with Windows DLLs, or at least not without a bunch of work: (https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-st...) - To get and set the thread-local error from the last function we must call WSAGetLastError and WSASetLastError. - Error codes are integers with superficial similarity to errno codes but different symbols and values (eg. WSAEINTR == 10004 is similar to EINTR). - I'm not sure if this is just a mingw thing or a Windows thing, but mingw defines an <errno.h> header which has its own values for errno like EINTR, and for some reason perror(3) cannot print many of those values. Still looking into this one ... nbdkit does approximately 5 different things with errno: (1a) It reads them by reading the thread-local variable errno. (1b) It reads and compares them, eg. errno == EINTR. (2) It writes to them to set particular errno values, eg. errno = EIO. (3) It saves them around functions such as nbdkit_error(), which is (1) + (2) but might be a distinct operation eg. using cleanups. (4) It prints them with perror and %m. (5) It handles plugins which claim errno_is_preserved. My first thought was we could define a macro which does: #ifdef WIN32 #define errno (translate_to_errno (WSAGetLastError ())) #endif which reads the last error and translates WSA* to E* codes. This would solve (1) and is not very invasive for existing code. We'd have to then need to wrap all assignments to errno in a macro like: #ifndef WIN32 #define set_errno(v) (errno = (v)) #else #define set_errno(v) (WSASetLastError (translate_from_errno (v))) #endif 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. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html