[Libguestfs] [nbdkit] Windows errno handling

[Eric Blake]

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-h-errno-and-wsagetlasterror-2
>> https://docs.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2
>>

>>
>> 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).

> 
> 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
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org