[Libguestfs] [libnbd PATCH v3 07/29] lib/utils: add async-signal-safe assert()

[Eric Blake]

On Fri, Feb 24, 2023 at 01:57:37PM +0100, Laszlo Ersek wrote:
> On 2/21/23 07:33, Laszlo Ersek wrote:
> > On 2/20/23 19:21, Laszlo Ersek wrote:
> >> On 2/15/23 21:57, Eric Blake wrote:
> >>> On Wed, Feb 15, 2023 at 03:11:36PM +0100, Laszlo Ersek wrote:
> > 
> >>>> +
> >>>> +  xwrite (STDERR_FILENO, file, strlen (file));
> >>>> +  xwrite (STDERR_FILENO, ":", 1);
> >>>
> >>> Presumably, if our first best-effort xwrite() fails to produce full
> >>> output, all later xwrite() will hopefully hit the same error condition
> >>> so that we aren't producing even more-mangled output where later
> >>> syscalls succeed despite missing context earlier in the overall
> >>> output.  If it were something we truly wanted to worry about, the
> >>> solution would be pre-loading the entire message into a single buffer,
> >>> then calling xwrite() just once - but that's far more effort for
> >>> something we don't anticipate hitting in normal usage anyways.  I'm
> >>> happy if you ignore this whole paragraph of mine.
> >>
> >> Any single buffer presents the problem of sizing the buffer
> >> appropriately, which we can't do in this context :)
> > 
> > Actually, we *can* do better:
> > 
> > https://pubs.opengroup.org/onlinepubs/9699919799/functions/writev.html
> > 
> > What we're doing here is the textbook use case for scatter-gather (well,
> > in this case, gather). It's strange that it has taken me one night to
> > realize this (it occurred to me before falling asleep last night), given
> > that I heavily used scatter-gather in other, not-so-old, code. (Namely
> > the edk2 virtiofs driver.)
> > 
> > I'll attempt to replace xwrite() with xwritev().
> 
> No, I won't -- writev() is not required to be async-signal-safe, so it's
> not good enough for NBD_INTERNAL_FORK_SAFE_ASSERT().

Actually, that's a bug in POSIX; writev() is async-safe on all known
platforms:

https://www.austingroupbugs.net/view.php?id=1455

In part because writev() is documented as being "identical to write()
except for..." where the exception does NOT mention async safety, and
therefore writev() must be async safe if write() is.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org