[Libguestfs] [PATCH nbdkit] server: Add -D nbdkit.environ=1 to dump the environment

[Eric Blake]

On Mon, May 08, 2023 at 10:33:25PM +0100, Richard W.M. Jones wrote:
> > >  
> > 
> > I was surprised not to see any assignment to the new global variable,
> > but then I found "server/debug-flags.c"; that's some serious wizardry.
> 
> With the side effect, as it turns out, that it's very hard to
> implement for language plugins.  Rust is fine.  OCaml can be done by
> adding a C object to the link.  Other languages, not really :-(

And not today's problem to solve.

> 
> > Side thought: should we not make both "nbdkit_debug_environ" (and
> > friends), and the target of the "sym" pointer in "apply_debug_flags",
> > volatile? Aliasing doesn't get much more obscure than this, and I wonder
> > if this has the potential to blow up later, when compilers (and
> > processors) get even more clever.
> > 
> > Of course POSIX could always say "this is just required to work", which
> > I guess would be fine, but currently it doesn't seem to say it.
> 
> I guess Eric will know for sure, but isn't it generally true that all
> global variables can be modified by any function call?  My model is
> that they can modified at any time by any called function; even
> through a pointer so not really visible to the compiler.

Marking a variable as volatile will force the compiler to reload it on
each access, which can slow down loops that read the debug variable.
But in general, these are not variables we are frequently changing (we
do it at most once, during startup, based on the environment); and a
compiler can't completely eliminate global variables unless it is
doing some heavy LTO.  I see no reason why we would want to mark it
volatile, because the only thing that would guarantee is that the
compiler no longer hoists repeated reads of the globals outside of
loops, even though our loops won't be changing the debug state.

I suppose something like LTO could provide a compiler an opportunity
to say that no .o linked into the final image modifies the variable
directly, which may hurt nbdkit.* debug variables in the main
executable, but I don't see how the compiler could ever optimize away
access to globals in .so plugins (since the very nature of .so's is
that they will be dynamically run by code that the compiler didn't
see).  This is all waaaaay outside of the bounds of what POSIX says,
so at best we are just relying on "does it work for how we are using
it" - but since we are only using it for debugging, where it is doing
what we want, I'm not too worried about trying to make this comply
with some unwritten spec.  Sometimes, looking the other way when there
is dark magic present is the prudent action.

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