[Libguestfs] [PATCH 3/5] threads: Use thread-local storage for errors.

[Daniel P. Berrange]

On Thu, Jun 11, 2015 at 11:32:59AM +0100, Richard W.M. Jones wrote:
> On Thu, Jun 11, 2015 at 11:27:23AM +0100, Daniel P. Berrange wrote:
> > On Sat, Jun 06, 2015 at 02:20:39PM +0100, Richard W.M. Jones wrote:
> > > We permit the following constructs in libguestfs code:
> > > 
> > >   if (guestfs_some_call (g) == -1) {
> > >     fprintf (stderr, "failed: error is %s\n", guestfs_last_error (g));
> > >   }
> > > 
> > > and:
> > > 
> > >   guestfs_push_error_handler (g, NULL, NULL);
> > >   guestfs_some_call (g);
> > >   guestfs_pop_error_handler (g);
> > > 
> > > Neither of these would be safe if we allowed the handle to be used
> > > from threads concurrently, since the error string or error handler
> > > could be changed by another thread.
> > > 
> > > Solve this in approximately the same way that libvirt does: by making
> > > the error, current error handler, and stack of error handlers use
> > > thread-local storage (TLS).
> > > 
> > > The implementation is not entirely straightforward, mainly because
> > > POSIX doesn't give us useful destructor behaviour, so effectively we
> > > end up creating our own destructor using a linked list.
> > > 
> > > Note that you have to set the error handler in each thread separately,
> > > which is an API change (eg: if you set the error handler in one
> > > thread, then pass the handle 'g' to another thread, the error handler
> > > in the second thread appears to have reset itself back to the default
> > > error handler).  I haven't yet worked out a better way to solve this.
> > 
> > Do you have a feeling whether the error handler function push/pop
> > is a commonly used feature or not ?
> 
> All the time, in our code:
> 
> $ git grep -E 'guestfs_(push|pop)_error_handler' -- *.c | wc -l
> 83
> 
> Of course we could change those, but it is still an API guarantee.

Right but you have some flexibility here. The API guarantee means
that you'll never break existing applications - which will all be
using any single connection handle from a single thread. Introducing
new rules for multi-threaded handle usage is OK, as long as you don't
regress the single-threaded handle usage.

> > In libvirt we originally had
> > 
> >   virGetLastError     (global error)
> >   virConnGetLastError (per connection error)
> >   virSetErrorFunc     (global error handler func)
> > 
> > When we introduced thread support we didn't try to make the
> > virConnGetLastError/SetErrorFun work. We just updated the docs
> > to tell applications to /never/ use them in threaded applications,
> > and always use the virGetLastError which was thread-local
> > backed. For back compatibility we do copy the error from
> > the global thread local into the per-connection object, so we
> > didn't break single-threaded apps using that old function.
> >
> > IOW I think you could avoid alot of the complexity here if you
> > just marked the existing error handler functions as deprecated
> > in the context of multi-threaded usage and just introduced a
> > single global "get error" function that was backed by a static
> > allocated thread local.

You could maintain the push/pop behaviour too, but have it via a new
API pair that uses a static thread local for the handle stack, instead
of per handle

Regards,
Daniel
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