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

[Daniel P. Berrange]

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 ?

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.

Regards,
Daniel
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