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

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.