Re: [Libguestfs] [PATCH v3 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.