Re: [Libguestfs] [nbdkit PATCH 2/4] truncate: Factor out reading real_size under mutex

Add a helper function get_real_size() to make it easier to add sanity checking that mutex calls don't fail.

I'm a bit unsure why truncate.c needs a lock anyway. I guess it's because we are storing 'size' and 'real_size' as globals rather than per-connection values in the handle, and we're worrying about parallel connections where we don't want one thread reading inconsistent values while another thread is in the middle of truncate_get_size()? At any rate, as long as we don't have dynamic resize in the NBD protocol, size can't change within the confines of a single connection. And even if we DO assume that the underlying plugin reports different sizes for different connections, our use of a global does not play well (if client A sees size 1k, then client B sees size 2k, that changes client A's use of 'real_size' to be something different than client A was expecting). So, I think we have to move 'size' and 'real_size' into the per-connection handle, at which point, can't we just set them once during truncate_prepare by moving the existing guts of truncate_get_size there, and then letting truncate_get_size just return h->size while all other truncate_* functions just access h->real_size without a lock?