[Libguestfs] [PATCH libnbd v2 0/3] fuse: Enable multithread support

v1 was here: https://listman.redhat.com/archives/libguestfs/2021-May/msg00081.html Compared to v1, patches 1/3 and 3/3 are identical. The new version of patch 2/3 cleans up parallel support. In v1 (https://listman.redhat.com/archives/libguestfs/2021-May/msg00083.html) the thread model was ... interesting. Each FUSE thread would race poll(2) calls on the same file descriptor against each other, and then when one succeeded it would grab a global lock, rerun poll on the file descriptor (to get the true picture of the fd's state), and run the NBD state machine within that global lock but on the FUSE thread. In this version there is a single background thread which dispatches NBD requests on the handle. FUSE threads only start the asynch command, then block waiting for the background thread to finish off their command. Perhaps not too surprisingly v1 is quite a bit faster than v2. v2 suffers from the overhead of coordination between the FUSE threads and the single thread performing the NBD operations (as well as there being only a single thread doing the work). But I think v2 is more maintainable. Multi-conn would help here and also be easier to add with v2. The FIO results presented in the commit messages are stable. Rich.