[Libguestfs] [PATCH nbdkit v2] tmpdisk: Generalize the tmpdisk plugin

An evolution of v1 here: https://www.redhat.com/archives/libguestfs/2020-April/msg00035.html I want to generalize the tmpdisk plugin, particularly so you can use commands like ‘qemu-img create’ or even ‘virt-builder’. (Actually virt-builder really works - I tested it - but of course it causes a 30+ second delay when connecting to the server.) You can now use commands such as: nbdkit tmpdisk 16G command=' truncate -s $size "$disk" ' The way it works is it creates a temporary directory under $TMPDIR, runs the external command with disk=$TMPDIR/<random>/disk, then when the external command finishes the plugin opens the disk, finds out the size and serves it. At the same time the plugin deletes the disk and temporary subdirectory so it is all automatically cleaned up even if nbdkit crashes. It seems this is secure because mkdtemp(3) creates the subdirectory with 0700 permissions, so no other user on the same machine should be able to monkey around with the disk. Hopefully. But I suggest Eric takes a closer look with his much wider experience :-) Rich.