Re: [Libguestfs] nbdkit / mingw support

On 3/25/20 7:16 AM, Richard W.M. Jones wrote: >I think I understand now that libnbdkit.so won't break the ABI for >existing plugins. Does it require that plugins for newer nbdkit use >-lnbdkit (which would be a source API break) or would it still be >possible to compile without this? I guess as long as plugins do not >start using -no-undefined then it would still work, so it wouldn't be >a source API break. Yes, that's what I'm thinking - it's an API break if you have to add in -lnbdkit; but you only have to add that if you want to use -no-undefined. Plugins compiled without -lnbdkit (either because they predate the library, or because they specifically did not care about -no-undefined) should still be viable. > >I had a look into how we might implement libnbdkit.so. Some functions >are obviously self-contained (eg. nbdkit_parse_*, nbdkit_realpath, >nbdkit_debug, nbdkit_error, nbdkit_*extents). > >Unfortunately some functions depend themselves on internals >of the server: > > * nbdkit_nanosleep, nbdkit_export_name, nbdkit_peer_name call > threadlocal_get_conn > * nbdkit_set_error calls threadlocal_set_error > * nbdkit_shutdown must set the quit global (or call a server function) Yeah, there's some awkward dependencies to figure out. It's obvious the library has to export public nbdkit_* interfaces for the sake of plugins, but can it also export one additional symbol _nbdkit_init() for internal use? Then we can have the nbdkit binary pass whatever additional hooks are needed for proper isolation of the public interface (a callback pointer to get at threadlocal_get_conn, threadlocal_set_error, the address of the quit global, ...), and leave the symbol undocumented (plus the fact that the leading _ will discourage plugins from trying to abuse it).

> >I guess we can deal with the first ones by moving threadlocal.c into >the same library, although it's a bit awkward. The quit flag is still >more awkward because you have to move a lot of quit pipe handling code >into the library which has knock-on effects all over. The other extreme is to have the entire nbdkit engine in libnbdkit.so, plus the addition of a single internal-only callback _nbdkit_main(), then the nbdkit binary becomes a bare-bones: int main (int argc, char *argv[]) { return _nbdkit_main (argc, argv); } at which point you don't have to decide which functionality lives where. After all, you already have a linker script that limits what the main binary exports; which really becomes what libnbdkit.so has to export. If we are worried about plugins trying to abuse that additional entry point, you can have _nbdkit_main set a flag on first execution so that subsequent attempts to call it fail immediately; but I'm not too worried about it. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org