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
  * 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

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