Re: proposal: nsswitch refactoring

[DJ Delorie <dj at redhat dot com>]

Florian Weimer <fw@deneb.enyo.de> writes:
> We currently bind lookup functions lazily, so the module data is not
> entirely write-once, more like a collection of ivars (called SyncVar
> in Concurrent ML, basically futures without blocking).

But once we bind those and record their addresses, they won't change, so
I put them in the "write once" group.  The rest of the data needs to be
write-flush-rewrite capable, in order to unload nsswitch.conf data and
reload it.

> One tricky aspect about the current framework is that the generic NSS
> code does not know which lookup functions exist.

I had noted that, and figured some sort of cache/hash/whatever would be
needed somewhere, but elided the problem for now :-)

> We already do this for the resolver configuration.  We could probably
> reuse the same mechanism, and even the same data structure.

Thanks, I'll look at it.

> Which aspect are you referring to?  The handling of struct pwd, struct
> hostend, etc., including parsing file data?  Or forwarding a lookup
> operation from a function such as getpwnam to an NSS module function
> like _nss_files_getpwnam_r?

This doesn't include the shared objects like nss_files, nss_ldap, etc.
Just the core code that calls them all.  I figure the arguments would
need to be stored in a struct, possibly with some other stateful
information.  Like this pseudo-code:

static int
my_so_caller (nss_handler *shared_object, my_data *data)
{
  /* call the shared_object's functions, passing my_data */
}

int
my_api_handler (arg1, arg2, arg3)
{
  struct { ... } my_data;
  my_data.arg1 = arg1;
  ...
  nss_logic_handler (NSS_PWNAM, my_so_caller, &my_data);
  return my_data.rv;
}

This means that none of the individual API functions need to have code
to do the logic of nsswitch.conf.

Alternately, the core logic could be a stateful machine:

int
my_api_handler (arg1, arg2, arg3)
{
  nss_logic_state state;
  nss_logic_initialize (NSS_PWNAM, &state);

  while (nss_logic_handler (&state))
    state->rv = state->shared_object->do_something (arg1, arg2, arg3);

  return nss_logic_finalize (&state);
}