Library layering

Problem

The three libraries libnetsnmpmibs, libnetsnmphelpers and libnetsnmpagent have circular dependencies instead of what should be a clean mibs -> helpers -> agent -> main library dependency chain:

• libnetsnmpmibs depends on libnetsnmphelpers
• both of these depend on libnetsnmpagent
• all 3 of these depend on libnetsnmp

There are only a few problematic calls/functions that violate this dependency chain:

Critical dependency	Symbol/function	implementation (under agent/)	caller(s) (under agent/)	Comment
agent -> mibs	make_tempfile	mibgroup/util_funcs.c	mibgroup/utilities/execute.c:run_shell_command()	fixed in MAIN by introducing netsnmp_mktemp
agent -> helpers	netsnmp_init_helpers	helpers/all_helpers.c	snmp_vars.c:init_agent()	The most difficult one?
agent -> helpers	netsnmp_register_null	helpers/null.c	agent_registry.c:setup_tree()	?
agent -> helpers	netsnmp_register_null_context	helpers/null.c	agent_registry.c:netsnmp_subtree_load()	?
agent -> helpers	netsnmp_register_old_api	helpers/old_api.c	agent_registry.c:register_mib_context()	?
agent -> helpers	netsnmp_bulk_to_next_fix_requests	helpers/bulk_to_next.c	mibgroup/agentx/master.c:agentx_got_response()	?
agent -> helpers	netsnmp_get_bulk_to_next_handler	helpers/bulk_to_next.c	agent_handler.c:netsnmp_register_handler()	?

Discussion

The brute-force approach is to move all three mibs/helpers/agent libraries into one huge libagent library again. Downside: even a minimal subagent needs to link in all the mib code which it shouldn't need. That's like it effectively already is today, but it lowers the chance to ever sort it out.

The clever way would be to sensibly restructure the library while maintaining backward-compatibility and not causing too much overhead for agent developers. This needs to be investigated and discussed. Not sure if it's possible at all, though.

See the references below for the full discussion and details to date.

Proposal

• rework Makefiles to build libnetsnmp, libnetsnmpagent, libnetsnmphelpers and libnetsnmpmibs in that order (done for MAIN)
• rework Makefiles/configure to fullfil internal and external library dependencies correctly
• move:
  • make_tempfile from libnetsnmpmibs to libnetsnmp (patch proposed on -coders)
  • ...
• ...

References

• 2006-10-19 #net-snmp IRC log (starting at 07:12:29)
• Bug #1565496: library layering violation
• Bug #1619827: building with --as-needed is broken