Library layering
Contents
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 5.4+ 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() | ? |
| other strange dependencies | ||||
| libnetsnmptrapd -> snmptrapd | extern int dropauth | apps/snmptrapd.c | apps/snmptrapd_handlers.c:print_handler() | ? |
| libnetsnmptrapd -> snmptrapd | event_input | apps/snmptrapd.c | apps/snmptrapd_handlers.c:event_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
| what? | who? | status? |
| rework Makefiles to build libnetsnmp, libnetsnmpagent, libnetsnmphelpers and libnetsnmpmibs in that order | tanders | done for 5.4+ |
| duplicate libnetsnmpmibs' make_tempfile as netsnmp_mktemp in libnetsnmp (now fixed in 5.4+) | tanders | done for 5.4+ |
| introduce callbacks for the other dependencies | rstory has offered to come up with a patch | TODO |
| rework Makefiles/configure to fullfil internal and external library dependencies correctly (see below) | tanders | TODO |
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
