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 in ... (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() | Propose that 'dropauth' is moved from snmptrapd.c to snmptrapd_handlers.c (i.e. exchange 'int' and 'extern int') |
| libnetsnmptrapd -> snmptrapd | extern int SyslogTrap | apps/snmptrapd.c | apps/snmptrapd_handlers.c:syslog_handler() | Propose that 'SyslogTrap' is moved from snmptrapd.c to snmptrapd_handlers.c (i.e. exchange 'int' and 'extern int') |
| libnetsnmptrapd -> snmptrapd | event_input | apps/snmptrapd.c | apps/snmptrapd_handlers.c:event_handler() | Propose this special-case handling is dropped altogether. |
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 | tanders | done for 5.4+ |
| introduce callbacks for the agent->helpers dependencies | rstory has proposed a patch on -coders | TODO |
| fix libnetsnmptrapd->snmptrapd dependencies by moving integer variables to the library and dropping the event_input code | dts12, tanders | TODO |
| rework Makefiles/configure to fullfil internal and external library dependencies correctly (see below) | tanders | TODO |
Library linking
Here are the details of what library linking dependencies need to be cleaned up.
| shared object/library/binary | currently linked against | dependencies to be added/removed | Comment |
| libnetsnmp.so | -lcrypto | - | ok |
| libnetsnmpagent.so | - | -lnetsnmp -lperl | TODO |
| libnetsnmphelpers.so | - | -lnetsnmpagent -lnetsnmp | TODO |
| libnetsnmpmibs.so | - | -lnetsnmphelpers -lnetsnmpagent -lnetsnmp -ldl -lrpm -lrpmio -lpopt -lz ... | TODO |
| libnetsnmptrapd.so | - | -lnetsnmpmibs -lnetsnmp | TODO |
| Perl SNMP/SNMP.so | -lnetsnmp -lcrypto | - | ok |
| Perl NetSNMP/ASN/ASN.so | -lnetsnmp -lcrypto | - | ok |
| Perl NetSNMP/OID/OID.so | TODO | ||
| Perl NetSNMP/agent/agent.so | almost everything | remove: most external references ? (dependencies are in libnetsnmpmibs instead)
You can't do this unfortunately... perl doesn't use libtool, and only libtool is protecting you against having to know dependencies on every platform. --Wes |
TODO |
| Perl NetSNMP/agent/default_store/default_store.so | -lnetsnmp -lcrypto | remove: both ? | TODO |
| Perl NetSNMP/TrapReceiver/TrapReceiver.so | - | -lnetsnmptrapd -lnetsnmpagent -lnetsnmp | TODO |
| Perl NetSNMP/default_store/default_store.so | -lnetsnmp -lcrypto | remove: both ? | TODO |
| Python netsnmp/client_intf.so | -lnetsnmp -lcrypto | - | ok |
| agent/snmpd | almost everything | remove: most external references ? (dependencies are in libnetsnmpmibs instead) | TODO |
| apps/snmptrapd | almost everything | remove: most external references ? (dependencies are in libnetsnmpmibs instead) | TODO |
| apps/snmp{get,set,walk,...} | almost everything | remove: most external references ? (not needed at all) | 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
