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()	fix by e.g. merging agent and helpers libs (see below)
agent -> helpers	netsnmp_register_null	helpers/null.c	agent_registry.c:setup_tree()	ditto
agent -> helpers	netsnmp_register_null_context	helpers/null.c	agent_registry.c:netsnmp_subtree_load()	ditto
agent -> helpers	netsnmp_register_old_api	helpers/old_api.c	agent_registry.c:register_mib_context()	ditto
agent -> helpers	netsnmp_bulk_to_next_fix_requests	helpers/bulk_to_next.c	mibgroup/agentx/master.c:agentx_got_response()	ditto
agent -> helpers	netsnmp_get_bulk_to_next_handler	helpers/bulk_to_next.c	agent_handler.c:netsnmp_register_handler()	ditto
other strange dependencies
libnetsnmptrapd -> snmptrapd	extern int dropauth	apps/snmptrapd.c	apps/snmptrapd_handlers.c:print_handler()	Move 'dropauth' from snmptrapd.c to snmptrapd_handlers.c. Done for MAIN.
libnetsnmptrapd -> snmptrapd	extern int SyslogTrap	apps/snmptrapd.c	apps/snmptrapd_handlers.c:syslog_handler()	Move 'SyslogTrap' from snmptrapd.c to snmptrapd_handlers.c. Done for MAIN.
libnetsnmptrapd -> snmptrapd	event_input	apps/snmptrapd.c	apps/snmptrapd_handlers.c:event_handler()	Drop "-e"/printEventNumbers support altogether. Done for MAIN.

Discussion

Most library layering issues have been solved without any drastic changes (see below for details). There's just one issue left: the circular dependency between the agent and helpers lib.

As of May 2010, we seem to have three choices:

• Merge agent and helpers libraries into one libagent library again. Pro: maintain backward-compatibility. Con: agent lib will have increased footprint (+50%). Need to keep empty helpers lib around which is odd.
• Remove the netsnmp_init_helpers() call from init_agent() and require agent developers to add an explicit call to netsnmp_init_helpers() after every call to init_agent(). Pro: keep libraries separate. Small change for developers. Con: backwards-incompatible change.
• Clever compiler constructs to check if helpers lib has been loaded before agent lib. Con: non-portable (gcc-ism)!! Pro: maintain backward-compatibility, keep libraries separate.

The choice under discussion is basically between the first and second approach, unless somebody can come up with a portable third approach.

Status

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+
resolve circular dependency between agent and helpers libs (e.g. by merging them)	tbd	TODO (trunk only)
fix libnetsnmptrapd->snmptrapd dependencies by moving integer variables to the library and dropping the event_input code	dts12, tanders	done for MAIN
rework Makefiles/configure to fullfil internal and external library dependencies correctly (see below)	tanders	done (5.4+)
link libperl to libnetsnmpagent instead of snmpd/snmptrapd. How to check at configure time whether linking of libperl to a shared library works?	tanders	forced for 5.5 (w/o additional configure check, r16529); protected by --enable-as-needed in 5.4 (r16569)

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	ditto	ok
libnetsnmpagent.so	-	-lnetsnmp -lperl Linking a shared library against libperl turns out to be tricky since existing PERLLDOPTS reference the static DynaLoader.a which isn't always portable.	r16529 (5.5) + r16569 (5.4.1.c1)
libnetsnmphelpers.so	-	-lnetsnmpagent -lnetsnmp	r16419 (5.4+)
libnetsnmpmibs.so	-	-lnetsnmphelpers -lnetsnmpagent -lnetsnmp	r16419 (5.4+)
		-ldl -lrpm -lrpmio -lpopt -lz ...	r16399 (5.4+)
libnetsnmptrapd.so	-	-lnetsnmpmibs -lnetsnmp -lperl	r16419 (5.4+), r16529 (Perl, 5.5)
Perl SNMP/SNMP.so	-lnetsnmp -lcrypto (net-snmp-config --libs)	remove "-crypto" in "net-snmp-config --libs"	r16419 (5.4+)
Perl NetSNMP/ASN/ASN.so	see SNMP.so	see SNMP.so	r16419 (5.4+)
Perl NetSNMP/OID/OID.so	see SNMP.so	see SNMP.so	r16419 (5.4+)
Perl NetSNMP/agent/default_store/default_store.so	see SNMP.so	see SNMP.so	r16419 (5.4+)
Perl NetSNMP/default_store/default_store.so	see SNMP.so	see SNMP.so	r16419 (5.4+)
Perl NetSNMP/TrapReceiver/TrapReceiver.so	-	-lnetsnmptrapd -lnetsnmpagent -lnetsnmp	r16419 (5.4+)
Perl NetSNMP/agent/agent.so	almost everything	remove: most external references (dependencies are in libnetsnmpmibs instead)	fixed for 5.4.1.pre3/5.5
Python netsnmp/client_intf.so	see SNMP.so	see SNMP.so	r16419 (5.4+)
agent/snmpd	almost everything	remove: most external references (incl. Perl; dependencies are in libnetsnmpmibs/libnetsnmpagent instead)	fixed for 5.4.1.pre3/5.5
apps/snmptrapd	almost everything	remove: most external references (incl. Perl; dependencies are in libnetsnmpmibs/libnetsnmpagent instead)	fixed for 5.4.1.pre3/5.5
apps/snmp{get,set,walk,...}	almost everything	remove: most external references (not needed at all)	fixed for 5.4.1.pre2/5.5

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
• perl5-porters thread "how to best link libperl to a shared library?"