lib: fix build error on MacOS

[httpstorm]

Sections use a different syntax for Mach-O executables.

The frr package has a host build step in OpenWRT, which can be cross-compiled on macOS. This commit fixes errors while building OpenWRT on macOS host.

Fixes:

# OpenWRT
make package/feeds/packages/frr/host/{clean,compile} -j 1 V=s
…
lib/command_graph.c:16:1: error: argument to 'section' attribute is not valid for this target: mach-o section specifier requires a segment and section separated by a comma DEFINE_MTYPE_STATIC(LIB, CMD_TOKENS, "Command Tokens"); ^
./lib/memory.h:139:2: note: expanded from macro 'DEFINE_MTYPE_STATIC'
        DEFINE_MTYPE_ATTR(group, name, static, desc)                           \
        ^
./lib/memory.h:109:26: note: expanded from macro 'DEFINE_MTYPE_ATTR'
                __attribute__((section(".data.mtypes"))) = { {                 \

Compile tested as part of OpenWRT buildroot: macOS 14.4.1, Ubuntu 22.04.4 LTS.
Changes from the original version [1]: rebased on master, fixed formatting style. Please let me know if any changes are needed.

[1] #6032

Maintainer: @eqvinox
@polychaeta @qlyoung @donaldsharp @rzalamena @rubenk @robimarko

[eqvinox]

This is only the tip of the iceberg that you would need to tackle to get FRR to work on macOS. The FRR community has decided it's not worth the effort. If you really want, I suppose you could try to fix all of the entire iceberg, but — again, this is only the tip.

Take a look at clippy and what it does with libelf, that's not gonna work on macOS full stop…

[httpstorm]

@eqvinox
The frr package has a host build step in OpenWRT, which can be cross-compiled on macOS. I'm not sure where it is used and why. This commit fixes errors while building OpenWRT on macOS host. I hope this clarifies the reason behind my contribution.
openwrt/packages#24053

[eqvinox]

host build step in OpenWRT, which can be cross-compiled on macOS

Aaaaah that makes much more sense. Please mention that the fix is for that ;)

[httpstorm]

@eqvinox

Aaaaah that makes much more sense. Please mention that the fix is for that ;)

Ha hah, done. Sorry for the confusion. I changed the description. 😆

[eqvinox]

Hmm, since this affects only clippy, the easier fix would be to just remove the section attribute. It's not needed there.

(The primary purpose of this is to place the MTYPE variables close to each other to get [page] cache locality, so technically the entire thing is optional—nothing will break if you remove those specific bits. To be clear, this applies only here for MTYPE/MGROUP. There are other section attributes in xref_* and those are absolutely needed. Just to note in case someone is looking at porting FRR to macOS.)

[eqvinox]

To be clear, the xref stuff is not used for clippy (which is the host build step.)

[eqvinox]

↑ Unrelated whitespace change

[eqvinox]

the easier fix would be to just remove the section attribute.

Hmm. I guess there's no benefit to change it to remove the attribute, this should be fine as is. Please remove that added whitespace though 😃

[httpstorm]

@eqvinox

Hmm. I guess there's no benefit to change it to remove the attribute, this should be fine as is. Please remove that added whitespace though 😃

Ready.

[eqvinox]

LGTM

[eqvinox]

Merging when CI completes (which will take a bit because it's Tuesday which is "CI overloaded" day for FRR)

[httpstorm]

@eqvinox
Thank you for your help!