python3-netifaces: build error caused by linking with -L/usr/lib in configure script

[cotequeiroz]

Maintainer: @BKPepe
Environment: mvebu, arm_cortex-a9+vfpu3, current master

Description:
I couldn't find the bug in the bots, but it fails here consistently. The failure output is this:

if [ "" == "HOST" ]; then export PYTHONPATH="/home/equeiroz/src/openwrt/staging_dir/hostpkg/lib/python3.7:/home/equeiroz/src/openwrt/staging_dir/hostpkg/lib/python3.7/site-packages"; export PYTHONDONTWRITEBYTECODE=0; else export PYTHONPATH="/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/lib/python3.7:/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi//usr/lib/python3.7/site-packages:/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9/ipkg-install//usr/lib/python3.7/site-packages"; export PYTHONDONTWRITEBYTECODE=1; export _python_sysroot="/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi"; export _python_prefix="/usr"; export _python_exec_prefix="/usr"; fi; export PYTHONOPTIMIZE="";   cd /home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9/; CC="arm-openwrt-linux-muslgnueabi-gcc" CCSHARED="arm-openwrt-linux-muslgnueabi-gcc -fpic" CXX="arm-openwrt-linux-muslgnueabi-g++" LD="arm-openwrt-linux-muslgnueabi-gcc" LDSHARED="arm-openwrt-linux-muslgnueabi-gcc -shared" CFLAGS="-Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro" CPPFLAGS="-I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7" LDFLAGS="-L/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/lib -L/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/lib -L/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/lib -L/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/lib -znow -zrelro -lpython3.7" _PYTHON_HOST_PLATFORM=linux2 __PYVENV_LAUNCHER__="/usr/bin/python3.7"      /home/equeiroz/src/openwrt/staging_dir/hostpkg/bin/python3.7   ./setup.py   install --prefix="/usr" --root="/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9/ipkg-install" --single-version-externally-managed ;
running install
running build
running build_ext
checking for getifaddrs...not found.
checking for getnameinfo...not found.
checking for socket IOCTLs...not found.
checking for optional header files...netpacket/packet.h linux/atm.h linux/llc.h linux/tipc.h linux/dn.h.
checking whether struct sockaddr has a length field...no.
checking which sockaddr_xxx structs are defined...none!
checking for routing socket support...no.
checking for sysctl(CTL_NET...) support...no.
checking for netlink support...no.
building 'netifaces' extension
arm-openwrt-linux-muslgnueabi-gcc -DNDEBUG -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/Python-3.7.2:Python-3.7.2 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7 -fPIC -DNETIFACES_VERSION=0.10.9 -DHAVE_NETPACKET_PACKET_H=1 -DHAVE_LINUX_ATM_H=1 -DHAVE_LINUX_LLC_H=1 -DHAVE_LINUX_TIPC_H=1 -DHAVE_LINUX_DN_H=1 -I/home/equeiroz/src/openwrt/staging_dir/hostpkg/include/python3.7 -c netifaces.c -o build/temp.linux2-3.7/netifaces.o
cc1: note: someone does not honour COPTS correctly, passed 2 times
netifaces.c:210:6: error: #error You need to add code for your platform.
 #    error You need to add code for your platform.
      ^~~~~
error: command 'arm-openwrt-linux-muslgnueabi-gcc' failed with exit status 1
make[2]: *** [Makefile:47: /home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9/.built] Error 1
make[2]: Leaving directory '/home/equeiroz/src/openwrt/feeds/packages/lang/python/python3-netifaces'

I've checked the build dir, and found out where the problem seems to be:

~/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9/build/temp.linux2-3.7 $ cat conftest.out
creating build/temp.linux2-3.7/build
creating build/temp.linux2-3.7/build/temp.linux2-3.7
arm-openwrt-linux-muslgnueabi-gcc -DNDEBUG -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/Python-3.7.2:Python-3.7.2 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/equeiroz/src/openwrt/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/home/equeiroz/src/openwrt/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7 -fPIC -I/home/equeiroz/src/openwrt/staging_dir/hostpkg/include/python3.7 -c build/temp.linux2-3.7/conftest-0.c -o build/temp.linux2-3.7/build/temp.linux2-3.7/conftest-0.o
cc1: note: someone does not honour COPTS correctly, passed 2 times
arm-openwrt-linux-muslgnueabi-gcc build/temp.linux2-3.7/build/temp.linux2-3.7/conftest-0.o -L/usr/lib -o build/temp.linux2-3.7/conftest
/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/../../../../arm-openwrt-linux-muslgnueabi/bin/ld: skipping incompatible /usr/lib/libc.so when searching for -lc
/home/equeiroz/src/openwrt/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/../../../../arm-openwrt-linux-muslgnueabi/bin/ld: /usr/lib/libc.a: error adding symbols: file format not recognized
collect2: error: ld returned 1 exit status

If I link the file manually, omitting -L/usr/lib, it succeeds; if I hack TARGET_CC+= $(TARGET_LDFLAGS) in the Makefile, the package builds, but I don't believe this is a solution, so I'm not opening a PR. I'm not familiar with python's package build system, so I could not identify where the flag is being added. @jefferyto, @commodo, do you have any thoughts regarding this?

[BKPepe]

This one doesn't happen for me. I'm able to compile it locally for Turris Omnia (same arch) and it is even built on Turris servers - https://repo.turris.cz/master/omnia/packages/packages/python3-netifaces_0.10.9-3.7-1_arm_cortex-a9_vfpv3.ipk

Would be good if I can reproduce it.

[cotequeiroz]

Can you compare your build/temp.linux2-3.7/conftest.out (from the package's build dir) with the one I posted above?

[BKPepe]

Here it is:

✔ ~/work/turrisos40/omnia-hbd/build/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9/build/temp.linux2-3.7 [master L|…17] 
22:11 $ cat conftest.out
creating build/temp.linux2-3.7/build
creating build/temp.linux2-3.7/build/temp.linux2-3.7
ccache_cc -DNDEBUG -Os -pipe -mcpu=cortex-a9 -mfpu=vfpv3-d16 -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/pepe/work/turrisos40/omnia-hbd/build/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/Python-3.7.2:Python-3.7.2 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -Os -pipe -mcpu=cortex-a9 -mfpu=vfpv3-d16 -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/home/pepe/work/turrisos40/omnia-hbd/build/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7 -fPIC -I/home/pepe/work/turrisos40/omnia-hbd/build/staging_dir/hostpkg/include/python3.7 -c build/temp.linux2-3.7/conftest-0.c -o build/temp.linux2-3.7/build/temp.linux2-3.7/conftest-0.o
cc1: note: someone does not honour COPTS correctly, passed 2 times
ccache_cc build/temp.linux2-3.7/build/temp.linux2-3.7/conftest-0.o -L/usr/lib -o build/temp.linux2-3.7/conftest

[cotequeiroz]

Thanks! That helps, as it seems to be closer to gcc/binutils/toolchain than to python. Nonetheless, it doesn't seem right that /usr/lib is used there.

[commodo]

I'll take a look these days.

But

Nonetheless, it doesn't seem right that /usr/lib is used there.

yes, it isn't right;
but this is too common for Python & Python packages;
the way all the build elements are stacked-up, some of these absolute/system paths [ /usr/lib, /usr/include, etc] sometimes slip through the cracks;
i guess that's one reason why the whole Python/Python3 packaging in OpenWrt looks so messy;

i sometimes like to re-post this article [ I did not write it ], because it resonates so much even today (7 years after it was written ) with Python packaging
http://lucumr.pocoo.org/2012/6/22/hate-hate-hate-everywhere/

[jefferyto]

I tried compiling this, here is the relevant make output:

running build_ext
checking for getifaddrs...found.
checking for getnameinfo...found.
checking for IPv6 socket IOCTLs...not found.
checking for optional header files...netpacket/packet.h linux/atm.h linux/llc.h linux/tipc.h linux/dn.h.
checking whether struct sockaddr has a length field...no.
checking which sockaddr_xxx structs are defined...in in6 un ll atmpvc atmsvc dn llc.
checking for routing socket support...no.
checking for sysctl(CTL_NET...) support...no.
checking for netlink support...yes.
will use netlink to read routing table
building 'netifaces' extension
arm-openwrt-linux-muslgnueabi-gcc -DNDEBUG -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/media/jeff/Jekyll/Files/openwrt/testing/mvebu/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/Python-3.7.2:Python-3.7.2 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/media/jeff/Jekyll/Files/openwrt/testing/mvebu/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7 -fPIC -DNETIFACES_VERSION=0.10.9 -DHAVE_GETIFADDRS=1 -DHAVE_GETNAMEINFO=1 -DHAVE_NETPACKET_PACKET_H=1 -DHAVE_LINUX_ATM_H=1 -DHAVE_LINUX_LLC_H=1 -DHAVE_LINUX_TIPC_H=1 -DHAVE_LINUX_DN_H=1 -DHAVE_SOCKADDR_IN=1 -DHAVE_SOCKADDR_IN6=1 -DHAVE_SOCKADDR_UN=1 -DHAVE_SOCKADDR_LL=1 -DHAVE_SOCKADDR_ATMPVC=1 -DHAVE_SOCKADDR_ATMSVC=1 -DHAVE_SOCKADDR_DN=1 -DHAVE_SOCKADDR_LLC=1 -DHAVE_PF_NETLINK=1 -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/hostpkg/include/python3.7 -c netifaces.c -o build/temp.linux2-3.7/netifaces.o
cc1: note: someone does not honour COPTS correctly, passed 2 times
creating build/lib.linux2-3.7
arm-openwrt-linux-muslgnueabi-gcc -shared -L/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/lib -L/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/lib -L/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/lib -L/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/lib -znow -zrelro -lpython3.7 -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/media/jeff/Jekyll/Files/openwrt/testing/mvebu/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7 build/temp.linux2-3.7/netifaces.o -L/usr/lib -lpython3.7 -o build/lib.linux2-3.7/netifaces.cpython-37.so

And conftest.out:

creating build/temp.linux2-3.7/build
creating build/temp.linux2-3.7/build/temp.linux2-3.7
arm-openwrt-linux-muslgnueabi-gcc -DNDEBUG -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/media/jeff/Jekyll/Files/openwrt/testing/mvebu/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/Python-3.7.2:Python-3.7.2 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -mfloat-abi=hard -iremap/media/jeff/Jekyll/Files/openwrt/testing/mvebu/build_dir/target-arm_cortex-a9+vfpv3_musl_eabi/netifaces-0.10.9:netifaces-0.10.9 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/usr/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include/fortify -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.4.0_musl_eabi/include -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/python3.7 -fPIC -I/media/jeff/Jekyll/Files/openwrt/testing/mvebu/staging_dir/hostpkg/include/python3.7 -c build/temp.linux2-3.7/conftest-0.c -o build/temp.linux2-3.7/build/temp.linux2-3.7/conftest-0.o
cc1: note: someone does not honour COPTS correctly, passed 2 times
arm-openwrt-linux-muslgnueabi-gcc build/temp.linux2-3.7/build/temp.linux2-3.7/conftest-0.o -L/usr/lib -o build/temp.linux2-3.7/conftest

The -L/usr/lib is certainly not correct, but because it was able to detect the API(s) it needs (presumable from my build system), it continued with the build, using the correct LDFLAGS to build the C extension.

I think the reason why the test link doesn't include the right LDFLAGS is because netifaces uses compiler.link_executable() for the test, whereas setuptools/distutils (when building the actual extension) uses compiler.link_shared_object(). The linker command these two uses are different (linker_exe and linker_so), and whereas LDFLAGS is added to linker_so, linker_exec only gets the CC command. As for why LDFLAGS is not added when linking executables... I have no idea 😂

I also haven't been able to track down where/why -L/usr/lib is added.

[cotequeiroz]

I also haven't been able to track down where/why -L/usr/lib is added.

I have some answers, but I'm no expert in this. I was just lucky and guessed them right 😆.

I know the /usr/lib string was taken from $(STAGING_DIR)/usr/lib/python3.7/_sysconfigdata.py:632:

 'LIBDIR': '/usr/lib',

My guess is that the file contents are generated at build time from Makefile.pre, and stored in $(STAGING_DIR)/usr/lib/python3.7/config/Makefile as well. Remember my note about being lucky.
If I change the LIBDIR line to point where my staging dir is, it works.

LIBDIR gets added to the library_dirs list in $(STAGING_DIR)/usr/lib/python3.7/distutils/command/build_ext.py:236:

        # For building extensions with a shared Python library,
        # Python's library directory must be appended to library_dirs
        # See Issues: #1600860, #4366
        if (sysconfig.get_config_var('Py_ENABLE_SHARED')):
            if not sysconfig.python_build:
                # building third party extensions
                self.library_dirs.append(sysconfig.get_config_var('LIBDIR'))
            else:
                # building python standard extensions
                self.library_dirs.append('.')

I haven't guessed the answer to the LDFLAGS omission. I don't like to push my luck that far. Now the question becomes what's the best way to fix/patch/go around this?

[cotequeiroz]

Found it. Unlike Python2, Python3 does not apply 008-distutils-use-python-sysroot.patch.