Go on Gentoo SPARC

Preface

Gc (Go's most widely-used compiler) isn't available for GOOS=linux && GOARCH=sparc (Gentoo SPARC) for now; what makes matters worse is that Gccgo (sys-devel/gcc[go]) is not working on Gentoo (it hangs in early initialization). Debian Ports comes to the rescue, as although no Gc as well, their Gccgo (at least gccgo-6; gccgo-7 panics in early stages) is working. Yet Debian's SPARC port is on architecture sparc64, which means it'll run on a Gentoo kernel (UltraSPARC T2 requires a 64bit kernel), yet the executables it creates won't run outside without some tricks. (Gentoo SPARC has 32bit userland)

EDIT: as of Feb 2, 2018, gccgo-7 works as well. It seems like they've fixed it. Kudos for Debian team!

Set up Debian Ports environment

Install debootstrap:

emerge --ask --verbose dev-util/debootstrap

Make the debian root and create the chroot with debootstrap:

mkdir -p /var/debian/
debootstrap --arch=sparc64 --variant=buildd --verbose sid /var/debian/ https://deb.debian.org/debian-ports

Download the required keychain package. Chroot into the new debian environemnt, install keychain, and install required packages:

cd /var/debian/root
wget http://deb.debian.org/debian-ports/pool/main/d/debian-ports-archive-keyring/debian-ports-archive-keyring_2018.01.05_all.deb
chroot /var/debian /bin/su
chsh -s /bin/bash
dpkg -i /root/debian-ports-archive-keyring_2018.01.05_all.deb
apt update
apt install gccgo-6

Symlink the required go tools, make the GOPATH, and set it upon entering the chroot:

for a in go{,fmt}; do ln -sf /usr/bin/$a-6 /usr/bin/$a; done
mkdir -p /root/go
echo "export GOPATH=/root/go" >> /root/.bashrc

Exit the chroot, enter it once again, and verify that everything's working:

exit
chroot /var/debian /bin/su
go env

Test the toolchain via a simple Hello, world!:

cd
cat > helloworld.go << EOF
package main

import "fmt"

func main() {
    fmt.Println("Hello, world!")
}
EOF
go run helloworld.go
go build helloworld.go
./helloworld

Set up Gentoo to use the executables from Debian chroot

Now that we can produce executables, we need to set up the Gentoo system to be able to use the executables from Debian chroot. Debian has 64bit libc:

root@hikari:~# ldd helloworld
        libgo.so.9 => /usr/lib/sparc64-linux-gnu/libgo.so.9 (0xffff800100230000)
        libm.so.6 => /lib/sparc64-linux-gnu/libm.so.6 (0xffff8001015ac000)
        libgcc_s.so.1 => /lib/sparc64-linux-gnu/libgcc_s.so.1 (0xffff800101794000)
        libc.so.6 => /lib/sparc64-linux-gnu/libc.so.6 (0xffff8001018a8000)
        libpthread.so.0 => /lib/sparc64-linux-gnu/libpthread.so.0 (0xffff800101b18000)
        /lib64/ld-linux.so.2 (0xffff800100000000)

And for gccgo-7:

root@hikari:~# ldd helloworld
        libgo.so.11 => /usr/lib/sparc64-linux-gnu/libgo.so.11 (0xffff800100230000)
        libm.so.6 => /lib/sparc64-linux-gnu/libm.so.6 (0xffff800101b60000)
        libgcc_s.so.1 => /lib/sparc64-linux-gnu/libgcc_s.so.1 (0xffff800101d44000)
        libc.so.6 => /lib/sparc64-linux-gnu/libc.so.6 (0xffff800101e58000)
        libz.so.1 => /lib/sparc64-linux-gnu/libz.so.1 (0xffff8001020c0000)
        libpthread.so.0 => /lib/sparc64-linux-gnu/libpthread.so.0 (0xffff8001021dc000)
        /lib64/ld-linux.so.2 (0xffff800100000000)

We need to copy those to the Gentoo host's /lib64 and add it to ld's search paths.

mkdir -p /lib64
for a in /usr/lib/sparc64-linux-gnu/libgo.so.{9,11} /lib/sparc64-linux-gnu/{libm.so.6,libgcc_s.so.1,libc.so.6,libz.so.1,libpthread.so.0}; do cp -Lv /var/debian$a /lib64/$(basename $a); done
cp -v /var/debian/lib/sparc64-linux-gnu/ld-2.26.so /lib64/ld-linux.so.2
cp -Pv /var/debian/lib/sparc64-linux-gnu/libnss* /lib64 # needed for proper user / dns support
sed -i -e 's|/lib:/usr/lib|/lib:/lib64:/usr/lib|' /etc/env.d/00basic
env-update

Note: if you upgrade the debian chroot, watch closely if libc and friends have been updated. If so, remember to copy the libraries over to avoid possible breakage.

We can now check if the executable works outside the chroot:

cd /var/debian/root
ldd helloworld
./helloworld

If any libraries are missing, we need to copy it over (to /lib64) and re-generate ld.so.cache with env-update. From this point we're able to compile the Go source code in the Debian chroot, and copy it over for use.

Extra setup for ease of use

Copying the executable outside the chroot outside every time is exhausting. The following setup will ease the process of compiling source and using the executable.

Set up fstab entries for the debian chroot, so that it will be ready for use right after boot. Failing to mount system psuedo-filesystems (especially /proc) will result in mysterious errors with libbacktrace.

# debian chroot mounts
/dev /var/debian/dev none defaults,rbind,rslave 0 0
/sys /var/debian/dev none defaults,rbind,rslave 0 0
/proc /var/debian/proc none defaults,rbind,rslave 0 0
# mount to expose the executables built to the system outside
/var/debian/root/go/bin /usr/local/go/bin none defaults,bind 0 0

The last entry exposes ${GOPATH}/bin inside the debian chroot to the system outside, so that we can add /usr/local/go/bin to PATH, and easily use them outside (as we have set up appropriate libc outside).

Add the /usr/local/go/bin to PATH:

echo "PATH=\"/usr/local/go/bin\"\nROOTPATH=\"/usr/local/go/bin\"" | sudo tee /etc/env.d/40debian-golang
env-update

And add the following alias to your shell config (.bashrc or .zshrc):

alias gobuild='sudo chroot /var/debian /bin/bash --login'

We can now issue gobuild from Gentoo, go into the Debian chroot, issue

go get path/to/your/great/package

to install your binary, and then you can access them outside the chroot as well, thanks to the bind mount and the PATH settings.

Credits

The method to resolve the incompatiable libc libraries comes from lilydjwg, while the suggestion of installing the keychain package instead of importing raw keys comes from zhsj. Thanks.