contrib/rootfs-builder: Support multiple architectures

The new Make handling is using:

* Default values [1] for ARCHES ?= amd64
* Substitution references [2] for $(ARCHES:%=rootfs-%.tar.gz).
* % [3] and $* [4] for pattern rules like rootfs-%.tar.gz.
* .PRECIOUS [5] to avoid errors like:

    rm downloads/stage3-amd64-current.tar.bz2 rootfs/amd64/bin/busyboxmake: unlink: rootfs/amd64/bin/busybox: Permission denied
    rootfs/amd64/bin/echomake: unlink: rootfs/amd64/bin/echo: Permission denied

  when Make completes.

Where Gentoo's architecture names don't match, the per-platform
tarballs should be renamed to use the Go names [6] when copying into
the root project directory.

[1]: https://www.gnu.org/software/make/manual/html_node/Setting.html#Setting
[2]: https://www.gnu.org/software/make/manual/html_node/Substitution-Refs.html
[3]: https://www.gnu.org/software/make/manual/html_node/Pattern-Rules.html
[4]: https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html
[5]: https://www.gnu.org/software/make/manual/html_node/Special-Targets.html#index-precious-targets
[6]: https://golang.org/doc/install/source#introduction

Signed-off-by: W. Trevor King <[email protected]>

contrib/rootfs-builder/.gitignore

@@ -0,0 +1 @@
+*.gz

contrib/rootfs-builder/Makefile

@@ -1,26 +1,32 @@
-rootfs.tar.gz: rootfs/bin/echo
-	tar -czf $@ -C rootfs .
+ARCHES ?= amd64
 
-rootfs/bin/busybox: downloads/stage3-amd64-current.tar.bz2 rootfs-files
+all: $(ARCHES:%=rootfs-%.tar.gz)
+
+rootfs-%.tar.gz: rootfs/%/bin/echo
+	tar -czf $@ -C rootfs/$* .
+
+.PRECIOUS: rootfs/%/bin/busybox
+rootfs/%/bin/busybox: downloads/stage3-%-current.tar.bz2 rootfs-files
 	gpg --verify $<.DIGESTS.asc
 	(cd downloads && \
-		grep -A1 '^# SHA512 HASH' stage3-amd64-current.tar.bz2.DIGESTS.asc | \
+		grep -A1 '^# SHA512 HASH' stage3-$*-current.tar.bz2.DIGESTS.asc | \
 		grep -v '^--' | \
 		sha512sum -c)
-	sudo rm -rf rootfs
-	sudo mkdir rootfs
-	sudo tar -xvf downloads/stage3-amd64-current.tar.bz2 -C rootfs \
+	sudo rm -rf rootfs/$*
+	sudo mkdir -p rootfs/$*
+	sudo tar -xvf downloads/stage3-$*-current.tar.bz2 -C rootfs/$* \
 		--no-recursion --wildcards $$(< rootfs-files)
 	sudo touch $@
 
-rootfs/bin/echo: rootfs/bin/busybox
+.PRECIOUS: rootfs/%/bin/echo
+rootfs/%/bin/echo: rootfs/%/bin/busybox
 	sudo sh -c 'COMMANDS=$$($< --list) || exit 1; for COMMAND in $${COMMANDS}; do \
-		test -L "rootfs/bin/$${COMMAND}" || ln -rs $< "rootfs/bin/$${COMMAND}" || exit; \
+		test -L "rootfs/$*/bin/$${COMMAND}" || ln -rs $< "rootfs/$*/bin/$${COMMAND}" || exit; \
 	done'
 
-downloads/stage3-amd64-current.tar.bz2: get-stage3.sh
-	./$<
-	touch downloads/stage3-amd64-*.tar.bz2
+downloads/stage3-%-current.tar.bz2: get-stage3.sh
+	STAGE3_ARCH=$* ./$<
+	touch downloads/stage3-$*-*.tar.bz2
 
 clean:
 	rm -f downloads/*

contrib/rootfs-builder/README.md

@@ -1,21 +1,43 @@
-Building `rootfs.tar.gz`
-------------------------
+# Building `rootfs-*.tar.gz`
 
-The root filesystem tarball is based on [Gentoo][]'s [amd64
-stage3][stage3-amd64] (which we check for a valid [GnuPG
+The root filesystem tarball is based on [Gentoo][]'s stage3s (e.g. the
+[amd64 stage3][stage3-amd64]).  We check for a valid [GnuPG
 signature][gentoo-signatures]), copying a [minimal
 subset](rootfs-files) to the root filesytem, and adding symlinks for
-all BusyBox commands. To rebuild the tarball based on a newer stage3,
-just run:
+all BusyBox commands.  To rebuild the tarball based on a newer stage3,
+run:
 
 ```
 $ touch get-stage3.sh
-$ make rootfs.tar.gz
+$ make rootfs-amd64.tar.gz
 ```
 
-### Getting Gentoo's Release Engineering public key
+## Platform tarballs
 
-If `make rootfs.tar.gz` gives an error like:
+The extraction requires a local machine capable of executing the
+extracted BusyBox.  Extraction assumes amd64, but if your local
+machine is a different platform, set `ARCHES`.  For example:
+
+```
+$ make ARCHES='i486 i686 amd64'
+```
+
+The architecture identifiers can be found by browsing Gentoo's
+[releases][] for `autobuilds/latest-stage3-{arch}.txt`.  Examples
+include:
+
+* `amd64` (the default)
+* `arm64`
+* `arm7a`
+* `i486`
+* `i686`
+* `ppc64-64ul`
+
+and many more.
+
+## Getting Gentoo's Release Engineering public key
+
+If `make rootfs-….tar.gz` gives an error like:
 
 ```
 gpg --verify downloads/stage3-amd64-current.tar.bz2.DIGESTS.asc
@@ -32,6 +54,7 @@ $ gpg --keyserver pool.sks-keyservers.net --recv-keys 2D182910
 ```
 
 [Gentoo]: https://www.gentoo.org/
-[stage3-amd64]: http://distfiles.gentoo.org/releases/amd64/autobuilds/
 [gentoo-signatures]: https://www.gentoo.org/downloads/signatures/
 [recv-keys]: https://www.gnupg.org/documentation/manuals/gnupg/Operational-GPG-Commands.html
+[releases]: http://distfiles.gentoo.org/releases/
+[stage3-amd64]: http://distfiles.gentoo.org/releases/amd64/autobuilds/

contrib/rootfs-builder/get-stage3.sh

@@ -17,11 +17,31 @@
 # limitations under the License.
 
 MIRROR="${MIRROR:-http://distfiles.gentoo.org/}"
-BASE_ARCH_URL="${BASE_ARCH_URL:-${MIRROR}releases/amd64/autobuilds/}"
+STAGE3_ARCH="${STAGE3_ARCH:-amd64}"
+
+if test -z "${BASE_ARCH}"
+then
+	case "${STAGE3_ARCH}" in
+		arm*)
+			BASE_ARCH=arm
+			;;
+		i[46]86)
+			BASE_ARCH=x86
+			;;
+		ppc*)
+			BASE_ARCH=ppc
+			;;
+		*)
+			BASE_ARCH="${STAGE3_ARCH}"
+			;;
+	esac
+fi
+
+BASE_ARCH_URL="${BASE_ARCH_URL:-${MIRROR}releases/${BASE_ARCH}/autobuilds/}"
 LATEST=$(wget -O - "${BASE_ARCH_URL}latest-stage3.txt")
-DATE=$(echo "${LATEST}" | sed -n 's|/stage3-amd64-[0-9]*[.]tar[.]bz2.*||p')
+DATE=$(echo "${LATEST}" | sed -n "s|/stage3-${STAGE3_ARCH}-[0-9]*[.]tar[.]bz2.*||p")
 ARCH_URL="${ARCH_URL:-${BASE_ARCH_URL}${DATE}/}"
-STAGE3="${STAGE3:-stage3-amd64-${DATE}.tar.bz2}"
+STAGE3="${STAGE3:-stage3-${STAGE3_ARCH}-${DATE}.tar.bz2}"
 STAGE3_CONTENTS="${STAGE3_CONTENTS:-${STAGE3}.CONTENTS}"
 STAGE3_DIGESTS="${STAGE3_DIGESTS:-${STAGE3}.DIGESTS.asc}"