Enable support for Gentoo overlays

This is intended to eventually support building musl-libc based images,
which need the musl overlay.

Change-Id: I8f5429ffa64e74c860772d9a00ff0b7eebb7721a

diskimage_builder/elements/gentoo/README.rst

@@ -44,3 +44,7 @@ Notes:
   which will be called by running the `python` command.  The
   `GENTOO_PYTHON_ACTIVE_VERSION` is used to set that mapping.  The variable
   contents can be something like `python3.5`.
+
+* You can enable overlays using the `GENTOO_OVERLAYS` variable.  In it you
+  should put a space separated list of overlays.  The overlays must be in the
+  official overlay list and must be git based.

diskimage_builder/elements/gentoo/environment.d/00-gentoo-envars.bash

@@ -4,3 +4,4 @@ export GENTOO_PROFILE=${GENTOO_PROFILE:-'default/linux/amd64/13.0'}
 export GENTOO_PORTAGE_CLEANUP=${GENTOO_PORTAGE_CLEANUP:-'False'}
 export GENTOO_PYTHON_TARGETS=${GENTOO_PYTHON_TARGETS:-'python2_7 python3_4'}
 export GENTOO_PYTHON_ACTIVE_VERSION=${GENTOO_PYTHON_ACTIVE_VERSION:-'python3.4'}
+export GENTOO_OVERLAYS=${GENTOO_OVERLAYS:-''}

diskimage_builder/elements/gentoo/finalise.d/99-cleanup

@@ -34,6 +34,13 @@ eselect news read all
 rm -Rf /root/.ccache/* /usr/src/* /var/cache/edb/dep/* /var/cache/genkernel/* /var/empty/* /var/run/* /var/state/* /var/tmp/*
 rm -Rf /etc/*- /etc/*.old /etc/ssh/ssh_host_* /root/.*history /root/.lesshst /root/.ssh/known_hosts /root/.viminfo /usr/share/genkernel /usr/lib64/python*/site-packages/gentoolkit/test/eclean/testdistfiles.tar.gz
 if [[ "${GENTOO_PORTAGE_CLEANUP}" != "False" ]]; then
+    # remove the overlays
+    if [[ ${GENTOO_OVERLAYS} != '' ]]; then
+        for OVERLAY in ${GENTOO_OVERLAYS}; do
+            layman -d "${OVERLAY}"
+        done
+    fi
+    # remove portage files
     rm -Rf /usr/portage/* /var/cache/portage/distfiles
 fi
 

diskimage_builder/elements/gentoo/pre-install.d/02-gentoo-03-enable-overlays

@@ -0,0 +1,41 @@
+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+if [[ ${GENTOO_OVERLAYS} != '' ]]; then
+    if mountpoint -q /dev/shm; then
+    echo "/dev/shm found in /proc/self/mountinfo"
+    elif [[ -k /dev/shm ]]; then
+        echo "/dev/shm exists and is stickied"
+    else
+        fix_shm
+    fi
+
+    if [[ ! -f /usr/portage/profiles ]]; then
+        emerge-webrsync -q
+    fi
+
+    # layman requires cryptography, which needs ecc crypto functions.
+    # binaries providing those functions have questionable legality for
+    # redistribution, so we have to use a version of openssl that works around
+    # it (using fedora's patchset) and also use a version of cryptography that
+    # depends on that version of openssl.
+    echo '=dev-python/cryptography-2.1.3 ~amd64' >> /etc/portage/package.keywords/layman
+    echo '=dev-libs/openssl-1.1.0g-r1 ~amd64' >> /etc/portage/package.keywords/layman
+    echo '=dev-libs/openssl-1.1.0g-r1' >> /etc/portage/package.unmask/layman
+    emerge -q --oneshot --jobs=2 openssl openssh
+    # install layman
+    USE="-build" emerge --deep -q --jobs=2 layman
+    # sync the initial overlay list
+    layman -S
+    # enable the various overlays
+    for OVERLAY in ${GENTOO_OVERLAYS}; do
+        layman -a "${OVERLAY}"
+    done
+
+    unfix_shm
+fi