sys-apps/nix: fix nix bootstrap, bug #693482

Fix `nix-channel --update` does not work on freshly-installed nix.

`nix-channel --update` does a few main things:
1. fetch new tarball with nix expressions
2. unpack that tarball as part of the build system using
   a shell script derived by unpack-channel.nix.
3. install the result into /nix/store

[2.] fails because tarball unpacking runs in a sandboxed environment
in a container with /nix/store and /bin/sh mounted.

unpack-channel.nix needs bash, tar and xz binaries in PATH.
These are normally provided by bash, tar and xz packages from /nix/store.

Unfortunately bash, tar and xz packages are not yet fetched as we
only have nix installed without nixpkgs tree.

1. we patch unpack-channel.nix to only require 'sh'
2. we use /bin/sh as a static busybox and point nix at it:
   ./configure --with-sandbox-shell=/bin/busybox

Reported-by: Kirill Zaborsky
Closes: https://bugs.gentoo.org/693482
Bug: NixOS/nix#2673
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

sys-apps/nix/files/nix-2.3-bootstrap.patch

@@ -0,0 +1,58 @@
+Fix `nix-channel --update` does not work on freshly-installed nix.
+
+# Why it does not work by default
+
+`nix-channel --update` does two main things:
+1. fetch new tarball with nix expressions
+2. unpack that tarball as part of the build system using
+   a shell script derived by unpack-channel.nix.
+3. install the result into /nix/store
+
+
+[2.] fails because tarball unpacking runs in a sandboxed environment
+a container with /nix/store and /bin/sh mounted.
+
+unpack-channel.nix needs bash, tar and xz binaries in PATH.
+These are normally provided by bash, tar and xz packages from /nix/store.
+
+Unfortunately bash, tar and xz packages are not yet fetched as we
+only have nix installed without nixpkgs tree.
+
+# How we get it working
+
+1. we patch unpack-channel.nix to only require 'sh'
+2. we use /bin/sh as a static busybox and point nix at it:
+   ./configure --with-sandbox-shell=/bin/busybox
+
+Bug: https://bugs.gentoo.org/693482
+Bug: https://github.com/NixOS/nix/issues/2673
+--- a/corepkgs/unpack-channel.nix
++++ b/corepkgs/unpack-channel.nix
+@@ -6,15 +6,9 @@ let
+     ''
+       mkdir $out
+       cd $out
+-      xzpat="\.xz\$"
+-      gzpat="\.gz\$"
+-      if [[ "$src" =~ $xzpat ]]; then
+-        ${xz} -d < $src | ${tar} xf - ${tarFlags}
+-      elif [[ "$src" =~ $gzpat ]]; then
+-        ${gzip} -d < $src | ${tar} xf - ${tarFlags}
+-      else
+-        ${bzip2} -d < $src | ${tar} xf - ${tarFlags}
+-      fi
++
++      tar xf $src
++
+       if [ * != $channelName ]; then
+         mv * $out/$channelName
+       fi
+@@ -26,7 +20,7 @@ in
+
+ derivation {
+   system = builtins.currentSystem;
+-  builder = shell;
++  builder = "/bin/sh";
+   args = [ "-e" builder ];
+   inherit name channelName src;
+

sys-apps/nix/nix-2.3-r1.ebuild

@@ -0,0 +1,138 @@
+# Copyright 1999-2019 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+
+inherit autotools flag-o-matic readme.gentoo-r1 user
+
+DESCRIPTION="A purely functional package manager"
+HOMEPAGE="https://nixos.org/nix"
+
+SRC_URI="http://nixos.org/releases/${PN}/${P}/${P}.tar.xz"
+LICENSE="LGPL-2.1"
+SLOT="0"
+KEYWORDS="~amd64 ~x86"
+IUSE="+etc-profile +gc doc s3 +sodium"
+
+# sys-apps/busybox is needed for sandbox mount of /bin/sh
+RDEPEND="
+	app-arch/brotli
+	app-arch/bzip2
+	app-arch/xz-utils
+	sys-apps/busybox[static]
+	dev-db/sqlite
+	dev-libs/editline:0=
+	dev-libs/openssl:0=
+	>=dev-libs/boost-1.66:0=[context]
+	net-misc/curl
+	sys-libs/libseccomp
+	sys-libs/zlib
+	gc? ( dev-libs/boehm-gc[cxx] )
+	doc? ( dev-libs/libxml2
+		dev-libs/libxslt
+		app-text/docbook-xsl-stylesheets
+	)
+	s3? ( dev-libs/aws-sdk-cpp )
+	sodium? ( dev-libs/libsodium:0= )
+"
+DEPEND="${RDEPEND}
+	>=sys-devel/bison-2.6
+	>=sys-devel/flex-2.5.35
+"
+
+PATCHES=(
+	"${FILESDIR}"/${PN}-2.0-user-path.patch
+	"${FILESDIR}"/${PN}-2.3-libpaths.patch
+	"${FILESDIR}"/${PN}-2.3-bootstrap.patch
+)
+
+DISABLE_AUTOFORMATTING=yes
+DOC_CONTENTS=" Quick start user guide on Gentoo:
+
+[as root] enable nix-daemon service:
+	[systemd] # systemctl enable nix-daemon
+	[openrc]  # rc-update add nix-daemon
+[as a user] relogin to get environment and profile update
+[as a user] fetch nixpkgs update:
+	\$ nix-channel --update
+[as a user] install nix packages:
+	\$ nix-env -i mc
+[as a user] configure environment:
+	Somewhere in .bash_profile you might want to set
+	LOCALE_ARCHIVE=\$HOME/.nix-profile/lib/locale/locale-archive
+	but please read https://github.com/NixOS/nixpkgs/issues/21820
+
+Next steps:
+	nix package manager user manual: http://nixos.org/nix/manual/
+"
+
+pkg_setup() {
+	enewgroup nixbld
+	for i in {1..10}; do
+		# we list 'nixbld' twice to
+		# both assign a primary group for user
+		# and add a user to /etc/group
+		enewuser nixbld${i} -1 -1 /var/empty nixbld,nixbld
+	done
+}
+
+src_prepare() {
+	default
+
+	eautoreconf
+}
+
+src_configure() {
+	if ! use s3; then
+		# Disable automagic depend: bug #670256
+		export ac_cv_header_aws_s3_S3Client_h=no
+	fi
+	econf \
+		--localstatedir="${EPREFIX}"/nix/var \
+		$(use_enable gc) \
+		--with-sandbox-shell=/bin/busybox
+}
+
+src_compile() {
+	emake V=1
+}
+
+src_install() {
+	# TODO: emacs highlighter
+	default
+
+	readme.gentoo_create_doc
+
+	# here we use an eager variant of something that
+	# is lazily done by nix-daemon and root nix-env
+
+	# TODO: will need a tweak for prefix
+	keepdir             /nix/store
+	fowners root:nixbld /nix/store
+	fperms 1775         /nix/store
+
+	keepdir             /nix/var/nix/channel-cache
+	fperms 0777         /nix/var/nix/channel-cache
+
+	keepdir             /nix/var/nix/profiles/per-user
+	fperms 1777         /nix/var/nix/profiles/per-user
+
+	# setup directories nix-daemon: /etc/profile.d/nix-daemon.sh
+	keepdir             /nix/var/nix/gcroots/per-user
+	fperms 1777         /nix/var/nix/gcroots/per-user
+
+	newinitd "${FILESDIR}"/nix-daemon.initd nix-daemon
+
+	if ! use etc-profile; then
+		rm "${ED}"/etc/profile.d/nix.sh || die
+		rm "${ED}"/etc/profile.d/nix-daemon.sh || die
+	fi
+}
+
+pkg_postinst() {
+	if ! use etc-profile; then
+		ewarn "${EROOT}/etc/profile.d/nix.sh was removed (due to USE=-etc-profile)."
+	fi
+
+	readme.gentoo_print_elog
+}