install: configure and bootstrap synthetic.conf on darwin

Starting macOS 10.15 /nix can't be creasted directly anymore due to the
readonly filesystem, but synthetic.conf was introduced to enable
creating mountpoints or symlinks for special usecases like package
managers.

scripts/create-darwin-volume.sh

@@ -0,0 +1,107 @@
+#!/bin/sh
+set -e
+
+root_disks() {
+    diskutil list -plist /
+}
+
+apfs_volumes_for() {
+    disk=$1
+    diskutil apfs list -plist "$disk"
+}
+
+disk_identifier() {
+    xpath "/plist/dict/key[text()='WholeDisks']/following-sibling::array[1]/string/text()" 2>/dev/null
+}
+
+volume_get() {
+    key=$1 i=$2
+    xpath "/plist/dict/array/dict/key[text()='Volumes']/following-sibling::array/dict[$i]/key[text()='$key']/following-sibling::string[1]/text()" 2> /dev/null
+}
+
+find_nix_volume() {
+    disk=$1
+    i=1
+    volumes=$(apfs_volumes_for "$disk")
+    while true; do
+        name=$(echo "$volumes" | volume_get "Name" "$i")
+        if [ -z "$name" ]; then
+            break
+        fi
+        case "$name" in
+            [Nn]ix*)
+                echo "$name"
+                break
+                ;;
+        esac
+        i=$((i+1))
+    done
+}
+
+test_fstab() {
+    grep -q "/nix" /etc/fstab 2>/dev/null
+}
+
+test_synthetic_conf() {
+    grep -q "^nix" /etc/synthetic.conf 2>/dev/null
+}
+
+test_nix() {
+    test -d "/nix"
+}
+
+main() {
+    (
+        echo ""
+        echo "     ------------------------------------------------------------------ "
+        echo "    | This installer will create a volume for the nix store and        |"
+        echo "    | configure it to mount at /nix.  Follow these steps to uninstall. |"
+        echo "     ------------------------------------------------------------------ "
+        echo ""
+        echo "  1. Remove the entry from fstab using 'sudo vifs'"
+        echo "  2. Destroy the data volume using 'diskutil apfs deleteVolume'"
+        echo "  3. Delete /etc/synthetic.conf"
+        echo ""
+    ) >&2
+
+    if [ -L "/nix" ]; then
+        echo "error: /nix is a symlink, please remove it or edit synthetic.conf (requires reboot)" >&2
+        echo "  /nix -> $(readlink "/nix")" >&2
+        exit 2
+    fi
+
+    if ! test_synthetic_conf; then
+        echo "Configuring /etc/synthetic.conf..." >&2
+        echo nix | sudo tee /etc/synthetic.conf
+        /System/Library/Filesystems/apfs.fs/Contents/Resources/apfs.util -B
+    fi
+
+    if ! test_nix; then
+        echo "Creating mountpoint for /nix..." >&2
+        sudo mkdir /nix
+    fi
+
+    disk=$(root_disks | disk_identifier)
+    volume=$(find_nix_volume "$disk")
+    if [ -z "$volume" ]; then
+        echo "Creating a Nix Store volume..." >&2
+        sudo diskutil apfs addVolume "$disk" APFS 'Nix Store' -mountpoint /nix
+        volume="Nix Store"
+    else
+        echo "Using existing '$volume' volume" >&2
+    fi
+
+    if ! test_fstab; then
+        echo "Configuring /etc/fstab..." >&2
+        label=$(echo "$volume" | sed 's/ /\\040/g')
+        printf "\$a\nLABEL=%s /nix apfs rw\n.\nwq\n" "$label" | EDITOR=ed sudo vifs
+    fi
+
+    echo "The following options can be enabled to disable spotlight indexing" >&2
+    echo "of the volume, which might be desirable." >&2
+    echo "" >&2
+    echo "   $ mdutil -i off /nix" >&2
+    echo "" >&2
+}
+
+main "$@"