installer: allow overriding of NIX_FIRST_BUILD_ID on darwin

because there are often already users in the 300 range and it's painful
to work around.

revives NixOS#6466

scripts/install-darwin-multi-user.sh

@@ -3,11 +3,13 @@
 set -eu
 set -o pipefail
 
+# System specific settings
+export NIX_FIRST_BUILD_UID="${NIX_FIRST_BUILD_UID:-301}"
+export NIX_BUILD_USER_NAME_TEMPLATE="_nixbld%d"
+
 readonly NIX_DAEMON_DEST=/Library/LaunchDaemons/org.nixos.nix-daemon.plist
 # create by default; set 0 to DIY, use a symlink, etc.
 readonly NIX_VOLUME_CREATE=${NIX_VOLUME_CREATE:-1} # now default
-NIX_FIRST_BUILD_UID="301"
-NIX_BUILD_USER_NAME_TEMPLATE="_nixbld%d"
 
 # caution: may update times on / if not run as normal non-root user
 read_only_root() {

scripts/install-multi-user.sh

@@ -25,9 +25,9 @@ readonly RED='\033[31m'
 readonly NIX_USER_COUNT=${NIX_USER_COUNT:-32}
 readonly NIX_BUILD_GROUP_ID="${NIX_BUILD_GROUP_ID:-30000}"
 readonly NIX_BUILD_GROUP_NAME="nixbld"
-# darwin installer needs to override these
-NIX_FIRST_BUILD_UID="${NIX_FIRST_BUILD_UID:-30001}"
-NIX_BUILD_USER_NAME_TEMPLATE="nixbld%d"
+# each system specific installer must set these:
+#   NIX_FIRST_BUILD_UID
+#   NIX_BUILD_USER_NAME_TEMPLATE
 # Please don't change this. We don't support it, because the
 # default shell profile that comes with Nix doesn't support it.
 readonly NIX_ROOT="/nix"
@@ -707,6 +707,12 @@ EOF
     fi
 }
 
+check_required_system_specific_settings() {
+    if [ -z "${NIX_FIRST_BUILD_UID+x}" ] || [ -z "${NIX_BUILD_USER_NAME_TEMPLATE+x}" ]; then
+        failure "Internal error: System specific installer for $(uname) ($1) does not export required settings."
+    fi
+}
+
 welcome_to_nix() {
     local -r NIX_UID_RANGES="${NIX_FIRST_BUILD_UID}..$((NIX_FIRST_BUILD_UID + NIX_USER_COUNT - 1))"
     local -r RANGE_TEXT=$(echo -ne "${BLUE}(uids [${NIX_UID_RANGES}])${ESC}")
@@ -726,7 +732,9 @@ manager. This will happen in a few stages:
    if you are ready to continue.
 
 3. Create the system users ${RANGE_TEXT} and groups ${GROUP_TEXT}
-   that the Nix daemon uses to run builds.
+   that the Nix daemon uses to run builds. To create system users
+   in a different range, exit and run this tool again with
+   NIX_FIRST_BUILD_UID set.
 
 4. Perform the basic installation of the Nix files daemon.
 
@@ -968,13 +976,16 @@ main() {
     if is_os_darwin; then
         # shellcheck source=./install-darwin-multi-user.sh
         . "$EXTRACTED_NIX_PATH/install-darwin-multi-user.sh"
+        check_required_system_specific_settings "install-darwin-multi-user.sh"
     elif is_os_linux; then
         # shellcheck source=./install-systemd-multi-user.sh
         . "$EXTRACTED_NIX_PATH/install-systemd-multi-user.sh" # most of this works on non-systemd distros also
+        check_required_system_specific_settings "install-systemd-multi-user.sh"
     else
         failure "Sorry, I don't know what to do on $(uname)"
     fi
 
+
     welcome_to_nix
 
     if ! is_root; then

scripts/install-systemd-multi-user.sh

@@ -3,6 +3,10 @@
 set -eu
 set -o pipefail
 
+# System specific settings
+export NIX_FIRST_BUILD_UID="${NIX_FIRST_BUILD_UID:-30001}"
+export NIX_BUILD_USER_NAME_TEMPLATE="nixbld%d"
+
 readonly SERVICE_SRC=/lib/systemd/system/nix-daemon.service
 readonly SERVICE_DEST=/etc/systemd/system/nix-daemon.service