add script to migrate macOS 15 Sequoia nixbld UIDs

While we don't have any easy way to forcibly notify everyone about the
impending breakage (or forcibly migrate the users on their system),
this script enables those who do hear about the problem to migrate
their systems before they take the macOS update.

It should also enable people who only discover it after the update
when a build fails to ~fix their installs without a full reinstall.

Author: abathur

scripts/sequoia-nixbld-user-migration.sh

@@ -2,17 +2,26 @@
 
 ((NEW_NIX_FIRST_BUILD_UID=331))
 
+nix_user_n() {
+	printf "_nixbld%d" "$1"
+}
+
 id_available(){
 	dscl . list /Users UniqueID | grep -E '\b'"$1"'\b' >/dev/null
 }
 
 change_nixbld_names_and_ids(){
-	local name uid next_id
+	local name next_id user_n
+
 	((next_id=NEW_NIX_FIRST_BUILD_UID))
+	((user_n=1))
+	name="$(nix_user_n "$user_n")"
+
 	echo "Attempting to migrate _nixbld users."
 	echo "Each _nixbld# user should have its UID moved to $next_id+"
-	while read -r name uid; do
-		echo "   Checking $name (uid: $uid)"
+
+	# start at _nixbld1 and increment until _nixbld<n> doesn't exist
+	while dscl . read "/Users/$name" &>/dev/null; do
 		# iterate for a clean ID
 		while id_available "$next_id"; do
 			((next_id++))
@@ -25,12 +34,18 @@ change_nixbld_names_and_ids(){
 			fi
 		done
 
-		# first 2 are cleanup, it's OK if they aren't here
-		sudo dscl . delete "/Users/$name" dsAttrTypeNative:_writers_passwd &>/dev/null || true
-		sudo dscl . change "/Users/$name" NFSHomeDirectory "/private/var/empty 1" "/var/empty" &>/dev/null || true
-		sudo dscl . change "/Users/$name" UniqueID "$uid" "$next_id"
+		sudo dscl . -create "/Users/$name" UniqueID "$next_id"
 		echo "      $name migrated to uid: $next_id"
-	done < <(dscl . list /Users UniqueID | grep _nixbld | sort -n -k2)
+		((user_n++))
+	done
+
+	if ((user_n == 1)); then
+		echo "Didn't find _nixbld1. Perhaps you have single-user Nix?"
+		exit 1
+	else
+		echo "Migrated $((user_n - 1)) users. If you want to double-check, try:"
+		echo "dscl . list /Users UniqueID | grep _nixbld | sort -n -k2"
+	fi
 }
 
 change_nixbld_names_and_ids