sequoia-nixbld-user-migration: nail down PATH

Fixes a user report of trouble with toybox grep and avoids
potential of same basic issue with other utils.

(cherry picked from commit 6a874c2865)

# Conflicts:
#	scripts/sequoia-nixbld-user-migration.sh

scripts/sequoia-nixbld-user-migration.sh

@@ -0,0 +1,172 @@
+#!/usr/bin/env bash
+
+set -eo pipefail
+
+# stock path to avoid unexpected command versions
+PATH="$(/usr/bin/getconf PATH)"
+
+((NEW_NIX_FIRST_BUILD_UID=351))
+((TEMP_NIX_FIRST_BUILD_UID=31000))
+
+nix_user_n() {
+	printf "_nixbld%d" "$1"
+}
+
+id_unavailable(){
+	dscl . list /Users UniqueID | grep -E '\b'"$1"'\b' >/dev/null
+}
+
+any_nixbld(){
+	dscl . list /Users UniqueID | grep -E '\b_nixbld' >/dev/null
+}
+
+dsclattr() {
+	dscl . -read "$1" | awk "/$2/ { print \$2 }"
+}
+
+re_create_nixbld_user(){
+	local name uid
+
+	name="$1"
+	uid="$2"
+	gid="$3"
+
+	sudo /usr/bin/dscl . -create "/Users/$name" "UniqueID" "$uid"
+	sudo /usr/bin/dscl . -create "/Users/$name" "IsHidden" "1"
+	sudo /usr/bin/dscl . -create "/Users/$name" "NFSHomeDirectory" "/var/empty"
+	sudo /usr/bin/dscl . -create "/Users/$name" "RealName" "Nix build user $name"
+	sudo /usr/bin/dscl . -create "/Users/$name" "UserShell" "/sbin/nologin"
+	sudo /usr/bin/dscl . -create "/Users/$name" "PrimaryGroupID" "$gid"
+}
+
+hit_id_cap(){
+	echo "We've hit UID 400 without placing all of your users :("
+	echo "You should use the commands in this script as a starting"
+	echo "point to review your UID-space and manually move the"
+	echo "remaining users (or delete them, if you don't need them)."
+}
+
+# evacuate the role-uid space to simplify final placement logic
+temporarily_move_existing_nixbld_uids(){
+	local name uid next_id user_n
+
+	((next_id=TEMP_NIX_FIRST_BUILD_UID))
+
+	echo ""
+	echo "Step 1: move existing _nixbld users out of the destination UID range."
+
+	while read -r name uid; do
+		# iterate for a clean ID
+		while id_unavailable "$next_id"; do
+			((next_id++))
+			# We really want to get these all placed, but I guess there's
+			# some risk we iterate forever--so we'll give up after 9k uids.
+			if ((next_id >= 40000)); then
+				echo "We've hit UID 40000 without temporarily placing all of your users :("
+				echo "You should use the commands in this script as a starting"
+				echo "point to review your UID-space and manually move the"
+				echo "remaining users to any open UID over 1000."
+				exit 1
+			fi
+		done
+		sudo dscl . -create "/Users/$name" UniqueID "$next_id"
+		echo "   Temporarily moved $name from uid $uid -> $next_id"
+
+	done < <(dscl . list /Users UniqueID | grep _nixbld | sort -n -k2)
+}
+
+change_nixbld_uids(){
+	local existing_gid name next_id user_n
+
+	((next_id=NEW_NIX_FIRST_BUILD_UID))
+	((user_n=1))
+	name="$(nix_user_n "$user_n")"
+	existing_gid="$(dsclattr "/Groups/nixbld" "PrimaryGroupID")"
+
+	# we know that we have *some* nixbld users, but macOS may have
+	# already clobbered the first few users if this system has been
+	# upgraded
+
+	echo ""
+	echo "Step 2: re-create missing early _nixbld# users."
+
+	until dscl . read "/Users/$name" &>/dev/null; do
+		# iterate for a clean ID
+		while id_unavailable "$next_id"; do
+			((next_id++))
+			if ((next_id >= 400)); then
+				hit_id_cap
+				exit 1
+			fi
+		done
+
+		re_create_nixbld_user "$name" "$next_id" "$existing_gid"
+		echo "      $name was missing; created with uid: $next_id"
+
+		((user_n++))
+		name="$(nix_user_n "$user_n")"
+	done
+
+	echo ""
+	echo "Step 3: relocate remaining _nixbld# UIDs to $next_id+"
+
+	# start at first _nixbld# not re-created above and increment
+	# until _nixbld<n> doesn't exist
+	while dscl . read "/Users/$name" &>/dev/null; do
+		# iterate for a clean ID
+		while id_unavailable "$next_id"; do
+			((next_id++))
+			if ((next_id >= 400)); then
+				hit_id_cap
+				exit 1
+			fi
+		done
+
+		sudo dscl . -create "/Users/$name" UniqueID "$next_id"
+		echo "      $name migrated to uid: $next_id"
+
+		((user_n++))
+		name="$(nix_user_n "$user_n")"
+	done
+
+	if ((user_n == 1)); then
+		echo "Didn't find _nixbld1. Perhaps you have single-user Nix?"
+		exit 1
+	else
+		echo "Migrated $((user_n - 1)) users. If you want to double-check, try:"
+		echo "dscl . list /Users UniqueID | grep _nixbld | sort -n -k2"
+	fi
+}
+needs_migration(){
+	local name uid next_id user_n
+
+	((next_id=NEW_NIX_FIRST_BUILD_UID))
+	((user_n=1))
+
+	while read -r name uid; do
+		expected_name="$(nix_user_n "$user_n")"
+		if [[ "$expected_name" != "$name" ]]; then
+			return 0
+		fi
+		if [[ "$next_id" != "$uid" ]]; then
+			return 0
+		fi
+		((next_id++))
+		((user_n++))
+	done < <(dscl . list /Users UniqueID | grep _nixbld | sort -n -k2)
+	return 1
+}
+
+
+if any_nixbld; then
+	if needs_migration; then
+		echo "Attempting to migrate _nixbld users."
+		temporarily_move_existing_nixbld_uids
+		change_nixbld_uids
+	else
+		echo "_nixbld users already appear to be migrated."
+	fi
+else
+	echo "Didn't find any _nixbld users. Perhaps you have single-user Nix?"
+	exit 1
+fi