utils/service: add systemd_quote helper

We need a way to escape systemd command lines properly as systemd treats
unrecognised escape sequences as separate literal characters. This helper
function does that.

Library/Homebrew/test/utils/service_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require "utils/service"
+
+RSpec.describe Utils::Service do
+  describe "::systemd_quote" do
+    it "quotes empty strings correctly" do
+      expect(described_class.systemd_quote("")).to eq '""'
+    end
+
+    it "quotes strings with special characters escaped correctly" do
+      expect(described_class.systemd_quote("\a\b\f\n\r\t\v\\"))
+        .to eq '"\\a\\b\\f\\n\\r\\t\\v\\\\"'
+      expect(described_class.systemd_quote("\"' ")).to eq "\"\\\"' \""
+    end
+
+    it "does not escape characters that do not need escaping" do
+      expect(described_class.systemd_quote("daemon off;")).to eq '"daemon off;"'
+      expect(described_class.systemd_quote("--timeout=3")).to eq '"--timeout=3"'
+      expect(described_class.systemd_quote("--answer=foo bar"))
+        .to eq '"--answer=foo bar"'
+    end
+  end
+end

Library/Homebrew/utils/service.rb

@@ -48,5 +48,40 @@ module Utils
     def self.systemctl?
       !systemctl.nil?
     end
+
+    # Quote a string for use in systemd command lines, e.g., in `ExecStart`.
+    # https://www.freedesktop.org/software/systemd/man/latest/systemd.syntax.html#Quoting
+    sig { params(str: String).returns(String) }
+    def self.systemd_quote(str)
+      result = +"\""
+      # No need to escape single quotes and spaces, as we're always double
+      # quoting the entire string.
+      str.each_char do |char|
+        result << case char
+        when "\a"
+          "\\a"
+        when "\b"
+          "\\b"
+        when "\f"
+          "\\f"
+        when "\n"
+          "\\n"
+        when "\r"
+          "\\r"
+        when "\t"
+          "\\t"
+        when "\v"
+          "\\v"
+        when "\\"
+          "\\\\"
+        when "\""
+          "\\\""
+        else
+          char
+        end
+      end
+      result << "\""
+      result.freeze
+    end
   end
 end