Fix systemd command line quoting

Library/Homebrew/service.rb

@@ -455,7 +455,7 @@ def to_plist
     sig { returns(String) }
     def to_systemd_unit
       # command needs to be first because it initializes all other values
-      cmd = command&.map { |arg| Utils::Shell.sh_quote(arg) }
+      cmd = command&.map { |arg| Utils::Service.systemd_quote(arg) }
                    &.join(" ")
 
       options = []

Library/Homebrew/test/service_spec.rb

@@ -727,7 +727,7 @@ def stub_formula_with_service_sockets(sockets_var)
 
         [Service]
         Type=simple
-        ExecStart=#{HOMEBREW_PREFIX}/opt/#{name}/bin/beanstalkd test
+        ExecStart="#{HOMEBREW_PREFIX}/opt/#{name}/bin/beanstalkd" "test"
         Restart=always
         RestartSec=30
         WorkingDirectory=#{HOMEBREW_PREFIX}/var
@@ -760,7 +760,7 @@ def stub_formula_with_service_sockets(sockets_var)
 
         [Service]
         Type=oneshot
-        ExecStart=#{HOMEBREW_PREFIX}/opt/#{name}/bin/beanstalkd
+        ExecStart="#{HOMEBREW_PREFIX}/opt/#{name}/bin/beanstalkd"
       SYSTEMD
       expect(unit).to eq(unit_expect)
     end
@@ -783,7 +783,7 @@ def stub_formula_with_service_sockets(sockets_var)
 
         [Service]
         Type=simple
-        ExecStart=#{HOMEBREW_PREFIX}/opt/#{name}/bin/beanstalkd
+        ExecStart="#{HOMEBREW_PREFIX}/opt/#{name}/bin/beanstalkd"
         WorkingDirectory=#{Dir.home}
       SYSTEMD
       expect(unit).to eq(unit_expect)

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 +48,30 @@ def self.launchctl?
     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" then "\\a"
+        when "\b" then "\\b"
+        when "\f" then "\\f"
+        when "\n" then "\\n"
+        when "\r" then "\\r"
+        when "\t" then "\\t"
+        when "\v" then "\\v"
+        when "\\" then "\\\\"
+        when "\"" then "\\\""
+        else char
+        end
+      end
+      result << "\""
+      result.freeze
+    end
   end
 end