(IAC-967) Append null terminators since it is no longer part of the data

Early ruby versions did not handle wide terminators correctly and would sometime
read past the end of the string causing segfaults. To prevent that, puppet
started embedding a wide terminator within the string. So the ruby string 'hi'
would be encoded as the three character string 'hi\0', and then that would be
encoded as UTF-16LE.

While this works, it creates ambiguity as to whether the "\0" character is part
of the data or not. For example, "hi\n\0".chop is "hi\n" whereas "hi\n".chop is
"hi", and is probably what was intended.

Ruby has since fixed its wide terminator handling, and as of Puppet 7, the
`wide_string` method no longer embeds a terminator in the string. So update the
registry module to explicitly add the terminator(s) before calling
RegSetValueExW`.

Author: joshcooper

lib/puppet/provider/registry_value/registry.rb

@@ -178,16 +178,30 @@ def write_value
     raise error
   end
 
+  def wide_string_to_bytes(data)
+    bytes = Puppet::Util::Windows::String.wide_string(data).bytes.to_a
+    # versions prior to 7 embedded a wide null in the string content to work
+    # around ruby bugs, see PUP-3970
+    if Puppet::PUPPETVERSION[0].to_i >= 7
+      bytes << 0 << 0
+    else
+      bytes
+    end
+  end
+
+  # This method must include wide null terminators in the returned
+  # byte array for string-based registry values like REG_SZ. In
+  # addition REG_MULTI_SZ must append another wide null character
+  # to signify there are no more entries in the array.
   def data_to_bytes(type, data)
     bytes = []
 
     case type
     when Win32::Registry::REG_SZ, Win32::Registry::REG_EXPAND_SZ
-      bytes = Puppet::Util::Windows::String.wide_string(data).bytes.to_a
+      bytes = wide_string_to_bytes(data)
     when Win32::Registry::REG_MULTI_SZ
-      # each wide string is already NULL terminated
-      bytes = data.map { |s| Puppet::Util::Windows::String.wide_string(s).bytes.to_a }.flat_map { |a| a }
-      # requires an additional NULL terminator to terminate properly
+      bytes = data.map { |s| wide_string_to_bytes(s) }.flat_map { |a| a }
+      # requires an additional wide NULL terminator
       bytes << 0 << 0
     when Win32::Registry::REG_BINARY
       bytes = data.bytes.to_a
@@ -209,6 +223,8 @@ def write(reg, _name, type, data)
       bytes = data_to_bytes(type, data)
       FFI::MemoryPointer.new(:uchar, bytes.length) do |data_ptr|
         data_ptr.write_array_of_uchar(bytes)
+        # From https://docs.microsoft.com/en-us/windows/win32/api/winreg/nf-winreg-regsetvalueexw
+        # "cbData must include the size of the terminating null character or characters"
         if RegSetValueExW(reg.hkey, name_ptr, 0,
                           type, data_ptr, data_ptr.size) != 0
           raise Puppet::Util::Windows::Error, 'Failed to write registry value'