Process.parse_arguments for Windows

[HertzDevil]

Process.parse_arguments uses the POSIX parsing rules, so it can fail to parse Windows command-line arguments (that use Microsoft's C runtime, which is virtually everything). A common example is unquoted arguments containing backslashes, the preferred path separator on Windows:

lib LibC
  fun CommandLineToArgvW(lpCmdLine : LPWSTR, pNumArgs : Int*) : LPWSTR*
  fun LocalFree(hMem : Void*) : Void*
end

class Process
  def self.parse_arguments_native(line : String) : Array(String)
    # `CommandLineToArgvW` always takes a program name, possibly empty,
    # and the program name doesn't obey the same escaping rules, so
    # we have to insert a space before *line* and then ignore the empty program name
    arg_ptrs = LibC.CommandLineToArgvW((' ' + line).to_utf16, out arg_count)
    raise RuntimeError.from_winerror("CommandLineToArgvW") unless arg_ptrs

    Array.new(arg_count - 1) do |i|
      str, _ = String.from_utf16(arg_ptrs[i + 1])
      str
    end
  ensure
    LibC.LocalFree(arg_ptrs) if arg_ptrs
  end
end

Process.parse_arguments %q(/LIBPATH:C:\crystal\lib)        # => ["/LIBPATH:C:crystallib"]
Process.parse_arguments_native %q(/LIBPATH:C:\crystal\lib) # => ["/LIBPATH:C:\\crystal\\lib"]

This was found while trying to get the interpreter compiling on Windows, where the compiler builds the list of linker arguments for passing to a Crystal::Loader. It is not possible to split the arguments beforehand, because we need to do this again anyway when there is any backtick expansion.

The standard library should provide a portable implementation of the above method. I also suggest renaming the method so that it matches .quote's API:

class Process
  # this naming suggests it is the inverse of `.quote`
  def self.unquote(line : String) : Array(String)
    {% if flag?(:win32) %}
      unquote_windows(args)
    {% else %}
      unquote_posix(args)
    {% end %}
  end

  def self.unquote_posix(line : String) : Array(String); end

  # equivalent to `CommandLineToArgvW`
  # note that *line* refers to the one for functions like `CreateProcess`,
  # _not_ the CMD shell arguments, nor PowerShell for that matter
  def self.unquote_windows(line : String) : Array(String); end

  @[Deprecated("Use `.unquote_posix` instead")]
  def self.parse_arguments(line : String) : Array(String); end
end

Deprecation of the existing method is likely, because directly putting unquote's body in it would cause a breaking change for Windows.

[straight-shoota]

I like parse_arguments better than unquote. This method does not only remove quotes (which could just return a single string), but it also parses the individual arguments.

Breaking changes on Windows should be fine because it's still not fully supported yet.