Allow /SUBSYSTEM:WINDOWS on Windows

src/crystal/system/win32/file_descriptor.cr

@@ -54,8 +54,19 @@ module Crystal::System::FileDescriptor
   end
 
   private def windows_handle
+    FileDescriptor.windows_handle!(fd)
+  end
+
+  def self.windows_handle(fd)
+    ret = LibC._get_osfhandle(fd)
+    return LibC::INVALID_HANDLE_VALUE if ret == -1 || ret == -2
+    LibC::HANDLE.new(ret)
+  end
+
+  def self.windows_handle!(fd)
     ret = LibC._get_osfhandle(fd)
     raise RuntimeError.from_errno("_get_osfhandle") if ret == -1
+    raise RuntimeError.new("_get_osfhandle returned -2") if ret == -2
     LibC::HANDLE.new(ret)
   end
 
@@ -154,9 +165,7 @@ module Crystal::System::FileDescriptor
   end
 
   def self.pread(fd, buffer, offset)
-    handle = LibC._get_osfhandle(fd)
-    raise IO::Error.from_errno("_get_osfhandle") if handle == -1
-    handle = LibC::HANDLE.new(handle)
+    handle = windows_handle!(fd)
 
     overlapped = LibC::OVERLAPPED.new
     overlapped.union.offset.offset = LibC::DWORD.new(offset)
@@ -172,9 +181,8 @@ module Crystal::System::FileDescriptor
 
   def self.from_stdio(fd)
     console_handle = false
-    handle = LibC._get_osfhandle(fd)
-    if handle != -1
-      handle = LibC::HANDLE.new(handle)
+    handle = windows_handle(fd)
+    if handle != LibC::INVALID_HANDLE_VALUE
       # TODO: use `out old_mode` after implementing interpreter out closured var
       old_mode = uninitialized LibC::DWORD
       if LibC.GetConsoleMode(handle, pointerof(old_mode)) != 0
@@ -187,7 +195,7 @@ module Crystal::System::FileDescriptor
       end
     end
 
-    io = IO::FileDescriptor.new(fd)
+    io = IO::FileDescriptor.new(fd, blocking: true)
     # Set sync or flush_on_newline as described in STDOUT and STDERR docs.
     # See https://crystal-lang.org/api/toplevel.html#STDERR
     if console_handle

src/crystal/system/win32/process.cr

@@ -182,9 +182,7 @@ struct Crystal::System::Process
   end
 
   private def self.handle_from_io(io : IO::FileDescriptor, parent_io)
-    ret = LibC._get_osfhandle(io.fd)
-    raise RuntimeError.from_winerror("_get_osfhandle") if ret == -1
-    source_handle = LibC::HANDLE.new(ret)
+    source_handle = FileDescriptor.windows_handle!(io.fd)
 
     cur_proc = LibC.GetCurrentProcess
     if LibC.DuplicateHandle(cur_proc, source_handle, cur_proc, out new_handle, 0, true, LibC::DUPLICATE_SAME_ACCESS) == 0