[ruby/fiddle] Fix Fiddle.last_error on FFI backend and improve test

to work for all
(https://github.com/ruby/fiddle/pull/173)

https://github.com/ruby/fiddle/commit/ef2382a7ef

ext/fiddle/lib/fiddle.rb

@@ -10,22 +10,30 @@ require 'fiddle/function'
 require 'fiddle/version'
 
 module Fiddle
-  case RUBY_ENGINE
-  when 'jruby'
+  if RUBY_ENGINE != 'ruby' # FFI backend
     def self.last_error
-      FFI.errno.nonzero?
+      FFI.errno
     end
 
-    def self.last_error= error
+    def self.last_error=(error)
       FFI.errno = error || 0
     end
 
     if WINDOWS
       class << self
-        alias win32_last_error last_error
-        alias win32_last_error= last_error=
-        alias win32_last_socket_error last_error
-        alias win32_last_socket_error= last_error=
+        def win32_last_error
+          FFI.errno.nonzero?
+        end
+        def win32_last_error=(error)
+          FFI.errno = error || 0
+        end
+
+        def win32_last_socket_error
+          FFI.errno.nonzero?
+        end
+        def win32_last_socket_error=(error)
+          FFI.errno = error || 0
+        end
       end
     end
   else

test/fiddle/test_function.rb

@@ -8,7 +8,7 @@ module Fiddle
   class TestFunction < Fiddle::TestCase
     def setup
       super
-      Fiddle.last_error = nil
+      Fiddle.last_error = 0
       if WINDOWS
         Fiddle.win32_last_error = nil
         Fiddle.win32_last_socket_error = nil
@@ -126,14 +126,20 @@ module Fiddle
     end
 
     def test_last_error
-      if ffi_backend?
-        omit("Fiddle.last_error doesn't work with FFI backend")
+      if RUBY_ENGINE == 'jruby' && WINDOWS
+        omit("Fiddle.last_error doesn't work well on JRuby on Windows")
       end
 
-      func = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
+      require 'rbconfig/sizeof'
 
-      assert_nil Fiddle.last_error
-      func.call(+"000", "123")
+      strtol = Function.new(@libc['strtol'], [TYPE_VOIDP, TYPE_VOIDP, TYPE_INT], TYPE_LONG)
+
+      assert_equal 0, Fiddle.last_error
+
+      assert_equal RbConfig::LIMITS["LONG_MAX"], strtol.call((2**128).to_s, nil, 10) # overflow
+      assert_equal Errno::ERANGE::Errno, Fiddle.last_error
+
+      assert_equal 123, strtol.call("123", nil, 10)
       refute_nil Fiddle.last_error
     end