Fix Fiddle::Pointer#ref behavior for FFI backend (#185)

Fixes #182 by making the pointer address dynamic.

```ruby
require "fiddle"

ptr = Fiddle::Pointer["hello"]
ptr.ref[0, Fiddle::SIZEOF_VOIDP] = Fiddle::Pointer["world"].ref
p ptr.to_s
```

The code above previously returned `"hello"` with the FFI backend.

Author: ankane

lib/fiddle/ffi_backend.rb

@@ -218,8 +218,8 @@ class Error < StandardError; end
   class DLError < Error; end
   class ClearedReferenceError < Error; end
 
+  # Pointer isn't thread safe for now
   class Pointer
-    attr_reader :ffi_ptr
     extend FFI::DataConverter
     native_type FFI::Type::Builtin::POINTER
 
@@ -285,15 +285,15 @@ def []=(*args, value)
           value = value.to_str(args[1])
         end
 
-        @ffi_ptr.put_bytes(args[0], value, 0, args[1])
+        ffi_ptr.put_bytes(args[0], value, 0, args[1])
       elsif args.size == 1
         if value.is_a?(Fiddle::Pointer)
           value = value.to_str(args[0] + 1)
         else
           value = value.chr
         end
 
-        @ffi_ptr.put_bytes(args[0], value, 0, 1)
+        ffi_ptr.put_bytes(args[0], value, 0, 1)
       end
     rescue FFI::NullPointerError
       raise DLError.new("NULL pointer access")
@@ -332,9 +332,14 @@ def initialize(addr, size = nil, free = nil)
       end
       @free = free
       @ffi_ptr = ptr
+      @addr_ptr = nil
       @freed = false
     end
 
+    def ffi_ptr
+      @addr_ptr ? @addr_ptr.get_pointer(0) : @ffi_ptr
+    end
+
     module LibC
       extend FFI::Library
       ffi_lib FFI::Library::LIBC
@@ -363,11 +368,11 @@ def self.malloc(size, free = nil)
     end
 
     def null?
-      @ffi_ptr.null?
+      ffi_ptr.null?
     end
 
     def to_ptr
-      @ffi_ptr
+      ffi_ptr
     end
 
     def size
@@ -386,9 +391,9 @@ def call_free
       return if @free.nil?
       return if @freed
       if @free == RUBY_FREE
-        LibC::FREE.call(@ffi_ptr)
+        LibC::FREE.call(ffi_ptr)
       else
-        @free.call(@ffi_ptr)
+        @free.call(ffi_ptr)
       end
       @freed = true
     end
@@ -495,10 +500,8 @@ def -@
     end
 
     def ref
-      cptr = Pointer.malloc(FFI::Type::POINTER.size, RUBY_FREE)
-      cptr.ffi_ptr.put_pointer(0, ffi_ptr)
-      cptr.size = 0
-      cptr
+      @addr_ptr ||= FFI::MemoryPointer.new(:pointer).put_pointer(0, @ffi_ptr)
+      Pointer.new(@addr_ptr, 0)
     end
   end
 

test/fiddle/test_pointer.rb

@@ -193,6 +193,19 @@ def test_cmp
       assert_nil(ptr <=> 10, '10 should not be comparable')
     end
 
+    def test_ref
+      ptr = Fiddle::Pointer["hello"]
+      ref = ptr.ref
+      assert_equal(0, ref.size)
+      assert_nil(ref.free)
+      assert_equal(ptr, ref.ptr)
+
+      ptr2 = Fiddle::Pointer["world"]
+      ptr.ref[0, Fiddle::SIZEOF_VOIDP] = ptr2.ref
+      assert_equal("world", ptr.to_s)
+      assert_equal(ptr.to_i, ptr2.to_i)
+    end
+
     def test_ref_ptr
       if ffi_backend?
         omit("Fiddle.dlwrap([]) isn't supported with FFI backend")