Add spec for a function returning a function

* Used by the sassc gem.

Author: eregon

spec/ffi/callback_spec.rb

@@ -75,6 +75,7 @@ class S8F32S32 < FFI::Struct
     attach_function :testCallbackVrS64, :testClosureVrLL, [ :cbVrS64 ], :long_long
     attach_function :testCallbackVrU64, :testClosureVrLL, [ :cbVrU64 ], :ulong_long
     attach_function :testCallbackVrP, :testClosureVrP, [ :cbVrP ], :pointer
+    attach_function :testCallbackReturningFunction, :testClosureVrP, [ :cbVrP ], :cbVrP
     attach_function :testCallbackVrY, :testClosureVrP, [ :cbVrY ], S8F32S32.ptr
     # struct by value
     # attach_function :testCallbackVrT, :testClosureVrT, [ :cbVrT ], S8F32S32.by_value
@@ -267,6 +268,12 @@ class S8F32S32 < FFI::Struct
     expect(LibTest.testCallbackVrP { p }).to eq(p)
   end
 
+  it "returning a callback function" do
+    ret = LibTest.testCallbackReturningFunction { FFI::Pointer.new(42) }
+    expect(ret).to be_kind_of(FFI::Function)
+    expect(ret.address).to eq(42)
+  end
+
   it "returning struct by value" do
     next # struct by value
     skip "Segfault on 32 bit MINGW" if RUBY_PLATFORM == 'i386-mingw32'