Warn on a finalizer that captures the object to be finalized

Also improve specs and documentation for finalizers and more clearly
recommend a safe code pattern to use them.

spec/ruby/core/objectspace/define_finalizer_spec.rb

@@ -1,24 +1,42 @@
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
-# NOTE: A call to define_finalizer does not guarantee that the
-# passed proc or callable will be called at any particular time.
+# Why do we not test that finalizers are run by the GC? The documentation
+# says that finalizers are never guaranteed to be run, so we can't
+# spec that they are. On some implementations of Ruby the finalizers may
+# run asyncronously, meaning that we can't predict when they'll run,
+# even if they were guaranteed to do so. Even on MRI finalizers can be
+# very unpredictable, due to conservative stack scanning and references
+# left in unused memory.
+
 describe "ObjectSpace.define_finalizer" do
   it "raises an ArgumentError if the action does not respond to call" do
     -> {
-      ObjectSpace.define_finalizer("", mock("ObjectSpace.define_finalizer no #call"))
+      ObjectSpace.define_finalizer(Object.new, mock("ObjectSpace.define_finalizer no #call"))
     }.should raise_error(ArgumentError)
   end
 
   it "accepts an object and a proc" do
-    handler = -> obj { obj }
-    ObjectSpace.define_finalizer("garbage", handler).should == [0, handler]
+    handler = -> id { id }
+    ObjectSpace.define_finalizer(Object.new, handler).should == [0, handler]
+  end
+
+  it "accepts an object and a bound method" do
+    handler = mock("callable")
+    def handler.finalize(id) end
+    finalize = handler.method(:finalize)
+    ObjectSpace.define_finalizer(Object.new, finalize).should == [0, finalize]
   end
 
   it "accepts an object and a callable" do
     handler = mock("callable")
-    def handler.call(obj) end
-    ObjectSpace.define_finalizer("garbage", handler).should == [0, handler]
+    def handler.call(id) end
+    ObjectSpace.define_finalizer(Object.new, handler).should == [0, handler]
+  end
+
+  it "accepts an object and a block" do
+    handler = -> id { id }
+    ObjectSpace.define_finalizer(Object.new, &handler).should == [0, handler]
   end
 
   it "raises ArgumentError trying to define a finalizer on a non-reference" do
@@ -31,7 +49,7 @@ describe "ObjectSpace.define_finalizer" do
   it "calls finalizer on process termination" do
     code = <<-RUBY
       def scoped
-        Proc.new { puts "finalized" }
+        Proc.new { puts "finalizer run" }
       end
       handler = scoped
       obj = "Test"
@@ -39,18 +57,104 @@ describe "ObjectSpace.define_finalizer" do
       exit 0
     RUBY
 
-    ruby_exe(code).should == "finalized\n"
+    ruby_exe(code, :args => "2>&1").should include("finalizer run\n")
   end
 
-  it "calls finalizer at exit even if it is self-referencing" do
+  ruby_version_is "2.8" do
+    it "warns if the finalizer has the object as the receiver" do
+      code = <<-RUBY
+        class CapturesSelf
+          def initialize
+            ObjectSpace.define_finalizer(self, proc {
+              puts "finalizer run"
+            })
+          end
+        end
+        CapturesSelf.new
+        exit 0
+      RUBY
+
+      ruby_exe(code, :args => "2>&1").should include("warning: finalizer references object to be finalized\n")
+    end
+
+    it "warns if the finalizer is a method bound to the receiver" do
+      code = <<-RUBY
+        class CapturesSelf
+          def initialize
+            ObjectSpace.define_finalizer(self, method(:finalize))
+          end
+          def finalize(id)
+            puts "finalizer run"
+          end
+        end
+        CapturesSelf.new
+        exit 0
+      RUBY
+
+      ruby_exe(code, :args => "2>&1").should include("warning: finalizer references object to be finalized\n")
+    end
+
+    it "warns if the finalizer was a block in the reciever" do
+      code = <<-RUBY
+        class CapturesSelf
+          def initialize
+            ObjectSpace.define_finalizer(self) do
+              puts "finalizer run"
+            end
+          end
+        end
+        CapturesSelf.new
+        exit 0
+      RUBY
+
+      ruby_exe(code, :args => "2>&1").should include("warning: finalizer references object to be finalized\n")
+    end
+  end
+
+  it "calls a finalizer at exit even if it is self-referencing" do
     code = <<-RUBY
       obj = "Test"
-      handler = Proc.new { puts "finalized" }
+      handler = Proc.new { puts "finalizer run" }
       ObjectSpace.define_finalizer(obj, handler)
       exit 0
     RUBY
 
-    ruby_exe(code).should == "finalized\n"
+    ruby_exe(code).should include("finalizer run\n")
+  end
+
+  it "calls a finalizer at exit even if it is indirectly self-referencing" do
+    code = <<-RUBY
+      class CapturesSelf
+        def initialize
+          ObjectSpace.define_finalizer(self, finalizer(self))
+        end
+        def finalizer(zelf)
+          proc do
+            puts "finalizer run"
+          end
+        end
+      end
+      CapturesSelf.new
+      exit 0
+    RUBY
+
+    ruby_exe(code, :args => "2>&1").should include("finalizer run\n")
+  end
+
+  it "calls a finalizer defined in a finalizer running at exit" do
+    code = <<-RUBY
+      obj = "Test"
+      handler = Proc.new do
+        obj2 = "Test"
+        handler2 = Proc.new { puts "finalizer 2 run" }
+        ObjectSpace.define_finalizer(obj2, handler2)
+        exit 0
+      end
+      ObjectSpace.define_finalizer(obj, handler)
+      exit 0
+    RUBY
+
+    ruby_exe(code, :args => "2>&1").should include("finalizer 2 run\n")
   end
 
   it "allows multiple finalizers with different 'callables' to be defined" do