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.
This commit is contained in:
Chris Seaton 2020-09-16 19:59:36 +01:00 коммит произвёл Aaron Patterson
Родитель f75009c122
Коммит 8e173d8b27
4 изменённых файлов: 190 добавлений и 15 удалений

64
gc.c
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@ -3384,6 +3384,57 @@ should_be_finalizable(VALUE obj)
* as an argument to <i>aProc</i>. If <i>aProc</i> is a lambda or
* method, make sure it can be called with a single argument.
*
* The return value is an array <code>[0, aProc]</code>.
*
* The two recommended patterns are to either create the finaliser proc
* in a non-instance method where it can safely capture the needed state,
* or to use a custom callable object that stores the needed state
* explicitly as instance variables.
*
* class Foo
* def initialize(data_needed_for_finalization)
* ObjectSpace.define_finalizer(self, self.class.create_finalizer(data_needed_for_finalization))
* end
*
* def self.create_finalizer(data_needed_for_finalization)
* proc {
* puts "finalizing #{data_needed_for_finalization}"
* }
* end
* end
*
* class Bar
* class Remover
* def initialize(data_needed_for_finalization)
* @data_needed_for_finalization = data_needed_for_finalization
* end
*
* def call(id)
* puts "finalizing #{@data_needed_for_finalization}"
* end
* end
*
* def initialize(data_needed_for_finalization)
* ObjectSpace.define_finalizer(self, Remover.new(data_needed_for_finalization))
* end
* end
*
* Note that if your finalizer references the object to be
* finalized it will never be run on GC, although it will still be
* run at exit. You will get a warning if you capture the object
* to be finalized as the receiver of the finalizer.
*
* class CapturesSelf
* def initialize(name)
* ObjectSpace.define_finalizer(self, proc {
* # this finalizer will only be run on exit
* puts "finalizing #{name}"
* })
* end
* end
*
* Also note that finalization can be unpredictable and is never guaranteed
* to be run except on exit.
*/
static VALUE
@ -3400,6 +3451,10 @@ define_final(int argc, VALUE *argv, VALUE os)
should_be_callable(block);
}
if (rb_callable_receiver(block) == obj) {
rb_warn("finalizer references object to be finalized");
}
return define_final0(obj, block);
}
@ -12101,9 +12156,9 @@ rb_gcdebug_remove_stress_to_class(int argc, VALUE *argv, VALUE self)
*
* ObjectSpace also provides support for object finalizers, procs that will be
* called when a specific object is about to be destroyed by garbage
* collection.
*
* require 'objspace'
* collection. See the documentation for
* <code>ObjectSpace.define_finalizer</code> for important information on
* how to use this method correctly.
*
* a = "A"
* b = "B"
@ -12111,6 +12166,9 @@ rb_gcdebug_remove_stress_to_class(int argc, VALUE *argv, VALUE self)
* ObjectSpace.define_finalizer(a, proc {|id| puts "Finalizer one on #{id}" })
* ObjectSpace.define_finalizer(b, proc {|id| puts "Finalizer two on #{id}" })
*
* a = nil
* b = nil
*
* _produces:_
*
* Finalizer two on 537763470

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@ -46,6 +46,7 @@ VALUE rb_method_call_with_block(int, const VALUE *, VALUE, VALUE);
VALUE rb_method_call_with_block_kw(int, const VALUE *, VALUE, VALUE, int);
int rb_mod_method_arity(VALUE, ID);
int rb_obj_method_arity(VALUE, ID);
VALUE rb_callable_receiver(VALUE);
VALUE rb_protect(VALUE (*)(VALUE), VALUE, int*);
RBIMPL_SYMBOL_EXPORT_END()

12
proc.c
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@ -2739,6 +2739,18 @@ rb_obj_method_arity(VALUE obj, ID id)
return rb_mod_method_arity(CLASS_OF(obj), id);
}
VALUE
rb_callable_receiver(VALUE callable) {
if (rb_obj_is_proc(callable)) {
VALUE binding = rb_funcall(callable, rb_intern("binding"), 0);
return rb_funcall(binding, rb_intern("receiver"), 0);
} else if (rb_obj_is_method(callable)) {
return method_receiver(callable);
} else {
return Qundef;
}
}
const rb_method_definition_t *
rb_method_def(VALUE method)
{

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@ -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