[ruby-core:112849] [Ruby master Bug#19436] Call Cache for singleton methods can lead to "memory leaks"

Issue #19436 has been updated by ufuk (Ufuk Kayserilioglu). Let me add a few points in this conversation: I agree with @Eregon on this point that this is totally acceptable. When I was proposing `Class#attached_object` the argument was that it would be helpful in cases where the objects are still supposed to be in memory for introspection purposes. There is very little value in trying to introspect an attached object that was already garbage collected, and it would not be great to retain objects just because they had a singleton class defined for them. So, I am +1 for slightly changing the semantics of `Class#attached_object` before it becomes more widely used. I should also say, though, that the purist in me still considers that the cache should be the one holding weak references, since, after all it is a cache and a miss should be relatively inconsequential, as opposed to the link between a singleton class and its attached object. I do understand, however, that it might not be the desired implementation and pragmatism over purism might be more warranted here. ---------------------------------------- Bug #19436: Call Cache for singleton methods can lead to "memory leaks" https://bugs.ruby-lang.org/issues/19436#change-102361 * Author: byroot (Jean Boussier) * Status: Open * Priority: Normal * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN ---------------------------------------- Using "memory leaks" with quotes, because strictly speaking the memory isn't leaked, but it can nonetheless lead to large memory overheads. ### Minimal Reproduction ```ruby module Foo def bar end end def call_bar(obj) # Here the call cache we'll keep a ref on the method_entry # which then keep a ref on the singleton_class, making that # instance immortal until the method is called again with # another instance. # The reference chain is IMEMO(callcache) -> IMEMO(ment) -> ICLASS -> CLASS(singleton) -> OBJECT obj.bar end obj = Object.new obj.extend(Foo) call_bar(obj) id = obj.object_id obj = nil 4.times { GC.start } p ObjectSpace._id2ref(id) ``` ### Explanation Call caches keep a strong reference onto the "callable method entry" (CME), which itself keeps a strong reference on the called object class and in the cache of a singleton class, it keeps a strong reference onto the `attached_object` (instance). This means that any call site that calls a singleton method, will effectively keep a strong reference onto the last receiver. If the method is frequently called it's not too bad, but if it's infrequently called, it's effectively a (bounded) memory leak. And if the `attached_object` is big, the wasted memory can be very substantial. ### Practical Implications Once relative common API impacted by this is [Rails' `extending` API](https://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#met…. This API allow to extend a "query result set" with a module. These query results set can sometimes be very big, especially since they keep references to the instantiated `ActiveRecord::Base` instances etc. ### Possible Solutions #### Only keep a weak reference to the CME The fairly "obvious" solution is to keep a weak reference to the CME, that's what I explored in https://github.com/ruby/ruby/pull/7272, and it seems to work. However in debug mode It does fail on an assertion during compaction, but it's isn't quite clear to me what the impact is. Additionally, something that makes me think this would be the right solution, is that call caches already try to avoid marking the class: ```c # vm_callinfo.h:275 struct rb_callcache { const VALUE flags; /* inline cache: key */ const VALUE klass; // should not mark it because klass can not be free'd // because of this marking. When klass is collected, // cc will be cleared (cc->klass = 0) at vm_ccs_free(). ``` So it appears that the class being also marked through the CME is some kind of oversight? #### Don't cache based on some heuristics If the above isn't possible or too complicated, an alternative would be to not cache CMEs found in singleton classes, except if it's the the singleton class of a `Class` or `Module`. It would make repeated calls to such methods slower, but the assumption is that it's unlikely that these CME would live very long. #### Make `Class#attached_object` a weak reference Alternatively we could make the `attached_object` a weak reference, which would drastically limit the amount of memory that may be leaked in such scenario. The downside is that `Class#attached_object` was very recently exposed in Ruby 3.2.0, so it means changing its semantic a bit. cc @peterzhu2118 @ko1 -- https://bugs.ruby-lang.org/