Bug #17146: Queue operations are allowed after it is frozen

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.6.6p146 (2020-03-31 revision 67876) [x86_64-linux]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[Eregon (Benoit Daloze)]

[1] pry(main)> q = Queue.new
=> #<Thread::Queue:0x000056263683aee8>
[2] pry(main)> q.freeze
=> #<Thread::Queue:0x000056263683aee8>
[3] pry(main)> q << 1
=> #<Thread::Queue:0x000056263683aee8>
[4] pry(main)> q.pop
=> 1
[5] pry(main)> q.frozen?
=> true

Found by @ko1 (Koichi Sasada) in https://bugs.ruby-lang.org/issues/17100#note-28

I think it's a bug, since those are clear mutations.
I guess old Thread::Queue implemented in Ruby did not have this bug.

[ko1 (Koichi Sasada)]

I guess old Thread::Queue implemented in Ruby did not have this bug.

require "thread"; q=Queue.new.freeze; q.push 1 works without error from Ruby 1.6.0.
Surprisingly before 1.6.0 there is no Queue#freeze.

[jeremyevans0 (Jeremy Evans)]

If a Queue cannot push/pop, it is useless. A Queue doesn't have any operations on that would make sense if you cannot push/pop. This makes it different from most freezable objects, where the object is usable read-only in a frozen state.

In relation to Ractor/deep_freeze discussion, if deep_freeze is called on an object that contains a Queue (directly or transitively), and Queue#freeze makes the Queue unusable, it seems very dangerous. It would be best if Queue was an shareable object whose operations worked across Ractors. For example, assuming the object was sharable, a push of the object onto the Queue on Ractor A, while Ractor B, C, and D were waiting in Queue#pop, would result in only one of B, C, D popping the object. However, I don't know whether or not that is feasible.

I think the best solution for Queue#freeze is Queue.send(:undef_method, :freeze).

[ko1 (Koichi Sasada)]

It seems "what is freeze mean" discussion.

[Eregon (Benoit Daloze)]

ko1 (Koichi Sasada) wrote in #note-1:

require "thread"; q=Queue.new.freeze; q.push 1 works without error from Ruby 1.6.0.

Ah, yes, because it's not deeply frozen:
https://github.com/ruby/ruby/blob/23ccbdf52188b78427f41d75b1a4b81959ac9876/lib/thread.rb#L145

At least #deep_freeze on Queue should prevent #push/#pop.
But I think #freeze should prevent #push/#pop too, just like for every other core collection.

jeremyevans0 (Jeremy Evans) wrote in #note-2:

If a Queue cannot push/pop, it is useless. A Queue doesn't have any operations on that would make sense if you cannot push/pop. This makes it different from most freezable objects, where the object is usable read-only in a frozen state.

One could still use Queue#empty? and Queue#size.
I think freezing a Queue should be similar to Queue#close, but also prevent #pop.
Many objects become severely limited when deeply frozen. I don't think Queue should be a special case.

In relation to Ractor/deep_freeze discussion, if deep_freeze is called on an object that contains a Queue (directly or transitively), and Queue#freeze makes the Queue unusable, it seems very dangerous. It would be best if Queue was an shareable object whose operations worked across Ractors. For example, assuming the object was sharable, a push of the object onto the Queue on Ractor A, while Ractor B, C, and D were waiting in Queue#pop, would result in only one of B, C, D popping the object. However, I don't know whether or not that is feasible.

What if it's a non-shareable object?
It seems unacceptable to copy the object in that case, because how would we know if it is the same or different Ractor that will Queue#pop that element?

So IMHO the simplest thing makes sense: Queue#freeze prevents modifications to the Queue (which is the general contract of #freeze for collections). A Queue cannot be copied by Ractor (neither by #dup nor Marshal.dump). The Queue can be frozen but then indeed is of limited value.

Ractor communicate via their internal channels.
Queues don't work with Ractor (i.e., cannot be used to communicate between Ractors), and they cannot for compatibility (would copy non-shareable elements, which would be incompatible).

I think the best solution for Queue#freeze is Queue.send(:undef_method, :freeze).

AFAIK no other class undefines #freeze, that seems unfriendly.
I don't see any real-world motivation to prevent freezing a Queue (and it would be inconsistent).
So I think this is a plain bug and we should fix it.

[ko1 (Koichi Sasada)]

• Status changed from Open to Feedback

At last dev-meeting we skipped this issue because it is difficult, but no benefit to change the current behavior.

BTW

AFAIK no other class undefines #freeze, that seems unfriendly.

ENV.freeze raises an error (`freeze': cannot freeze ENV (TypeError)).

[jeremyevans0 (Jeremy Evans)]

• Status changed from Feedback to Closed

[Eregon (Benoit Daloze)]

• Status changed from Closed to Open

Why was this closed?

I think there is only one rational answer here:
Queue#freeze should prevent mutations to the Queue just like for Array/Hash/etc.

There is no other "nested object" (i.e., the queue contents) exposed to user code, so it would just be weird if Queue#deep_freeze would actually prevent mutation of the queue but Queue#freeze does not.

"deep freeze" should be equivalent to calling freeze on each object in an object graph, this can't work if Queue#freeze doesn't prevent mutations.

[jeremyevans0 (Jeremy Evans)]

Eregon (Benoit Daloze) wrote in #note-7:

Why was this closed?

Because there was no feedback after the last message from @ko1 (Koichi Sasada), and it was decided at the developer meeting last year that there was no benefit to changing the behavior.

You are welcome to bring this issue up again at another developer meeting. However, if the same conclusion is reached, I think this issue should be closed again.

[Eregon (Benoit Daloze)]

OK, that no benefit to changing the behavior. was not reported here but I guess it's in the dev meeting log.

Benefit is consistency and being able to design deep freezing/immutable without making an exception for Queue.
Every time deep freezing/immutable comes up, this comes up as well as a problem.

I argue the current behavior is useful to nobody, and it's highly inconsistent, i.e., let's fix it.

[Eregon (Benoit Daloze)]

(an alternative would be to have some other way to freeze the Queue for the purpose of deep_freeze/Immutable, but I see no value to that, the way is called freeze and it already works for most objects).

[jeremyevans0 (Jeremy Evans)]

Eregon (Benoit Daloze) wrote in #note-9:

OK, that no benefit to changing the behavior. was not reported here but I guess it's in the dev meeting log.

ko1 (Koichi Sasada) wrote in #note-5:

At last dev-meeting we skipped this issue because it is difficult, but no benefit to change the current behavior.

[Eregon (Benoit Daloze)]

Indeed, sorry, I missed that somehow.

[ioquatix (Samuel Williams)]

I think there is only one rational answer here: Queue#freeze should prevent mutations to the Queue just like for Array/Hash/etc.

I agree with this for consistency reasons. Anyone who is freezing a queue and expecting it to work has buggy code.

[matz (Yukihiro Matsumoto)]

Since frozen queue is meaningless, if you care about consistency, we should prohibit freeze queues, like ENV.

Matz.

[mame (Yusuke Endoh)]

I would like to add a point that came up during the dev meeting: if a thread is waiting for Queue#pop, and another thread freezes the Queue, what should the first thread do? Wait forever? Raise a FrozenError?

To raise a FrozenError, it needs to be told that the Queue was asynchronously frozen by another thread. In other words, Queue#freeze needs to wake up the thread waiting for the Queue, which is likely to do more than #freeze would normally be expected to do.

[jeremyevans0 (Jeremy Evans)]

I submitted a pull request to have {Queue,SizedQueue}#freeze raise TypeError, as ENV.freeze does: https://github.com/ruby/ruby/pull/8515

• Status changed from Open to Closed

Applied in changeset git|62181e17da2db729eb6d95f6edff9e028ff557b6.

---

Make {Queue,SizedQueue}#freeze raise TypeError

Fixes [Bug #17146]