#18157 closed defect (fixed)
Random failure in coerce_action.pyx
Reported by:  vbraun  Owned by:  

Priority:  major  Milestone:  sage6.6 
Component:  coercion  Keywords:  random_fail 
Cc:  SimonKing  Merged in:  
Authors:  Vincent Delecroix, Simon King  Reviewers:  Simon King 
Report Upstream:  N/A  Work issues:  
Branch:  60fcedc (Commits)  Commit:  
Dependencies:  Stopgaps: 
Description (last modified by )
Zmod(6) can be collected at the wrong time
sage t long warnlong 47.9 src/sage/structure/coerce_actions.pyx ********************************************************************** File "src/sage/structure/coerce_actions.pyx", line 78, in sage.structure.coerce_actions.LAction Failed example: sage.structure.coerce_actions.GenericAction(QQ, Zmod(6), True, check=False) Expected: Left action by Rational Field on Ring of integers modulo 6 Got: <repr(<sage.structure.coerce_actions.GenericAction at 0x7f8650aca938>) failed: RuntimeError: This action acted on a set that became garbage collected> ********************************************************************** 1 item had failures: 1 of 4 in sage.structure.coerce_actions.LAction [100 tests, 1 failure, 0.82 s]
Idem with
sage t long src/sage/structure/coerce.pyx ********************************************************************** File "src/sage/structure/coerce.pyx", line 1452, in sage.structure.coerce.CoercionModel_cache_maps.discover_action Failed example: cm.discover_action(GF(5)['x'], ZZ, operator.div) Expected: Right inverse action by Finite Field of size 5 on Univariate Polynomial Ring in x over Finite Field of size 5 with precomposition on right by Natural morphism: From: Integer Ring To: Finite Field of size 5 Got: <repr(<sage.categories.action.PrecomposedAction at 0x2b41eb2a2050>) failed: RuntimeError: This action acted on a set that became garbage collected> **********************************************************************
Change History (24)
comment:1 Changed 5 years ago by
comment:2 Changed 5 years ago by
 Description modified (diff)
comment:3 followup: ↓ 4 Changed 5 years ago by
cm.discover_action(GF(5)['x'], ZZ, operator.div)
Also for this one: the coercion framework will never be asked to discover an action on an otherwise unreferenced parent. At the same time, the action object does get cached (globally!) somewhere in the coercion framework and hence should probably not hold a strong reference (at least to one of the sets it acts on). The container it gets stored in is probably a TripleDict, keyed on the two parents involved. It takes care of having a callback to delete the action object when one of the domains gets deleted, so the action objects will normally not outlive the shortest lifetime of the parents involved.
If we have an absolute need of handling actions outside the coercion framework, we should probably make other action objects that do keep strong references. We do this for maps that get used for conversion and coercion. But these actions are showing desirable behaviour for use in the coercion framework. I think the bug is in the doctests.
comment:4 in reply to: ↑ 3 Changed 5 years ago by
comment:5 Changed 5 years ago by
Somebody please write a patch instead of metooing ;)
comment:6 Changed 5 years ago by
 Branch set to public/18157
 Commit set to ced7bfcc760e2fb362b2a382a22a94e02f996f9b
 Status changed from new to needs_review
Might be other wrong doctests... not sure how to find them.
New commits:
ced7bfc  Trac 18157: fix garbage collected objects in doctest

comment:7 Changed 5 years ago by
There is another one in src/sage/matrix/action.pyx
; it is already fixed in #10513, so no need to treat that one here.
comment:8 followup: ↓ 9 Changed 5 years ago by
Would be nice if people would run the testsuite with this patch (and #10513)
for i in $(seq w 1 100) do sage t long structure/ rings/ modular/ matrix/ tee ~/testlong${i}.log done
Or even better, does somebody know how to call gc.collect()
after each command in doctests?
comment:9 in reply to: ↑ 8 ; followup: ↓ 10 Changed 5 years ago by
Replying to vdelecroix:
Or even better, does somebody know how to call
gc.collect()
after each command in doctests?
That wouldn't help, right?
comment:10 in reply to: ↑ 9 Changed 5 years ago by
Replying to jdemeyer:
Replying to vdelecroix:
Or even better, does somebody know how to call
gc.collect()
after each command in doctests?That wouldn't help, right?
Right! Not for the two doctests mentioned in the description...
comment:11 Changed 5 years ago by
 Cc SimonKing added
comment:12 followup: ↓ 15 Changed 5 years ago by
 Reviewers set to Simon King
 Status changed from needs_review to needs_work
 Work issues set to Find underlying problem for elliptic curve problem, or open a new ticket for it
With one exception, the flaky tests constitute a misuse: Objects that are normally used internally are suddenly put out into the wild. It is clear that that can lead to disaster unless precautions are taken. Hence, I support the general approach in the attached branch: In these tests, one should work with permanent rather than temporary objects, to prevent premature garbage collection.
However, one should also explicitly say in these tests that we are talking about things that normally happen in the innards of Sage, and that the tests do not show how to work with them!
One test is different:
@@ 606,7 +624,8 @@ cdef class IntegerMulAction(Action): This used to hang before :trac:`17844`::  sage: E = EllipticCurve(GF(5), [4,0]) + sage: GF5 = GF(5) + sage: E = EllipticCurve(GF5, [4,0]) sage: P = E.random_element() sage: (2^63)*P (0 : 1 : 0)
Defining an elliptic curve over a finite field should certainly be enough to prevent the finite field from garbage collection! So, here I do not agree with the fix. Apparently the test fails because of an internal bug, not because of misuse.
So, I put it to "needs work", because we should either find and fix the underlying bug here, or open a new ticket for the elliptic curve issue.
comment:13 Changed 5 years ago by
I don't see a problem with the elliptic curve example:
sage: import gc sage: E = EllipticCurve(GF(5), [4,0]) sage: gc.collect() 156 sage: P = E.random_element() sage: gc.collect() 0 sage: (2^63)*P (0 : 1 : 0)
So, why should it be changed?
comment:14 followup: ↓ 16 Changed 5 years ago by
Even if right now E is kept alive elsewhere (presumably another doctest leaked it) you shouldn't assume that this will not change in the future.
comment:15 in reply to: ↑ 12 Changed 5 years ago by
comment:16 in reply to: ↑ 14 Changed 5 years ago by
Replying to vbraun:
Even if right now E is kept alive elsewhere (presumably another doctest leaked it) you shouldn't assume that this will not change in the future.
What are you talking about? In the test in question, the elliptic curve E
certainly is kept alive in the test (not elsewhere), as it is strongly referenced. As I don't see this test fail, I agree with Vincent that the test should be reverted to its original state.
comment:17 Changed 5 years ago by
 Commit changed from ced7bfcc760e2fb362b2a382a22a94e02f996f9b to ef3bb5d985ee65e46d89277a3833af40953bc59c
Branch pushed to git repo; I updated commit sha1. New commits:
ef3bb5d  Add a warning not to use coerce actions outside of the coercion model

comment:18 Changed 5 years ago by
 Status changed from needs_work to needs_review
I added comments in appropriate places, telling to be careful when using coerce actions outside of coercion models, reverting the elliptic curve test, and fixing some more tests in which a premature garbage collection was possible.
From my point of view, it is a positive review, but of course someone should have a look at my commit.
New commits:
ef3bb5d  Add a warning not to use coerce actions outside of the coercion model

comment:19 Changed 5 years ago by
 Commit changed from ef3bb5d985ee65e46d89277a3833af40953bc59c to 60fcedce48b8237353e72315976ed24ab7c236ca
comment:20 followup: ↓ 21 Changed 5 years ago by
Hi Simon,
I merge on sage6.7.beta1 (sorry if I did wrong). If you prefer you can only cherrypick 60fcedc.
I agree with your commits and just added references to your ticket. I am ok for the positive review.
Vincent
comment:21 in reply to: ↑ 20 Changed 5 years ago by
Replying to vdelecroix:
I merge on sage6.7.beta1 (sorry if I did wrong). If you prefer you can only cherrypick 60fcedc.
I suppose that Volker will tell you that (1) one should only merge if there is a good reason, and (2) one must not change the history, even if one has merged without a good reason.
So, just leave it as it is. I had a look at your changes, and they look fine to me. Provided that the patchbot gives a green blob, it should be positive review.
comment:22 Changed 5 years ago by
 Status changed from needs_review to positive_review
 Work issues Find underlying problem for elliptic curve problem, or open a new ticket for it deleted
The patchbot tells us that all tests pass, and based on the discussion above we have a positive review.
comment:23 Changed 5 years ago by
 Branch changed from public/18157 to 60fcedce48b8237353e72315976ed24ab7c236ca
 Resolution set to fixed
 Status changed from positive_review to closed
comment:24 Changed 5 years ago by
 Commit 60fcedce48b8237353e72315976ed24ab7c236ca deleted
Reliable reproduction of the error condition:
The reason is simple:
sage.categories.action.Action.__init__
only stores a weakref to the set acted on, and nothing else. If this "action" object is mainly meant to operate inside the coercion framework then this is probably the appropriate behaviour. Note that if the action ever needs to get used, then there is a group element to act and a set element to be acted upon, so the space has to be alive. Whoever is storing this action object should have a callback on the weakref, though, to throw away this action object to avoid a memory leak.It may just be that this action object does get used properly and that this doctest is broken. In that case the fix is: