Change:
} func (l *requestFIFO) Walk(f walkFunc) { <del> for current := l.Front(); current != nil; current = current.Next() { </del> <ins> var next *list.Element for current := l.Front(); current != nil; current = next { next = current.Next() // f is allowed to remove current </ins> if r, ok := current.Value.(*request); ok { if !f(r) { return

Discussion:
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/sig api-machinery
/triage accepted
adds a defensive check so we can avoid a panic. I was looking into why can be . This is the call chain, from the stack trace: makes sure there is at least one request waiting - acquires a lock on the queueset in the beginning. So should have at least one request waiting in a queue and that queue should be selected. could this be an issue with the locking somewhere, any thoughts
/cc
is the client version, not the server. In that commit, line 725 of is a comment.
check the first link at the top of the issue description -
Something is violating locking or counting discipline. Perhaps some cheap sanity checks on counts (like, never negative) would at least confirm that this is where the problem lies. With locking or counting being done wrong, a defense in one place will just make symptoms appear elsewhere. We need to find the problem. The needed discipline is very mechanical. A careful read of all the code should find any violations.
Also, we could make queueSet::UpdateObservations check that the queueSet has the right sums over its queues (under the queueSet lock, of course). I think it is called infrequently enough for that to be tolerable. The point would be to reveal something to us developers. We could even make it do repair if an error is found; this would not really solve the problem, of course, but might make a helpful reduction in the severity of the symptoms.
Also, some comments are outdated, incomplete, or missing. Some comments on the fields in reference a now-non-existent field . .. reads a bit odd, there is no "array" in sight. has no comment. The presence of "without ..." suggests that it is adding information, but as far as I can tell it is not. I would feel more comfortable with an explicit statement about concurrency and that returned .

Perhaps something like updating the comment on to say something like the following. .. should also describe the significance of the parameters. Since it is the same as for , a reference to that would suffice. Checking that locking discipline is followed would be easier if each non-Locked method of queueSet had a comment that explicitly says whether the method is lock-oblivious or an entry method. This would make it a local check to avoid the case of an entry method calling another entry method (which would cause a deadlock, not the symptom in this Issue). The function comment on uses the wrong capitalization for the function name. s/modify/modifying/
.. continued issues with comments. s/returns/returned/ The fields of should be re-ordered to start with immutable ones then have the mutable ones, and have a comment about locking. would be better named . The values of type should have comments defining their meaning. Note that both and can involve a timeout, and the comments/log-messages in say simply "time out", but they are different timeouts --- one is the timeout defined in APF and the other is the timeout/cancel from the request's context. should be named . Why are the and methods of public? The method of should be named .
and do not take into account the possibility that the decision was already set to . This obliviousness can lead to confusion about what is in and mis-counting. I.e., cause the symptom in this Issue.
I will propose a fix.
Thanks for your investigation. I still have some confusions need your clarification. the above issue occur under what conditions? As far as I know, will be set to just at the stage of for timeout, and this occurs at the , but will set decision prior to this and decision can only be set once. the can only be set once, is this parameter a bit redundant? two identical requests arrive at the same apiserver almost at the same time, what will happen and how to deal with it?
given particular request X might not get dispatched during its ; request X might have to wait a while if there are not enough seats available at the moment. The goroutine that calls for X then proceeds to call , which calls --- and that calls the method of X's . If X's context times out or is cancelled before X is dispatched or rejected by APF logic then X's gets set to .

hope my answer to 1. also answers this. am not sure I understand what potential problem you are thinking about here. Each request is a distinct , even if the client and request message are identical. In turn, each request gets a distinct .
Thank you for your clarification. Aout question 2, I mean the has been set to , the setting of will no longer work. maybe nil is better.
The attempt to set a request's decision to is made at dispatch time. This might happen after the request's context has been canceled or timed out and the method has thus already set the decision to .