Make the list of segments more abstract

[ndkoval]

Make the list of segments more abstract, so that it can be used for stacks and channels

[dkhalanskyjb]

There are some slightly troubling findings. They don't affect the semaphore implementation, but maybe they can still be considered bugs, in which case they shouldn't be hard to fix. Otherwise, the requirements for calling these methods should probably be stated clearly in the documentation.

[dkhalanskyjb]

It seems that there is no problem here, even though there used to be one and the fix isn't incorporated here.

Suggestions

Does the check for the prev.value do anything useful? Nothing bad happens if the null in prev is propagated to the right in the removed segments. Even if there are reasons to leave this check, the comment is misleading: if we believe that the head is the segment to which head points, assert { removed } ensures that this method could not have been called on a head; if, instead, we believe that the head is the rightmost segment with prev == null, then it isn't clear why it can't be removed.

Background

A quick explanation of what was the problem and why it doesn't matter any longer.

The problem that used to be present in this place is that the previous version of the remove() method had a (negligible) probability of a memory leak here, because it was possible that the segment after the one to which the head points could still have prev point to a segment to the left of head (for example, 3.prev could point to segment 1 even though head would point to 2 and 1 shouldn't have been accessible). Arbitrarily long links could be formed this way.

In this version, the problem is absent because head may only ever point to a non-removed segment, ensuring that segment removal can't propagate through the segments which are pointed to.

[ndkoval]

I have updated the data structure in a way so that it can be used for an efficient list of cancellable JobNodes implementation.

[dkhalanskyjb]

I didn't look at the tests, and the implementation itself looks fine. The most notable change is a major overhaul of remove , which doesn't break the invariants necessary for everything to be correct. The potentially dangerous bugs that were found earlier are properly fixed.