eaa5d3498c
This PR is a follow up to #8055 and implements a Selector for `Std.Channel` in order to allow multiplexing using channels. There is one subtlety to the implementation: Suppose we are in a situation where we run `select` in a loop on two channels. One of the channels is always quiet while the other has data available occasionally (however not always as this would trigger the `tryFn` fast path and hide the issue). In this situation the select receivers that are enqueued on the silent channel would usually just remain there indefinitely as nothing ever happens, causing a memleak. To avoid this we want to make a channel select clean up after itself, even if it fails. In an imperative programming language we could implement the receive queue as a doubly linked list and simply make each receive select maintain a pointer to its element in the queue and then remove itself in `O(1)` upon failure. As that is not possible in Lean trivially we decided to go for another approach for now: simply filter the queue for selects that have failed in `unregisterFn`. While this approach is `O(n)` we expect the amount of receivers enqueued on a channel to not be terribly large and thus this to be a reasonably fast operation compared to the remaining overhead. If it ever ends up becoming an issue, we could switch to an approach that uses a `TreeMap` with numbered receivers instead at a certain wait queue size and go to `O(log(n))`. |
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