stream::ForEach can no longer block indefinitely #2224
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This PR removes the `loop` from `stream::ForEach`. Instead, the future makes at most 1 attempt to poll its interior, and if it can make more progress, it immediately calls `wake_by_ref()`, then returns Pending. In this way, under a fair executor, it can't accidentally starve the task queue.
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I'm skeptical of this approach-- see my the related discussion between myself, @jonhoo , and @carllerche in this thread: #2049 (comment) I also anticipate that this PR would have significant negative performance implications if we adopted this approach throughout the ecosystem: yielding back up to the top-level executor with every step seems like it would interact poorly with locality, and could be very inefficient depending on the top-level executor's queuing implementation. |
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This PR removes the
loopfromstream::ForEach. Instead, the futuremakes at most 1 attempt to poll its interior, and if it can make more
progress, it immediately calls
wake_by_ref(), then returns Pending.In this way, under a fair executor, it can't accidentally starve the
task queue.