[PAN-2196] Implement world state cancel #867
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Return an error if run() is called on an already running downloader.
| return future; | ||
| } | ||
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| public void cancel() { | ||
| // TODO | ||
| getFuture().cancel(true); |
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Given that the use case for this is likely to be cancelling one download and immediately starting a new one for a later block, how do we avoid a race condition where outstanding requests and/or handleCancellation are still happening after this method returns and a new download starts?
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Just to think this through a bit more...
When you can cancel, that same thread will be used to run any follow actions added via whenComplete, thenApply and friends. So if you call WorldStateDonwloader.cancel by the time that method returns we do actually know that handleCancellation has completed. However, we don't know if handleCancellation runs before or after anything else that was chained to that future.
I think that will probably work out ok but we may want to have createFuture return the future returned from whenComplete instead of the original to fix it (just need to make sure it doesn't interfere with whether the returned future completes exceptionally or not).
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Another good catch. I think I broke this when I removed the status var. I reverted that change and now you shouldn't be able to execute run() before the cancellation has finished because the checks are now based on the status variable which gets updated inside of the synchronized handleCancellation method.
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I went ahead and merged because I have another branch built on top of this one. But if you see any issues with my follow-up updates, I can fix them in another PR.
| try { | ||
| db.deleteRange(from, to); | ||
| lastDequeuedKey.set(0); | ||
| lastEnqueuedKey.set(0); |
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Do we need to set oldestKey to 0 as well?
This reverts commit f032879.
PR description
Implement world state download cancellation functionality. On cancellation, the
WorldStateDownloaderwill cancel any pending network tasks and clear its task queue. Also made a few other small changes:Removed the.statustracking from the downloader in favor of using the future as a signal on the statusrunis called on an already running instance.