shardtree panic about librustzcash HOT 5 CLOSED

@AArnott thanks very much for the report. Is there any chance that you can provide the contents of the scan queue (along with the current range being scanned) at the time that you encounter this error?

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zcash/incrementalmerkletree#107 removes the assertion that is at fault here, and improves the distinction between pruned nodes and inserted nodes. A librustzcash PR will follow from this, but the shardtree change should be sufficient to work around your problem. @AArnott can you try it out?

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Yes, I can confirm the panic goes away with that shardtree change.

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scan range at time of panic: 2516985..2523970

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The summary here is that the assertion that was added in conjunction with the previous bugfix was testing a flaky condition that doesn't work correctly with how subtree roots were previously being inserted. #1439 will change librustzcash to change how it does subtree root insertion to add a semantic distinction between fully-scanned-and-pruned subtree roots and inserted subtree roots, but I don't think it will be a good idea to reintroduce the assertion irrespective of this change.

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