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erroneously dropped pending transactions #135

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wanderingbort opened this issue Aug 3, 2017 · 4 comments
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erroneously dropped pending transactions #135

wanderingbort opened this issue Aug 3, 2017 · 4 comments
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@wanderingbort

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@wanderingbort
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I think this code will may cause us to improperly drop transactions from the pending transactions: code-in-question

particularly the notion that a transaction that previously did not throw an exception (because it made it on to the pending list) may throw one now (in light of new chain state AND any previous replayed pending transaction) and if it does so, may be dropped on the floor depending on where in push_transaction the exception is thrown from.

Incoming blocks may invalidate transactions in the mempool but the state created by re-applying pending transactions is not a real state, in that it has not been accepted as part of the chain and may never be. it is "guessed state" in light of the pending TXs and if it causes us to discard a transaction that would have been valid when it is this producers turn to schedule transactions and make a block then we will have erroneously dropped it.

I suppose we could "re-learn" it from the network but that seems inefficient. It also implies the ordering of the pending-transactions is important that between two producers and should be kept in sync.
Otherwise, producers may be dropping different transactions giving them a different view of what needs to be scheduled at any time and also a different view of which transactions from the outside world to accept.

I think the chain moves forward with this code regardless but the effect on a single set of transactions would be "strange" and unexpected to say the least.
@wanderingbort wanderingbort self-assigned this Aug 3, 2017
@bytemaster
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This comes down to probability of collision within a small time window.

Note that Steem and BTS are using this process with little harm.

@bytemaster
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At issue here is that the validity of a transaction may depend upon the order of execution. To prevent spam, nodes reject new pending transactions that conflict with current pending transactions. Other peers may have received the transactions in a different order and so have different pending states.

A block producer will decide the official order and the broadcast a block. Nodes that had a different order will apply the block and then attempt to apply the pending transactions. This time their pending transaction will fail. It is as-if a node received all pending transactions immediately after applying the block.

The node should no longer advertise or relay the now invalid pending transaction. Each node has a duty to its peers to only relay blocks and transactions it considers "good".

It is not computationally viable to test all permutations. If a failed transaction were to be kept around it should be kept in a separate cache used only by the P2P code just incase another producer finds an order where the transaction is valid. Even this may be over engineered.

@nathanielhourt
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Fwiw, I spent a while reviewing this code way back in the first couple weeks of this repo's life, and I was convinced it was behaving correctly and did not lose transactions.

@wanderingbort
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For now I am closing this issue. With changes coming down the pipe, it will have a much smaller window to manifest (if at all). If it manifests then we will know more about it at that time.

ljrprocc pushed a commit to bithacks-tech/myeosio that referenced this issue Jul 4, 2018
Merge pull request EOSIO#135 from enumivo/staging
dcd16db 
rename eosd to enudaemon
NorseGaud pushed a commit that referenced this issue Jul 30, 2019
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@nathanielhourt @bytemaster @wanderingbort