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netsync: Rework inventory announcement handling. #2548

Merged
merged 2 commits into from
Jan 15, 2021
Merged

netsync: Rework inventory announcement handling. #2548

merged 2 commits into from
Jan 15, 2021

Commits on Jan 15, 2021

  1. blockchain: Remove error from LatestBlockLocator. 

    This removes the error return from the LatestBlockLocator function since
it can never fail and ends up causing unneeded error checking.
    @davecgh
    davecgh committed Jan 15, 2021
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  2. netsync: Rework inventory announcement handling. 

    This modifies the way inventory announcements are handled to improve
efficiency and readability.

Specifically, it switches to a more explicit approach that handles the
specific recognized types (blocks and transactions) independently and
now determines if they are needed with unique logic for each type as
opposed to the more generic inventory-only based approach.

Next, and probably the most important change overall, is that recently
confirmed transactions are now tracked by the server and the logic which
determines if a transaction is needed now makes use of those tracked
transactions as opposed to the previous rather expensive utxo-based
query approach.

The reasoning for this change is that the only time honest nodes notify
others about transactions are when they view those transactions as
unconfirmed and they consider themselves to be current.  In practice
this means the only duplicate announcements are for unconfirmed and
recently confirmed transactions.  In the case of malicious nodes, the
transactions are rejected (either the transactions are actually invalid
or they are old transactions that were valid in the past but will now
fail due to trying to spend outputs they already spent).

So, given the above discussion, there are 3 cases to handle:

1) Duplicate announcements for unconfirmed transactions
2) Duplicate announcements for txns that have already been rejected
3) Duplicate announcements for recently-confirmed transactions

For the first case, the duplicate request is filtered by the mempool
since it already knows the unconfirmed transaction.  For the second
case, rejected transactions are tracked separately and filtered.  Thus,
only the third case of recently confirmed transactions remains.

Finally, the last block is now tracked by the hash instead of an index
into the index which allows the relevant updates to take place after the
main loop versus inside of it conditionally.
    @davecgh
    davecgh committed Jan 15, 2021
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