Commits on Jan 14, 2021

1. multi: Rework utxoset/view to use outpoints. …

   This modifies the utxoset in the database and related UtxoViewpoint to
   store and work with unspent transaction outputs on a per-output basis
   instead of at a transaction level.
   
   The primary motivation is to simplify the code, pave the way for a
   utxo cache, and generally focus on optimizing runtime performance.
   
   The tradeoff is that this approach does somewhat increase the size of
   the serialized utxoset since it means that the transaction hash is
   duplicated for each output as a part of the key and some additional
   details are duplicated in each output.  The details duplicated in each
   output include flags encoded into a single byte that specify whether the
   containing transaction is a coinbase, whether the containing transaction
   has an expiry, and the transaction type.  Additionally, the containing
   block height and index are stored in each output.
   
   However, in practice, the size difference isn't all that large, disk
   space is relatively cheap, certainly cheaper than memory, and it is much
   more important to provide more efficient runtime operation since that is
   the ultimate purpose of the daemon.
   
   While performing this conversion, it also simplifies the code to remove
   the transaction version information from the utxoset as well as the
   spend journal.  The logic for only serializing it under certain
   circumstances is complicated, and it was only used for the gettxout RPC,
   where it has already been removed.
   
   The utxo set and spend journal in the database are automatically
   migrated to the new format with this commit and it is possible to
   interrupt and resume the migration process.
   
   Finally, it also updates all references and tests that previously dealt
   with transaction hashes to use outpoints instead.
   
   An overview of the changes are as follows:
   
   - Remove transaction version from both spent and unspent output entries
     - Update utxo serialization format to exclude the version
     - Update spend journal serialization format to exclude the version
   - Convert UtxoEntry to represent a specific utxo instead of a
     transaction with all remaining utxos
     - Optimize for memory usage with an eye toward a utxo cache
       - Combine fields such as whether the containing transaction is a
         coinbase, whether the containing transaction has an expiry, and
         the transaction type into a single byte
       - Align entry fields to eliminate extra padding since ultimately
         there will be a lot of these in memory
       - Introduce a free list for serializing an outpoint to the database
         key format to significantly reduce pressure on the GC
     - Update entries to be keyed by a <hash><tree><output index> outpoint
       rather than just a tx hash
     - Update all related functions that previously dealt with transaction
       hashes to accept outpoints instead
     - Update all callers accordingly
     - Only add individually requested outputs from the mempool when
       constructing a mempool view
   - Modify the spend journal to always store the encoded flags with every
     spent txout
     - Combine fields such as whether the containing transaction is a
       coinbase, whether the containing transaction has an expiry, and the
       transaction type into a single byte
       - Use 4 bits instead of 3 for the transaction type to be consistent
         with utxos. The extra bit was already unused so this doesn't take
         any additional space
     - Remove the fully spent flag
   - Introduce ticketMinOuts in place of stakeExtra
     - Renamed stakeExtra as ticketMinOuts and updated all comments to make
       the purpose of the field clearer
     - Only store ticketMinOuts for ticket submission outputs
     - Add TicketMinimalOutputs function on UtxoEntry in place of
       ConvertUtxosToMinimalOutputs
   - Always decompress data loaded from the database now that a utxo entry
     only consists of a specific output
   - Introduce upgrade code to migrate the utxo set and spend journal to
     the new format
     - Update current database version to 9
     - Update current utxo set version to 3
     - Update current spend journal version to 3
     - Introduce the ability to run upgrades after the block index has been
       loaded
   - Update all tests to expect the correct encodings, remove tests that no
     longer apply, and add new ones for the new expected behavior
     - Convert old tests for the legacy utxo format deserialization code to
       test the new function that is used during upgrade
   - Introduce a few new functions on UtxoViewpoint
     - AddTxOut for adding an individual txout versus all of them
     - addTxOut to handle the common code between the new AddTxOut and
       existing AddTxOuts
     - RemoveEntry for removing an individual txout
   - Remove the ErrDiscordantTxTree error
     - Since utxos are now retrieved using an outpoint, which includes the
       tree, it is no longer possible to hit this error path

   

   rstaudt2 committed Jan 14, 2021
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