Multiple transactions (and packages)

TODO: This section should start from AcceptPackage() and flow through from there, including AcceptMultipleTransactions() as a sub-section.

It’s possible to consider multiple transactions for validation together, via AcceptMultipleTransactions() found in src/net_processing.cpp. It’s currently only available from tests (test/tx_package_tests.cpp) and the testmempoolaccept RPC (via ProcessNewPackage()), but the intention is for it to be available to packages received from the P2P network in the future.

This validation flow has been created for usage with Package Mempool Accept, which glozow has written up in a gist (archive).

The flow here is similar to AcceptSingleTransaction() in that we start by grabbing cs_main before initializing validation state and workspaces, however this time we use PackageValidationState and a vector of workspaces, std::vector<Workspace>. Each transaction therefore has it’s own workspace but all transactions in the package share a single validation state. This aligns with the goal of either accepting or rejecting the entire package as a single entity.

Next come two for loops over the vector of workspaces (i.e. transactions). The first performs the PreChecks(), but this time also freeing up coins to be spent by other transactions in this package. This would not usually be possible (nor make sense) within an AcceptTransaction() flow, but within a package we want to be able to validate transactions who use as inputs, other transactions not yet added to our mempool:

    // Make the coins created by this transaction available for subsequent transactions in the
    // package to spend. Since we already checked conflicts in the package and we don't allow
    // replacements, we don't need to track the coins spent. Note that this logic will need to be
    // updated if package replace-by-fee is allowed in the future.

If the PreChecks do not fail, we call m_viewmempool.PackageAddTransaction() passing in the workspace. This adds the transaction to a map in our Mempool called std::unordered_map<COutPoint, Coin, SaltedOutpointHasher> m_temp_added;, which is essentially a temporary cache somewhere in-between being validated and being fully added to the mempool.

TODO: Fix after adding section on AcceptPackage

After this first loop we perform PackageMempoolChecks() which first asserts that transactions are not already in the mempool, before checking the "PackageLimits".