SIMD-0186: Loaded Transaction Data Size Specification

proposals/0186-loaded-transaction-data-size-specification.md

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+---
+simd: '0186'
+title: Loaded Transaction Data Size Specification
+authors:
+  - Hanako Mumei
+category: Standard
+type: Core
+status: Review
+created: 2024-10-20
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+Before a transaction can be executed, every account it may read from or write to
+must be loaded, including any programs it may call. The amount of data a
+transaction is allowed to load is capped, and if it exceeds that limit, loading
+is aborted. This functionality is already implemented in the validator.
+
+This SIMD defines a new algorithm for calculating the consensus-enforced total
+size of loaded transaction data during transaction processing.
+
+## Motivation
+
+Transaction data size accounting is currently unspecified, and the
+implementation-defined algorithm used in the Agave client exhibits some
+surprising behaviors:
+
+* BPF loaders required by instructions' program IDs are counted against
+transaction data size. BPF loaders required by CPI programs are not. If a
+required BPF loader is also included in the accounts list, it is counted twice.
+* The size of a program owned by LoaderV3 may or may not include the size of its
+programdata depending on how the program account is used on the transaction.
+Programdata is also itself counted if included in the transaction accounts list.
+This means programdata may be counted zero, one, or two times per transaction.
+* Due to certain quirks of implementation, loader-owned accounts which do not
+contain valid programs for execution may or may not be counted against the
+transaction data size total depending on how they are used on the transaction.
+This includes, but is not limited to, LoaderV3 buffer accounts, and accounts
+which fail ELF validation.
+* Accounts can be included on a transaction account list without being an
+instruction account, fee-payer, or program ID. These accounts are presently
+loaded and counted against transaction data size, although they can never be
+used for any purpose by the transaction.
+
+All validator clients must arrive at precisely the same transaction data size
+for all transactions because a difference of one byte can determine whether a
+transaction is executed or failed, and thus affects consensus. Also, we want the
+calculated transaction data size to correspond well with the actual amount of
+data the transaction requests.
+
+Therefore, this SIMD seeks to specify an algorithm that is straightforward to
+implement in a client-agnostic way, while also accurately accounting for all
+account data required by the transaction.
+
+## New Terminology
+
+No new terms are introduced by this SIMD, however we define these for clarity:
+
+* Instruction account: an account passed to an instruction in its accounts
+array, which allows the program to view the actual bytes contained in the
+account. CPI can only happen through programs provided as instruction accounts.
+* Transaction accounts list: all accounts for the transaction, which includes
+instruction accounts, the fee-payer, program IDs, and any extra accounts added
+to the list but not used for any purpose.
+* LoaderV3 program account: an account owned by
+`BPFLoaderUpgradeab1e11111111111111111111111` which contains in its account data
+the first four bytes `02 00 00 00` followed by a pubkey which points to an
+account which is defined as the program's programdata account.
+
+For the purposes of this SIMD, we make no assumptions about the contents of the
+programdata account.
+
+## Detailed Design
+
+The proposed algorithm is as follows:
+
+1. Given a transaction, take the unique set of account keys which are used as:
+    * An instruction account.
+    * A program ID for an instruction.
+    * The fee-payer.
+2. Each account's size is determined solely by the byte length of its data prior
+to transaction execution, irrespective of it is used on the transaction.
+3. For any loaded account identified as a `LoaderV3` program account, add the
+size of the programdata account it references to its own size, irrespective of
+how the program account is used on the transaction.
+4. The total transaction size is the sum of these sizes.
+
+Transactions may include a
+`ComputeBudgetInstruction::SetLoadedAccountsDataSizeLimit` instruction to define
+a lower data size limit for the transaction. Otherwise, the default limit is
+64MiB (`64 * 1024 * 1024` bytes).
+
+If a transaction exceeds its data size limit, the transaction is failed. Fees
+will be charged once `enable_transaction_loading_failure_fees` is enabled.
+
+Adding required loaders to transaction data size is abolished. They are treated
+the same as any other account: counted if used in a manner described by 1, not
+counted otherwise.
+
+No account that falls outside of the three categories listed by 1 is counted
+against transaction data size. Validator clients are free to decline to load
+them.
+
+Read-only and writable accounts are treated the same. In the future, when direct
+mapping is enabled, this SIMD may be amended to count them differently.
+
+As a consequence of 1 and 3, for LoaderV3 programs, programdata is counted twice
+if a transaction explicitly references the program account and its programdata
+account. This is done partly for simplicity, and partly to account for the cost
+of maintaining the compiled program in addition to the actual bytes of
+the programdata account.
+
+We include programdata size in account size for LoaderV3 programs because using
+the program account on a transaction forces an unconditional load of programdata
+to compile the program for execution. We always count it, even when the program
+is an instruction account, because the program must be available for CPI.
+
+There is no special handling for any account owned by the native loader,
+LoaderV1, or LoaderV2.
+
+Account size for programs owned by LoaderV4 is left undefined. This SIMD should
+be amended to define the required semantics before LoaderV4 is enabled on any
+network.
+
+## Alternatives Considered
+
+* Transaction data size accounting is already enabled, so the null option is to
+enshrine the current Agave behavior in the protocol. This is undesirable because
+the current behavior is highly idiosyncratic, and LoaderV3 program sizes are
+routinely undercounted.
+* Builtin programs are backed by accounts that only contain the program name as
+a string, typically making them 15-40 bytes. We could impose a larger fixed cost
+for these. However, they must be made available for all programs anyway, and
+most of them are likely to be ported to BPF eventually, so this adds complexity
+for no real benefit.
+* Several slightly different algorithms were considered for handling LoaderV3
+programs in particular, for instance only counting programs that are valid for
+execution in the current slot. However, this would implicitly couple transaction
+data size with the results of ELF validation, which is highly undesirable.
+* We considered loading and counting sizes for accounts on the transaction
+account list which are not used for any purpose. This is the current behavior,
+but there is no reason to load such accounts at all.
+
+## Impact
+
+The primary impact is this SIMD makes correctly implementing transaction data
+size accounting much easier for other validator clients.
+
+It makes the calculated size of transactions which include program accounts for
+CPI somewhat larger, but given the generous 64MiB limit, it is unlikely that any
+existing users will be affected. Based on an investigation of a 30-day window,
+transactions larger than 30MiB are virtually never seen.
+
+## Security Considerations
+
+Security impact is minimal because this SIMD merely simplifies an existing
+feature. Care must be taken to implement the rules exactly.
+
+This SIMD requires a feature gate.
+
+## Backwards Compatibility
+
+Transactions that currently have a total transaction data size close to the
+64MiB limit, which call LoaderV3 programs via CPI, may now exceed it and fail.