SIMD-0083: Relax Entry Constraints (#83)

* Add XXXX-relax-entry-constraints.md

* Fill in SIMD-0083

* Explicitly specify consensus-break and sequential ordering

* Terminology, explicit constraints

* Clarify Tick entry terminology, and num hashes constraint

proposals/0083-relax-entry-constraints.md

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+---
+simd: '0083'
+title: Relax Entry Constraints
+authors:
+  - Andrew Fitzgerald (Solana Labs)
+category: Standard
+type: Core
+status: Draft
+created: 2023-11-02
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+Remove the constraint that transaction entries cannot contain conflicting
+transactions.
+
+## Motivation
+
+The current constraint that transactions within entries must not conflict with
+each other is unnecessary.
+It is a constraint, that made the current labs-client implementation easier,
+but is not required for the protocol to function correctly.
+The removal of this constraint simplifies the protocol, and allows more
+flexibility in the implementation of both block-produciton and
+block-validation.
+
+## Alternatives Considered
+
+1. Do nothing
+    - This is the simplest option, as we could leave the protocol as is.
+    However, this leaves the protocol more complex than it needs to be.
+2. Remove the constraint, but execute conflicting transactions by priority.
+    - This is a more complex option than the current proposal.
+    - It also gives less flexibility to the block-producer, since transactions
+    would be re-ordered.
+
+## New Terminology
+
+Conflicting transactions are defined as transactions that either:
+
+1. both require a write-lock on the same account, or
+2. one transaction requires a write-lock, and the other requires a read-lock on
+   the same account.
+
+Tick entries are defined as entries that contain no transactions.
+
+## Detailed Design
+
+Currently, if a transaction entry within a block contains transactions that
+conflict, the entire block is marked invalid.
+The proposal is that this constraint is removed entirely, and entries are
+allowed to contain transactions that conflict with each other.
+
+If transactions within an entry conflict with each other, they must be
+executed sequentially, in the order they appear in the entry.
+This decision gives the most freedom in block-production, as it allows for
+arbitrary ordering of transactions within an entry by the block-producer.
+
+If the proposal is accepted, the individual entry constraints are as follows:
+
+1. Each entry must be deserializable via `bincode` (or equivalent) into a
+   structure:
+
+   ```rust
+    struct Entry {
+      num_hashes: u64,
+      hash: Hash,
+      transactions: Vec<VersionedTransaction>,
+    }
+   ```
+
+   where `Hash` and `VersionedTransaction` are defined in `solana-sdk`.
+2. Tick entries are valid only if the sum of `num_hashes` in all entries,
+   including the tick entry itself, since the previous tick entry match the
+   `hashes_per_tick` field of the `Bank`. This value is subject to change with
+   feature gates, but is currently 12500. This means if a tick entry's
+   `num_hashes` field exceeds the `hashes_per_tick` field of the `Bank`, the
+   entry is invalid.
+
+If any of these constraints are violated, the entire block is invalid.
+
+## Impact
+
+- The protocol is simplified
+- Block-production is more flexible
+
+## Security Considerations
+
+This proposal changes the protocol and would break consensus.
+This proposal will require a feature-gate, so that all validators can change
+behavior at the same time.
+
+## Drawbacks
+
+None beyond the security considerations above.
+
+## Backwards Compatibility
+
+This proposal is backwards compatible with the current protocol, since it only
+relaxes constraints, and does not add any new constraints.
+All previously valid blocks would still be valid.
+However, newly produced blocks may no longer be valid under the old protocol.