docs: Transaction consensus rules: Coinbase rules (#3464)

* refactor coinbase transaction consensus rules docs

* add missing rule

* nit

Co-authored-by: teor <teor@riseup.net>

Co-authored-by: teor <teor@riseup.net>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

zebra-chain/src/transparent/serialize.rs

@@ -65,6 +65,21 @@ impl ZcashDeserialize for OutPoint {
 ///
 /// The height may consume `0..=5` bytes at the stat of the coinbase data.
 /// The genesis block does not include an encoded coinbase height.
+///
+/// # Consensus
+///
+/// > A coinbase transaction for a block at block height greater than 0 MUST have
+/// > a script that, as its first item, encodes the block height height as follows.
+/// > For height in the range {1..16}, the encoding is a single byte of value
+/// > 0x50 height. Otherwise, let heightBytes be the signed little-endian
+/// > representation of height, using the minimum nonzero number of bytes such that
+/// > the most significant byte is < 0x80.
+/// > The length of heightBytes MUST be in the range {1..5}.
+/// > Then the encoding is the length of heightBytes encoded as one byte,
+/// > followed by heightBytes itself. This matches the encoding used by Bitcoin in the
+/// > implementation of [BIP-34] (but the description here is to be considered normative).
+///
+/// <https://zips.z.cash/protocol/protocol.pdf#txnconsensus>
 pub(crate) fn parse_coinbase_height(
     mut data: Vec<u8>,
 ) -> Result<(block::Height, CoinbaseData), SerializationError> {

zebra-consensus/src/block/check.rs

@@ -184,10 +184,12 @@ pub fn miner_fees_are_valid(
     let block_subsidy = subsidy::general::block_subsidy(height, network)
         .expect("a valid block subsidy for this height and network");
 
-    // Consensus rule: The total value in zatoshi of transparent outputs from a
-    // coinbase transaction, minus vbalanceSapling, minus vbalanceOrchard, MUST NOT
-    // be greater than the value in zatoshi of block subsidy plus the transaction fees
-    // paid by transactions in this block.
+    // # Consensus
+    //
+    // > The total value in zatoshi of transparent outputs from a coinbase transaction,
+    // > minus vbalanceSapling, minus vbalanceOrchard, MUST NOT be greater than the value
+    // > in zatoshi of block subsidy plus the transaction fees paid by transactions in this block.
+    //
     // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
     let left = (transparent_value_balance - sapling_value_balance - orchard_value_balance)
         .map_err(|_| SubsidyError::SumOverflow)?;

zebra-consensus/src/transaction/check.rs

@@ -99,14 +99,20 @@ pub fn has_enough_orchard_flags(tx: &Transaction) -> Result<(), TransactionError
 
 /// Check that a coinbase transaction has no PrevOut inputs, JoinSplits, or spends.
 ///
-/// A coinbase transaction MUST NOT have any transparent inputs, JoinSplit descriptions,
-/// or Spend descriptions.
+/// # Consensus
 ///
-/// In a version 5 coinbase transaction, the enableSpendsOrchard flag MUST be 0.
+/// > A coinbase transaction MUST NOT have any transparent inputs with non-null prevout fields,
+/// > JoinSplit descriptions, or Spend descriptions.
+///
+/// > [NU5 onward] In a version 5 coinbase transaction, the enableSpendsOrchard flag MUST be 0.
 ///
 /// This check only counts `PrevOut` transparent inputs.
 ///
-/// https://zips.z.cash/protocol/protocol.pdf#txnencodingandconsensus
+/// > [Pre-Heartwood] A coinbase transaction also MUST NOT have any Output descriptions.
+///
+/// Zebra does not validate this last rule explicitly because we checkpoint until Canopy activation.
+///
+/// <https://zips.z.cash/protocol/protocol.pdf#txnconsensus>
 pub fn coinbase_tx_no_prevout_joinsplit_spend(tx: &Transaction) -> Result<(), TransactionError> {
     if tx.has_valid_coinbase_transaction_inputs() {
         if tx.contains_prevout_input() {
@@ -245,6 +251,8 @@ where
 ///
 /// This is used to validate coinbase transactions:
 ///
+/// # Consensus
+///
 /// > [Heartwood onward] All Sapling and Orchard outputs in coinbase transactions MUST decrypt to a note
 /// > plaintext, i.e. the procedure in § 4.19.3 ‘Decryption using a Full Viewing Key ( Sapling and Orchard )’ on p. 67
 /// > does not return ⊥, using a sequence of 32 zero bytes as the outgoing viewing key. (This implies that before
@@ -255,7 +263,8 @@ where
 /// > according to the preceding rule MUST have note plaintext lead byte equal to 0x02. (This applies even during
 /// > the "grace period" specified in [ZIP-212].)
 ///
-/// [3.10]: https://zips.z.cash/protocol/protocol.pdf#coinbasetransactions
+/// <https://zips.z.cash/protocol/protocol.pdf#txnconsensus>
+///
 /// [ZIP-212]: https://zips.z.cash/zip-0212#consensus-rule-change-for-coinbase-transactions
 ///
 /// TODO: Currently, a 0x01 lead byte is allowed in the "grace period" mentioned since we're
@@ -298,8 +307,12 @@ pub fn coinbase_expiry_height(
         // If Nu5 does not have a height, apply the pre-Nu5 rule.
         None => validate_expiry_height_max(expiry_height, true, block_height, coinbase),
         Some(activation_height) => {
-            // Consensus rule: from NU5 activation, the nExpiryHeight field of a
-            // coinbase transaction MUST be set equal to the block height.
+            // # Consensus
+            //
+            // > [NU5 onward] The nExpiryHeight field of a coinbase transaction MUST be equal
+            // > to its block height.
+            //
+            // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
             if *block_height >= activation_height {
                 match expiry_height {
                     None => Err(TransactionError::CoinbaseExpiryBlockHeight {

zebra-state/src/service/check/utxo.rs

@@ -168,16 +168,19 @@ fn transparent_spend_chain_order(
 
 /// Check that `utxo` is spendable, based on the coinbase `spend_restriction`.
 ///
-/// "A transaction with one or more transparent inputs from coinbase transactions
-/// MUST have no transparent outputs (i.e.tx_out_count MUST be 0)."
+/// # Consensus
 ///
-/// "A transaction MUST NOT spend a transparent output of a coinbase transaction
-/// from a block less than 100 blocks prior to the spend.
+/// > A transaction with one or more transparent inputs from coinbase transactions
+/// > MUST have no transparent outputs (i.e. tx_out_count MUST be 0).
+/// > Inputs from coinbase transactions include Founders’ Reward outputs and
+/// > funding stream outputs.
 ///
-/// Note that transparent outputs of coinbase transactions include Founders’
-/// Reward outputs and transparent funding stream outputs."
+/// > A transaction MUST NOT spend a transparent output of a coinbase transaction
+/// > from a block less than 100 blocks prior to the spend.
+/// > Note that transparent outputs of coinbase transactions include
+/// > Founders’ Reward outputs and transparent funding stream outputs.
 ///
-/// https://zips.z.cash/protocol/protocol.pdf#txnencodingandconsensus
+/// <https://zips.z.cash/protocol/protocol.pdf#txnconsensus>
 pub fn transparent_coinbase_spend(
     outpoint: transparent::OutPoint,
     spend_restriction: transparent::CoinbaseSpendRestriction,

zebra-state/src/service/finalized_state.rs

@@ -344,8 +344,11 @@ impl FinalizedState {
             batch.zs_insert(height_by_hash, hash, height);
             batch.zs_insert(block_by_height, height, &block);
 
-            // "A transaction MUST NOT spend an output of the genesis block coinbase transaction.
-            // (There is one such zero-valued output, on each of Testnet and Mainnet .)"
+            // # Consensus
+            //
+            // > A transaction MUST NOT spend an output of the genesis block coinbase transaction.
+            // > (There is one such zero-valued output, on each of Testnet and Mainnet.)
+            //
             // https://zips.z.cash/protocol/protocol.pdf#txnconsensus
             if block.header.previous_block_hash == GENESIS_PREVIOUS_BLOCK_HASH {
                 // Insert empty note commitment trees. Note that these can't be