Change mint tx format and coinbase rules to mitigate dust accumulation

src/protocol/tx-validity.md

@@ -103,6 +103,15 @@ def sum_inputs(tx, asset_id) -> int:
             total += input.amount
     return total
 
+"""
+Returns any minted amounts by the transaction
+"""
+def minted(tx, asset_id) -> int:
+  mint_amount: int = 0
+  if tx.type == TransactionType.Mint and asset_id == tx.mint_asset_id:
+    mint_amount = tx.mint_amount
+  return mint_amount
+
 def sum_outputs(tx, asset_id) -> int:
     total: int = 0
     for output in tx.outputs:
@@ -114,7 +123,7 @@ def available_balance(tx, asset_id) -> int:
     """
     Make the data message balance available to the script
     """
-    availableBalance = sum_inputs(tx, asset_id) + sum_data_messages(tx, asset_id)
+    availableBalance = sum_inputs(tx, asset_id) + sum_data_messages(tx, asset_id) + minted(tx, asset_id)
     return availableBalance
 
 def unavailable_balance(tx, asset_id) -> int:
@@ -206,11 +215,25 @@ Transaction processing is completed by removing spent UTXOs from the state and a
 
 ### Coinbase Transaction
 
-The coinbase transaction is a mechanism for block creators to convert fees into spendable UTXOs.
+The coinbase transaction is a mechanism for block creators to collect transaction fees.
 
 In order for a coinbase transaction to be valid:
 
 1. It must be a [Mint](../tx-format/transaction.md#TransactionMint) transaction.
-2. The coinbase transaction must be the first transaction within a block, even if there are no other transactions in the block and the fee is zero.
-3. The total output value of the coinbase transaction cannot exceed the total amount of fees processed from all other transactions within the same block.
-4. The `asset_id` for coinbase transaction outputs must match the `asset_id` that fees are paid in (`asset_id == 0`).
+1. The coinbase transaction must be the first transaction within a block, even if there are no other transactions in the block and the fee is zero.
+1. The `mintAmount` doesn't exceed the total amount of fees processed from all other transactions within the same block.
+1. The `mintAssetId` matches the `asset_id` that fees are paid in (`asset_id == 0`).
+1. the `gasPrice`, where relevant is assumed to be zero.
+
+A [Mint](../tx-format/transaction.md#TransactionMint) transaction is only valid within the context of these coinbase validity rules.
+
+Fee burning mechanisms are supported. The block creator may use a lower `mintAmount` than the total
+amount of fees paid within the block, leaving the excess fees forever unclaimed.
+
+The [Mint](../tx-format/transaction.md#TransactionMint) transaction type flexibly allows the block
+creator to decide how fees should be collected.
+For example, consider that a threshold amount exists for a UTXO to be considered non-spendable dust.
+If the coinbase amount is below this threshold, the block creator could use a script to transfer the amount 
+into a contract balance. After the contract balance is high enough, the block creator would sweep the balance out as 
+spendable coin UTXOs. If the coinbase amount is above this threshold, the block creator may use
+[OutputType.Coin](../tx-format/output.md#outputcoin) to avoid extra steps.

src/tx-format/transaction.md

@@ -131,15 +131,25 @@ Creates a contract with contract ID as computed [here](../identifiers/contract-i
 ## TransactionMint
 
 The transaction is created by the block producer and is not signed. Since it is not usable outside of block creation or execution, all fields must be fully set upon creation without any zeroing.
-
-| name           | type                         | description                                          |
-|----------------|------------------------------|------------------------------------------------------|
-| `txPointer`    | [TXPointer](./tx-pointer.md) | The location of the `Mint` transaction in the block. |
-| `outputsCount` | `uint8`                      | Number of outputs.                                   |
-| `outputs`      | [Output](./output.md)`[]`    | List of outputs.                                     |
+This means that the transaction ID must also include the correct `txPointer` value, not zeroed out.
+
+| name           | type                         | description                                               |
+|----------------|------------------------------|-----------------------------------------------------------|
+| `mintAmount`   | `uint64`                     | The amount of funds being minted within this transaction. |
+| `mintAssetId`  | `byte[32]`                   | The asset ID of funds minted within this transaction.     |
+| `gasLimit`     | `uint64`                     | Gas limit for transaction                                 |
+| `scriptLength` | `uint16`                     | Script length, in instructions.                           |
+| `inputsCount`  | `uint8`                      | Number of inputs.                                         |
+| `outputsCount` | `uint8`                      | Number of outputs.                                        |
+| `txPointer`    | [TXPointer](./tx-pointer.md) | The location of the `Mint` transaction in the block.      |
+| `receiptsRoot` | `byte[32]`                   | Merkle root of receipts.                                  |
+| `script`       | `byte[]`                     | Script to execute.                                        |
+| `inputs`       | [Input](./input.md)`[]`      | List of inputs.                                           |
+| `outputs`      | [Output](./output.md)`[]`    | List of outputs.                                          |
 
 Transaction is invalid if:
-
-- Any output is not of type `OutputType.Coin`
-- Any two outputs have the same `asset_id`
 - `txPointer` is zero or doesn't match the block.
+- Any output is of type `OutputType.ContractCreated`
+- `scriptLength > MAX_SCRIPT_LENGTH`
+- `scriptDataLength > MAX_SCRIPT_DATA_LENGTH`
+- `scriptLength * 4 != len(script)`