Add an UnbalancedMerkleTree primitive

The reasoning for it is documented with itself. The plan is to use it within
our header for committing to the DAG (allowing one header per epoch, yet
logarithmic proofs for any header within the epoch), the transactions
commitment (allowing logarithmic proofs of a transaction within a block,
without padding), and the events commitment (allowing logarithmic proofs of
unique events within a block, despite events not having a unique ID inherent).

This also defines transaction hashes and performs the necessary modifications
for transactions to be unique.

substrate/abi/src/transaction.rs

@@ -204,6 +204,26 @@ impl Transaction {
     explicit_context.serialize(&mut message).unwrap();
     message
   }
+
+  /// The unique hash of this transaction.
+  ///
+  /// No two transactions on the blockchain will share a hash, making this a unique identifier.
+  /// For signed transactions, this is due to the `(signer, nonce)` pair present within the
+  /// `ExplicitContext`. For unsigned transactions, this is due to inherent properties of their
+  /// execution (e.g. only being able to set a `ValidatorSet`'s keys once).
+  pub fn hash(&self) -> [u8; 32] {
+    sp_core::blake2_256(&match self {
+      Transaction::Unsigned { call } => borsh::to_vec(&call).unwrap(),
+      Transaction::Signed {
+        calls,
+        contextualized_signature: ContextualizedSignature { explicit_context, signature: _ },
+      } => {
+        // We explicitly don't hash the signature, so signatures can be replaced in the future if
+        // desired (such as with half-aggregated Schnorr signatures)
+        borsh::to_vec(&(calls, explicit_context)).unwrap()
+      }
+    })
+  }
 }
 
 #[cfg(feature = "substrate")]
@@ -276,16 +296,23 @@ mod substrate {
     ///
     /// Returns `None` if the time has yet to be set.
     fn current_time(&self) -> Option<u64>;
-    /// Get, and consume, the next nonce for an account.
-    fn get_and_consume_next_nonce(&self, signer: &SeraiAddress) -> u32;
+    /// Get the next nonce for an account.
+    fn next_nonce(&self, signer: &SeraiAddress) -> u32;
     /// If the signer can pay the SRI fee.
     fn can_pay_fee(
       &self,
       signer: &SeraiAddress,
       fee: Amount,
     ) -> Result<(), TransactionValidityError>;
+
+    /// Begin execution of a transaction.
+    fn start_transaction(&self);
+    /// Consume the next nonce for an account.
+    fn consume_next_nonce(&self, signer: &SeraiAddress);
     /// Have the transaction pay its SRI fee.
     fn pay_fee(&self, signer: &SeraiAddress, fee: Amount) -> Result<(), TransactionValidityError>;
+    /// End execution of a transaction.
+    fn end_transaction(&self, transaction_hash: [u8; 32]);
   }
 
   /// A transaction with the context necessary to evaluate it within Substrate.
@@ -402,7 +429,7 @@ mod substrate {
               Err(TransactionValidityError::Invalid(InvalidTransaction::Stale))?;
             }
           }
-          match self.1.get_and_consume_next_nonce(signer).cmp(nonce) {
+          match self.1.next_nonce(signer).cmp(nonce) {
             core::cmp::Ordering::Less => {
               Err(TransactionValidityError::Invalid(InvalidTransaction::Stale))?
             }
@@ -472,7 +499,12 @@ mod substrate {
       // We use 0 for the mempool priority, as this is no longer in the mempool so it's irrelevant
       self.validate_except_fee::<V>(TransactionSource::InBlock, 0)?;
 
-      match self.0 {
+      // Start the transaction
+      self.1.start_transaction();
+
+      let transaction_hash = self.0.hash();
+
+      let res = match self.0 {
         Transaction::Unsigned { call } => {
           let call = Context::RuntimeCall::from(call.0);
           V::pre_dispatch(&call)?;
@@ -487,7 +519,9 @@ mod substrate {
           contextualized_signature:
             ContextualizedSignature { explicit_context: ExplicitContext { signer, fee, .. }, .. },
         } => {
-          // Start by paying the fee
+          // Consume the signer's next nonce
+          self.1.consume_next_nonce(&signer);
+          // Pay the fee
           self.1.pay_fee(&signer, fee)?;
 
           let _res = frame_support::storage::transactional::with_storage_layer(|| {
@@ -514,7 +548,14 @@ mod substrate {
             pays_fee: Pays::Yes,
           }))
         }
-      }
+      };
+
+      // TODO: TransactionSuccess/TransactionFailure event?
+
+      // End the transaction
+      self.1.end_transaction(transaction_hash);
+
+      res
     }
   }
 }