feat: Disallow empty transaction batches

crates/miden-objects/src/batch/proposed_batch.rs

@@ -90,7 +90,7 @@ impl ProposedBatch {
     /// - The number of output notes exceeds [`MAX_OUTPUT_NOTES_PER_BATCH`].
     ///   - Note that output notes that are consumed in the same batch as unauthenticated input
     ///     notes do not count.
-    /// - Any note is consumed twice.
+    /// - Any note is consumed more than once.
     /// - Any note is created more than once.
     /// - The number of account updates exceeds [`MAX_ACCOUNTS_PER_BATCH`].
     ///   - Note that any number of transactions against the same account count as one update.
@@ -104,16 +104,23 @@ impl ProposedBatch {
     ///   the chain MMR.
     /// - The transactions in the proposed batch which update the same account are not correctly
     ///   ordered.
+    /// - The provided list of transactions is empty. An empty batch is pointless and would
+    ///   potentially result in the same [`BatchId`] for two empty batches which would mean batch
+    ///   IDs are no longer unique.
     /// - There are duplicate transactions.
     pub fn new(
         transactions: Vec<Arc<ProvenTransaction>>,
         block_header: BlockHeader,
         chain_mmr: ChainMmr,
         unauthenticated_note_proofs: BTreeMap<NoteId, NoteInclusionProof>,
     ) -> Result<Self, BatchProposeError> {
-        // Check for duplicate transactions.
+        // Check for empty or duplicate transactions.
         // --------------------------------------------------------------------------------------------
 
+        if transactions.is_empty() {
+            return Err(BatchProposeError::EmptyTransactionBatch);
+        }
+
         let mut transaction_set = BTreeSet::new();
         for tx in transactions.iter() {
             if !transaction_set.insert(tx.id()) {

crates/miden-objects/src/errors.rs

@@ -468,6 +468,9 @@ pub enum BatchProposeError {
   )]
     TooManyAccountUpdates(usize),
 
+    #[error("transaction batch must contain at least one transaction")]
+    EmptyTransactionBatch,
+
     #[error("transaction {transaction_id} appears twice in the proposed batch input")]
     DuplicateTransaction { transaction_id: TransactionId },
 

crates/miden-tx-batch-prover/src/tests/proposed_batch.rs

@@ -50,6 +50,20 @@ fn setup_chain() -> TestSetup {
     TestSetup { chain, account1, account2 }
 }
 
+/// Tests that a note created and consumed in the same batch are erased from the input and
+/// output note commitments.
+#[test]
+fn empty_transaction_batch() -> anyhow::Result<()> {
+    let TestSetup { chain, .. } = setup_chain();
+    let block1 = chain.block_header(1);
+
+    let error = ProposedBatch::new(vec![], block1, chain.chain(), BTreeMap::default()).unwrap_err();
+
+    assert_matches!(error, BatchProposeError::EmptyTransactionBatch);
+
+    Ok(())
+}
+
 /// Tests that a note created and consumed in the same batch are erased from the input and
 /// output note commitments.
 #[test]