fix(aztec-node): throw on existing nullifier in getLowNullifierMembershipWitness

yarn-project/aztec-node/src/aztec-node/server.test.ts

@@ -398,6 +398,33 @@ describe('aztec node', () => {
       });
     });
 
+    describe('getLowNullifierMembershipWitness', () => {
+      beforeEach(() => {
+        lastBlockNumber = BlockNumber(1);
+      });
+
+      it('throws when nullifier already exists in the tree', async () => {
+        const nullifier = Fr.random();
+        merkleTreeOps.getPreviousValueIndex.mockImplementation((treeId: MerkleTreeId, value: bigint) => {
+          if (treeId === MerkleTreeId.NULLIFIER_TREE && value === nullifier.toBigInt()) {
+            return Promise.resolve({ index: 42n, alreadyPresent: true });
+          }
+          return Promise.resolve(undefined);
+        });
+
+        await expect(node.getLowNullifierMembershipWitness('latest', nullifier)).rejects.toThrow(
+          /Cannot prove nullifier non-inclusion/,
+        );
+      });
+
+      it('returns undefined when nullifier not found', async () => {
+        merkleTreeOps.getPreviousValueIndex.mockResolvedValue(undefined);
+
+        const result = await node.getLowNullifierMembershipWitness('latest', Fr.random());
+        expect(result).toBeUndefined();
+      });
+    });
+
     describe('findLeavesIndexes', () => {
       const blockHash1 = Fr.random();
       const blockHash2 = Fr.random();

yarn-project/aztec-node/src/aztec-node/server.ts

@@ -1133,21 +1133,6 @@ export class AztecNodeService implements AztecNode, AztecNodeAdmin, Traceable {
     return new NullifierMembershipWitness(index, leafPreimage as NullifierLeafPreimage, path);
   }
 
-  /**
-   * Returns a low nullifier membership witness for a given nullifier at a given block.
-   * @param referenceBlock - The block parameter (block number, block hash, or 'latest') at which to get the data
-   * (which contains the root of the nullifier tree in which we are searching for the nullifier).
-   * @param nullifier - Nullifier we try to find the low nullifier witness for.
-   * @returns The low nullifier membership witness (if found).
-   * @remarks Low nullifier witness can be used to perform a nullifier non-inclusion proof by leveraging the "linked
-   * list structure" of leaves and proving that a lower nullifier is pointing to a bigger next value than the nullifier
-   * we are trying to prove non-inclusion for.
-   *
-   * Note: This function returns the membership witness of the nullifier itself and not the low nullifier when
-   * the nullifier already exists in the tree. This is because the `getPreviousValueIndex` function returns the
-   * index of the nullifier itself when it already exists in the tree.
-   * TODO: This is a confusing behavior and we should eventually address that.
-   */
   public async getLowNullifierMembershipWitness(
     referenceBlock: BlockParameter,
     nullifier: Fr,
@@ -1159,7 +1144,9 @@ export class AztecNodeService implements AztecNode, AztecNodeAdmin, Traceable {
     }
     const { index, alreadyPresent } = findResult;
     if (alreadyPresent) {
-      this.log.warn(`Nullifier ${nullifier.toBigInt()} already exists in the tree`);
+      throw new Error(
+        `Cannot prove nullifier non-inclusion: nullifier ${nullifier.toBigInt()} already exists in the tree`,
+      );
     }
     const preimageData = (await committedDb.getLeafPreimage(MerkleTreeId.NULLIFIER_TREE, index))!;
 

yarn-project/stdlib/src/interfaces/aztec-node.ts

@@ -122,6 +122,7 @@ export interface AztecNode
    * @param referenceBlock - The block parameter (block number, block hash, or 'latest') at which to get the data.
    * @param nullifier - Nullifier we try to find the low nullifier witness for.
    * @returns The low nullifier membership witness (if found).
+   * @throws If the nullifier already exists in the tree, since non-inclusion cannot be proven.
    * @remarks Low nullifier witness can be used to perform a nullifier non-inclusion proof by leveraging the "linked
    * list structure" of leaves and proving that a lower nullifier is pointing to a bigger next value than the nullifier
    * we are trying to prove non-inclusion for.