sdk: fix replace_signatures for variable signer ordering (anza-xyz#1608)

* Add failing test

* sdk: don't assume input key ordering matches tx when replacing signatures

* Refactor iterators

---------

Co-authored-by: Trent Nelson <[email protected]>

sdk/src/transaction/mod.rs

@@ -1060,13 +1060,21 @@ impl Transaction {
             return Err(TransactionError::InvalidAccountIndex);
         }
 
-        signers
+        for (index, account_key) in self
+            .message
+            .account_keys
             .iter()
             .enumerate()
-            .for_each(|(i, (pubkey, signature))| {
-                self.signatures[i] = *signature;
-                self.message.account_keys[i] = *pubkey;
-            });
+            .take(num_required_signatures)
+        {
+            if let Some((_pubkey, signature)) =
+                signers.iter().find(|(key, _signature)| account_key == key)
+            {
+                self.signatures[index] = *signature
+            } else {
+                return Err(TransactionError::InvalidAccountIndex);
+            }
+        }
 
         self.verify()
     }
@@ -1662,4 +1670,36 @@ mod tests {
 
         assert!(tx.is_signed());
     }
+
+    #[test]
+    fn test_replace_signatures() {
+        let program_id = Pubkey::default();
+        let keypair0 = Keypair::new();
+        let keypair1 = Keypair::new();
+        let pubkey0 = keypair0.pubkey();
+        let pubkey1 = keypair1.pubkey();
+        let ix = Instruction::new_with_bincode(
+            program_id,
+            &0,
+            vec![
+                AccountMeta::new(pubkey0, true),
+                AccountMeta::new(pubkey1, true),
+            ],
+        );
+        let message = Message::new(&[ix], Some(&pubkey0));
+        let expected_account_keys = message.account_keys.clone();
+        let mut tx = Transaction::new_unsigned(message);
+        tx.sign(&[&keypair0, &keypair1], Hash::new_unique());
+
+        let signature0 = keypair0.sign_message(&tx.message_data());
+        let signature1 = keypair1.sign_message(&tx.message_data());
+
+        // Replace signatures with order swapped
+        tx.replace_signatures(&[(pubkey1, signature1), (pubkey0, signature0)])
+            .unwrap();
+        // Order of account_keys should not change
+        assert_eq!(tx.message.account_keys, expected_account_keys);
+        // Order of signatures should match original account_keys list
+        assert_eq!(tx.signatures, &[signature0, signature1]);
+    }
 }