fix(nano): fix calls to get_balance on other contracts

hathor/nanocontracts/runner/runner.py

@@ -816,17 +816,27 @@ def _get_balance(
         before_current_call: bool,
     ) -> Balance:
         """Internal implementation of get_balance."""
-        if contract_id is None:
-            contract_id = self.get_current_contract_id()
         if token_uid is None:
             token_uid = TokenUid(HATHOR_TOKEN_UID)
 
         storage: NCContractStorage
-        if self._call_info is None:
-            storage = self.get_storage(contract_id)
-        else:
-            changes_tracker = self.get_current_changes_tracker(contract_id)
+        if contract_id is None:
+            # In this case we're getting the balance of the currently executing contract,
+            # so it's guaranteed that a changes tracker exists.
+            # Depending on `before_current_call`, we get the current changes tracker or its storage.
+
+            changes_tracker = self.get_current_changes_tracker()
             storage = changes_tracker.storage if before_current_call else changes_tracker
+        else:
+            # In this case we're getting the balance of another contract,
+            # so a changes tracker may not yet exist if the contract is not in the call chain.
+            # We cannot retrieve the balance before the current call because we don't know whether the latest
+            # changes tracker was created during the current call or not.
+
+            if before_current_call:
+                raise NCFail('getting the balance of another contract before the current call is not supported.')
+
+            storage = self.get_current_changes_tracker_or_storage(contract_id)
 
         return storage.get_balance(bytes(token_uid))
 
@@ -840,9 +850,10 @@ def get_current_contract_id(self) -> ContractId:
         call_record = self.get_current_call_record()
         return call_record.contract_id
 
-    def get_current_changes_tracker(self, contract_id: ContractId) -> NCChangesTracker:
+    def get_current_changes_tracker(self) -> NCChangesTracker:
         """Return the NCChangesTracker for the current method being executed."""
         assert self._call_info is not None
+        contract_id = self.get_current_contract_id()
         change_trackers = self._call_info.change_trackers[contract_id]
         assert len(change_trackers) > 0
         return change_trackers[-1]
@@ -950,7 +961,7 @@ def syscall_revoke_authorities(self, token_uid: TokenUid, *, revoke_mint: bool,
         if token_uid == HATHOR_TOKEN_UID:
             raise NCInvalidSyscall(f'contract {contract_id.hex()} cannot revoke authorities from HTR token')
 
-        changes_tracker = self.get_current_changes_tracker(contract_id)
+        changes_tracker = self.get_current_changes_tracker()
         assert changes_tracker.nc_id == call_record.contract_id
         balance = changes_tracker.get_balance(token_uid)
 
@@ -993,7 +1004,7 @@ def syscall_mint_tokens(
         if token_uid == HATHOR_TOKEN_UID:
             raise NCInvalidSyscall(f'contract {call_record.contract_id.hex()} cannot mint HTR tokens')
 
-        changes_tracker = self.get_current_changes_tracker(call_record.contract_id)
+        changes_tracker = self.get_current_changes_tracker()
         assert changes_tracker.nc_id == call_record.contract_id
 
         balance = changes_tracker.get_balance(token_uid)
@@ -1027,7 +1038,7 @@ def syscall_melt_tokens(
         if token_uid == HATHOR_TOKEN_UID:
             raise NCInvalidSyscall(f'contract {call_record.contract_id.hex()} cannot melt HTR tokens')
 
-        changes_tracker = self.get_current_changes_tracker(call_record.contract_id)
+        changes_tracker = self.get_current_changes_tracker()
         assert changes_tracker.nc_id == call_record.contract_id
 
         balance = changes_tracker.get_balance(token_uid)
@@ -1108,7 +1119,7 @@ def syscall_create_child_deposit_token(
         token_id = derive_child_token_id(parent_id, cleaned_token_symbol, salt=salt)
         token_version = TokenVersion.DEPOSIT
 
-        changes_tracker = self.get_current_changes_tracker(parent_id)
+        changes_tracker = self.get_current_changes_tracker()
         changes_tracker.create_token(
             token_id=token_id,
             token_name=token_name,
@@ -1165,7 +1176,7 @@ def syscall_create_child_fee_token(
         token_id = derive_child_token_id(parent_id, cleaned_token_symbol, salt=salt)
         token_version = TokenVersion.FEE
 
-        changes_tracker = self.get_current_changes_tracker(parent_id)
+        changes_tracker = self.get_current_changes_tracker()
         changes_tracker.create_token(
             token_id=token_id,
             token_name=token_name,
@@ -1209,7 +1220,7 @@ def syscall_change_blueprint(self, blueprint_id: BlueprintId) -> None:
         # exception.
         self.tx_storage.get_blueprint_class(blueprint_id)
 
-        nc_storage = self.get_current_changes_tracker(last_call_record.contract_id)
+        nc_storage = self.get_current_changes_tracker()
         nc_storage.set_blueprint_id(blueprint_id)
 
     def _get_token(self, token_uid: TokenUid) -> TokenDescription:
@@ -1220,7 +1231,7 @@ def _get_token(self, token_uid: TokenUid) -> TokenDescription:
             NCInvalidSyscall when the token isn't found.
         """
         call_record = self.get_current_call_record()
-        changes_tracker = self.get_current_changes_tracker(call_record.contract_id)
+        changes_tracker = self.get_current_changes_tracker()
         assert call_record.contract_id == changes_tracker.nc_id
 
         if changes_tracker.has_token(token_uid):
@@ -1275,7 +1286,7 @@ def _update_tokens_amount(
             AssertionError: If call_record.index_updates is None
         """
         call_record = self.get_current_call_record()
-        changes_tracker = self.get_current_changes_tracker(call_record.contract_id)
+        changes_tracker = self.get_current_changes_tracker()
 
         assert changes_tracker.nc_id == call_record.contract_id
         assert call_record.index_updates is not None

tests/nanocontracts/test_balance_syscalls.py

@@ -0,0 +1,109 @@
+#  Copyright 2025 Hathor Labs
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#  http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+import pytest
+
+from hathor import Blueprint, Context, ContractId, NCFail, TokenUid, public
+from tests.nanocontracts.blueprints.unittest import BlueprintTestCase
+
+
+class MyBlueprint(Blueprint):
+    token_uid: TokenUid
+
+    @public
+    def initialize(self, ctx: Context, token_uid: TokenUid) -> None:
+        self.token_uid = token_uid
+
+    @public
+    def test_balance_syscalls_on_self(self, ctx: Context) -> None:
+        self.syscall.get_current_balance()
+        self.syscall.get_balance_before_current_call()
+
+        self.syscall.can_mint(self.token_uid)
+        self.syscall.can_mint_before_current_call(self.token_uid)
+
+        self.syscall.can_melt(self.token_uid)
+        self.syscall.can_melt_before_current_call(self.token_uid)
+
+    @public
+    def test_current_balance_syscalls_on_another(self, ctx: Context, other_id: ContractId) -> None:
+        self.syscall.get_current_balance(contract_id=other_id)
+        self.syscall.can_mint(self.token_uid, contract_id=other_id)
+        self.syscall.can_melt(self.token_uid, contract_id=other_id)
+
+    @public
+    def test_get_balance_before_current_call_on_another(self, ctx: Context, other_id: ContractId) -> None:
+        self.syscall.get_balance_before_current_call(contract_id=other_id)
+
+    @public
+    def test_can_mint_before_current_call_on_another(self, ctx: Context, other_id: ContractId) -> None:
+        self.syscall.can_mint_before_current_call(self.token_uid, contract_id=other_id)
+
+    @public
+    def test_can_melt_before_current_call_on_another(self, ctx: Context, other_id: ContractId) -> None:
+        self.syscall.can_melt_before_current_call(self.token_uid, contract_id=other_id)
+
+
+class BalanceSyscallsTestCase(BlueprintTestCase):
+    def setUp(self) -> None:
+        super().setUp()
+
+        self.blueprint_id = self._register_blueprint_class(MyBlueprint)
+        self.contract_id1 = self.gen_random_contract_id()
+        self.contract_id2 = self.gen_random_contract_id()
+
+        token_uid = self.gen_random_token_uid()
+        self.runner.create_contract(self.contract_id1, self.blueprint_id, self.create_context(), token_uid)
+        self.runner.create_contract(self.contract_id2, self.blueprint_id, self.create_context(), token_uid)
+
+    def test_balance_syscalls_on_self(self) -> None:
+        self.runner.call_public_method(self.contract_id1, 'test_balance_syscalls_on_self', self.create_context())
+
+    def test_current_balance_syscalls_on_another(self) -> None:
+        self.runner.call_public_method(
+            self.contract_id1,
+            'test_current_balance_syscalls_on_another',
+            self.create_context(),
+            self.contract_id2,
+        )
+
+    def test_get_balance_before_current_call_on_another(self) -> None:
+        msg = 'getting the balance of another contract before the current call is not supported.'
+        with pytest.raises(NCFail, match=msg):
+            self.runner.call_public_method(
+                self.contract_id1,
+                'test_get_balance_before_current_call_on_another',
+                self.create_context(),
+                self.contract_id2,
+            )
+
+    def test_can_mint_before_current_call_on_another(self) -> None:
+        msg = 'getting the balance of another contract before the current call is not supported.'
+        with pytest.raises(NCFail, match=msg):
+            self.runner.call_public_method(
+                self.contract_id1,
+                'test_can_mint_before_current_call_on_another',
+                self.create_context(),
+                self.contract_id2,
+            )
+
+    def test_can_melt_before_current_call_on_another(self) -> None:
+        msg = 'getting the balance of another contract before the current call is not supported.'
+        with pytest.raises(NCFail, match=msg):
+            self.runner.call_public_method(
+                self.contract_id1,
+                'test_can_melt_before_current_call_on_another',
+                self.create_context(),
+                self.contract_id2,
+            )