Fix unused storage slots (vyperlang#2439)

* fix: don't alloc slots for repeated nonreentrancy keys

this is not a semantic bug but an optimization bug since we allocate
more slots than we actually need, leading to "holes" in the slot
allocator -- slots which are allocated but unused.

* make the test for nonreentrant slots harder

tests/cli/outputs/test_storage_layout.py

@@ -1,26 +1,39 @@
 from vyper.compiler import compile_code
 
 
-def test_method_identifiers():
+def test_storage_layout():
     code = """
 foo: HashMap[address, uint256]
-baz: Bytes[65]
-bar: uint256
 
 @external
 @nonreentrant("foo")
-def public_foo():
+def public_foo1():
     pass
 
+@external
+@nonreentrant("foo")
+def public_foo2():
+    pass
+
+
 @internal
 @nonreentrant("bar")
 def _bar():
     pass
 
+# mix it up a little
+baz: Bytes[65]
+bar: uint256
+
 @external
 @nonreentrant("bar")
 def public_bar():
     pass
+
+@external
+@nonreentrant("foo")
+def public_foo3():
+    pass
     """
 
     out = compile_code(
@@ -30,12 +43,12 @@ def public_bar():
 
     assert out["layout"] == {
         "nonreentrant.foo": {"type": "nonreentrant lock", "location": "storage", "slot": 0},
-        "nonreentrant.bar": {"type": "nonreentrant lock", "location": "storage", "slot": 2},
+        "nonreentrant.bar": {"type": "nonreentrant lock", "location": "storage", "slot": 1},
         "foo": {
             "type": "HashMap[address, uint256][address, uint256]",
             "location": "storage",
-            "slot": 3,
+            "slot": 2,
         },
-        "baz": {"type": "Bytes[65]", "location": "storage", "slot": 4},
-        "bar": {"type": "uint256", "location": "storage", "slot": 8},
+        "baz": {"type": "Bytes[65]", "location": "storage", "slot": 3},
+        "bar": {"type": "uint256", "location": "storage", "slot": 7},
     }

vyper/semantics/validation/data_positions.py

@@ -32,22 +32,30 @@ def set_storage_slots(vyper_module: vy_ast.Module) -> StorageLayout:
 
     for node in vyper_module.get_children(vy_ast.FunctionDef):
         type_ = node._metadata["type"]
-        if type_.nonreentrant is not None:
-            type_.set_reentrancy_key_position(StorageSlot(storage_slot))
-
-            # TODO this could have better typing but leave it untyped until
-            # we nail down the format better
-            variable_name = f"nonreentrant.{type_.nonreentrant}"
-            ret[variable_name] = {
-                "type": "nonreentrant lock",
-                "location": "storage",
-                "slot": storage_slot,
-            }
-
-            # TODO use one byte - or bit - per reentrancy key
-            # requires either an extra SLOAD or caching the value of the
-            # location in memory at entrance
-            storage_slot += 1
+        if type_.nonreentrant is None:
+            continue
+
+        variable_name = f"nonreentrant.{type_.nonreentrant}"
+
+        # a nonreentrant key can appear many times in a module but it
+        # only takes one slot. ignore it after the first time we see it.
+        if variable_name in ret:
+            continue
+
+        type_.set_reentrancy_key_position(StorageSlot(storage_slot))
+
+        # TODO this could have better typing but leave it untyped until
+        # we nail down the format better
+        ret[variable_name] = {
+            "type": "nonreentrant lock",
+            "location": "storage",
+            "slot": storage_slot,
+        }
+
+        # TODO use one byte - or bit - per reentrancy key
+        # requires either an extra SLOAD or caching the value of the
+        # location in memory at entrance
+        storage_slot += 1
 
     for node in vyper_module.get_children(vy_ast.AnnAssign):
         type_ = node.target._metadata["type"]