chore: add notes to selector table implementation (#3618)

and a couple sanity checks

vyper/codegen/function_definitions/common.py

@@ -73,6 +73,13 @@ class EntryPointInfo:
     min_calldatasize: int  # the min calldata required for this entry point
     ir_node: IRnode  # the ir for this entry point
 
+    def __post_init__(self):
+        # ABI v2 property guaranteed by the spec.
+        # https://docs.soliditylang.org/en/v0.8.21/abi-spec.html#formal-specification-of-the-encoding states:  # noqa: E501
+        # > Note that for any X, len(enc(X)) is a multiple of 32.
+        assert self.min_calldatasize >= 4
+        assert (self.min_calldatasize - 4) % 32 == 0
+
 
 @dataclass
 class ExternalFuncIR(FuncIR):

vyper/codegen/module.py

@@ -93,9 +93,12 @@ def _generate_external_entry_points(external_functions, global_ctx):
     for code in external_functions:
         func_ir = generate_ir_for_function(code, global_ctx)
         for abi_sig, entry_point in func_ir.entry_points.items():
+            method_id = method_id_int(abi_sig)
             assert abi_sig not in entry_points
+            assert method_id not in sig_of
+
             entry_points[abi_sig] = entry_point
-            sig_of[method_id_int(abi_sig)] = abi_sig
+            sig_of[method_id] = abi_sig
 
         # stick function common body into final entry point to save a jump
         ir_node = IRnode.from_list(["seq", entry_point.ir_node, func_ir.common_ir])