mypy plugin to check `@cached` return types (#14911)

Co-authored-by: David Robertson <davidr@element.io>
Co-authored-by: Patrick Cloke <patrickc@matrix.org>
Co-authored-by: Erik Johnston <erik@matrix.org>

Assert that the return type of callables wrapped in @cached
and @cachedList are cachable (aka immutable). 

1 files changed, 251 insertions, 38 deletions

diff --git a/scripts-dev/mypy_synapse_plugin.py b/scripts-dev/mypy_synapse_plugin.py
index a0b3854f1b..6592a4a6b7 100644
--- a/scripts-dev/mypy_synapse_plugin.py
+++ b/scripts-dev/mypy_synapse_plugin.py
@@ -16,13 +16,24 @@
 can crop up, e.g the cache descriptors.
 """
 
-from typing import Callable, Optional, Type
+from typing import Callable, Optional, Tuple, Type, Union
 
+import mypy.types
 from mypy.erasetype import remove_instance_last_known_values
-from mypy.nodes import ARG_NAMED_OPT
-from mypy.plugin import MethodSigContext, Plugin
+from mypy.errorcodes import ErrorCode
+from mypy.nodes import ARG_NAMED_OPT, TempNode, Var
+from mypy.plugin import FunctionSigContext, MethodSigContext, Plugin
 from mypy.typeops import bind_self
-from mypy.types import CallableType, Instance, NoneType, UnionType
+from mypy.types import (
+    AnyType,
+    CallableType,
+    Instance,
+    NoneType,
+    TupleType,
+    TypeAliasType,
+    UninhabitedType,
+    UnionType,
+)
 
 
 class SynapsePlugin(Plugin):
@@ -36,9 +47,37 @@ class SynapsePlugin(Plugin):
             )
         ):
             return cached_function_method_signature
+
+        if fullname in (
+            "synapse.util.caches.descriptors._CachedFunctionDescriptor.__call__",
+            "synapse.util.caches.descriptors._CachedListFunctionDescriptor.__call__",
+        ):
+            return check_is_cacheable_wrapper
+
         return None
 
 
+def _get_true_return_type(signature: CallableType) -> mypy.types.Type:
+    """
+    Get the "final" return type of a callable which might return an Awaitable/Deferred.
+    """
+    if isinstance(signature.ret_type, Instance):
+        # If a coroutine, unwrap the coroutine's return type.
+        if signature.ret_type.type.fullname == "typing.Coroutine":
+            return signature.ret_type.args[2]
+
+        # If an awaitable, unwrap the awaitable's final value.
+        elif signature.ret_type.type.fullname == "typing.Awaitable":
+            return signature.ret_type.args[0]
+
+        # If a Deferred, unwrap the Deferred's final value.
+        elif signature.ret_type.type.fullname == "twisted.internet.defer.Deferred":
+            return signature.ret_type.args[0]
+
+    # Otherwise, return the raw value of the function.
+    return signature.ret_type
+
+
 def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
     """Fixes the `CachedFunction.__call__` signature to be correct.
 
@@ -47,16 +86,17 @@ def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
         1. the `self` argument needs to be marked as "bound";
         2. any `cache_context` argument should be removed;
         3. an optional keyword argument `on_invalidated` should be added.
+        4. Wrap the return type to always be a Deferred.
     """
 
-    # First we mark this as a bound function signature.
-    signature = bind_self(ctx.default_signature)
+    # 1. Mark this as a bound function signature.
+    signature: CallableType = bind_self(ctx.default_signature)
 
-    # Secondly, we remove any "cache_context" args.
+    # 2. Remove any "cache_context" args.
     #
     # Note: We should be only doing this if `cache_context=True` is set, but if
     # it isn't then the code will raise an exception when its called anyway, so
-    # its not the end of the world.
+    # it's not the end of the world.
     context_arg_index = None
     for idx, name in enumerate(signature.arg_names):
         if name == "cache_context":
@@ -72,7 +112,7 @@ def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
         arg_names.pop(context_arg_index)
         arg_kinds.pop(context_arg_index)
 
-    # Third, we add an optional "on_invalidate" argument.
+    # 3. Add an optional "on_invalidate" argument.
     #
     # This is a either
     # - a callable which accepts no input and returns nothing, or
@@ -94,35 +134,16 @@ def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
     arg_names.append("on_invalidate")
     arg_kinds.append(ARG_NAMED_OPT)  # Arg is an optional kwarg.
 
-    # Finally we ensure the return type is a Deferred.
-    if (
-        isinstance(signature.ret_type, Instance)
-        and signature.ret_type.type.fullname == "twisted.internet.defer.Deferred"
-    ):
-        # If it is already a Deferred, nothing to do.
-        ret_type = signature.ret_type
-    else:
-        ret_arg = None
-        if isinstance(signature.ret_type, Instance):
-            # If a coroutine, wrap the coroutine's return type in a Deferred.
-            if signature.ret_type.type.fullname == "typing.Coroutine":
-                ret_arg = signature.ret_type.args[2]
-
-            # If an awaitable, wrap the awaitable's final value in a Deferred.
-            elif signature.ret_type.type.fullname == "typing.Awaitable":
-                ret_arg = signature.ret_type.args[0]
-
-        # Otherwise, wrap the return value in a Deferred.
-        if ret_arg is None:
-            ret_arg = signature.ret_type
-
-        # This should be able to use ctx.api.named_generic_type, but that doesn't seem
-        # to find the correct symbol for anything more than 1 module deep.
-        #
-        # modules is not part of CheckerPluginInterface. The following is a combination
-        # of TypeChecker.named_generic_type and TypeChecker.lookup_typeinfo.
-        sym = ctx.api.modules["twisted.internet.defer"].names.get("Deferred")  # type: ignore[attr-defined]
-        ret_type = Instance(sym.node, [remove_instance_last_known_values(ret_arg)])
+    # 4. Ensure the return type is a Deferred.
+    ret_arg = _get_true_return_type(signature)
+
+    # This should be able to use ctx.api.named_generic_type, but that doesn't seem
+    # to find the correct symbol for anything more than 1 module deep.
+    #
+    # modules is not part of CheckerPluginInterface. The following is a combination
+    # of TypeChecker.named_generic_type and TypeChecker.lookup_typeinfo.
+    sym = ctx.api.modules["twisted.internet.defer"].names.get("Deferred")  # type: ignore[attr-defined]
+    ret_type = Instance(sym.node, [remove_instance_last_known_values(ret_arg)])
 
     signature = signature.copy_modified(
         arg_types=arg_types,
@@ -134,6 +155,198 @@ def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
     return signature
 
 
+def check_is_cacheable_wrapper(ctx: MethodSigContext) -> CallableType:
+    """Asserts that the signature of a method returns a value which can be cached.
+
+    Makes no changes to the provided method signature.
+    """
+    # The true signature, this isn't being modified so this is what will be returned.
+    signature: CallableType = ctx.default_signature
+
+    if not isinstance(ctx.args[0][0], TempNode):
+        ctx.api.note("Cached function is not a TempNode?!", ctx.context)  # type: ignore[attr-defined]
+        return signature
+
+    orig_sig = ctx.args[0][0].type
+    if not isinstance(orig_sig, CallableType):
+        ctx.api.fail("Cached 'function' is not a callable", ctx.context)
+        return signature
+
+    check_is_cacheable(orig_sig, ctx)
+
+    return signature
+
+
+def check_is_cacheable(
+    signature: CallableType,
+    ctx: Union[MethodSigContext, FunctionSigContext],
+) -> None:
+    """
+    Check if a callable returns a type which can be cached.
+
+    Args:
+        signature: The callable to check.
+        ctx: The signature context, used for error reporting.
+    """
+    # Unwrap the true return type from the cached function.
+    return_type = _get_true_return_type(signature)
+
+    verbose = ctx.api.options.verbosity >= 1
+    # TODO Technically a cachedList only needs immutable values, but forcing them
+    # to return Mapping instead of Dict is fine.
+    ok, note = is_cacheable(return_type, signature, verbose)
+
+    if ok:
+        message = f"function {signature.name} is @cached, returning {return_type}"
+    else:
+        message = f"function {signature.name} is @cached, but has mutable return value {return_type}"
+
+    if note:
+        message += f" ({note})"
+    message = message.replace("builtins.", "").replace("typing.", "")
+
+    if ok and note:
+        ctx.api.note(message, ctx.context)  # type: ignore[attr-defined]
+    elif not ok:
+        ctx.api.fail(message, ctx.context, code=AT_CACHED_MUTABLE_RETURN)
+
+
+# Immutable simple values.
+IMMUTABLE_VALUE_TYPES = {
+    "builtins.bool",
+    "builtins.int",
+    "builtins.float",
+    "builtins.str",
+    "builtins.bytes",
+}
+
+# Types defined in Synapse which are known to be immutable.
+IMMUTABLE_CUSTOM_TYPES = {
+    "synapse.synapse_rust.acl.ServerAclEvaluator",
+    "synapse.synapse_rust.push.FilteredPushRules",
+    # This is technically not immutable, but close enough.
+    "signedjson.types.VerifyKey",
+}
+
+# Immutable containers only if the values are also immutable.
+IMMUTABLE_CONTAINER_TYPES_REQUIRING_IMMUTABLE_ELEMENTS = {
+    "builtins.frozenset",
+    "builtins.tuple",
+    "typing.AbstractSet",
+    "typing.Sequence",
+    "immutabledict.immutabledict",
+}
+
+MUTABLE_CONTAINER_TYPES = {
+    "builtins.set",
+    "builtins.list",
+    "builtins.dict",
+}
+
+AT_CACHED_MUTABLE_RETURN = ErrorCode(
+    "synapse-@cached-mutable",
+    "@cached() should have an immutable return type",
+    "General",
+)
+
+
+def is_cacheable(
+    rt: mypy.types.Type, signature: CallableType, verbose: bool
+) -> Tuple[bool, Optional[str]]:
+    """
+    Check if a particular type is cachable.
+
+    A type is cachable if it is immutable; for complex types this recurses to
+    check each type parameter.
+
+    Returns: a 2-tuple (cacheable, message).
+        - cachable: False means the type is definitely not cacheable;
+            true means anything else.
+        - Optional message.
+    """
+
+    # This should probably be done via a TypeVisitor. Apologies to the reader!
+    if isinstance(rt, AnyType):
+        return True, ("may be mutable" if verbose else None)
+
+    elif isinstance(rt, Instance):
+        if (
+            rt.type.fullname in IMMUTABLE_VALUE_TYPES
+            or rt.type.fullname in IMMUTABLE_CUSTOM_TYPES
+        ):
+            # "Simple" types are generally immutable.
+            return True, None
+
+        elif rt.type.fullname == "typing.Mapping":
+            # Generally mapping keys are immutable, but they only *have* to be
+            # hashable, which doesn't imply immutability. E.g. Mapping[K, V]
+            # is cachable iff K and V are cachable.
+            return is_cacheable(rt.args[0], signature, verbose) and is_cacheable(
+                rt.args[1], signature, verbose
+            )
+
+        elif rt.type.fullname in IMMUTABLE_CONTAINER_TYPES_REQUIRING_IMMUTABLE_ELEMENTS:
+            # E.g. Collection[T] is cachable iff T is cachable.
+            return is_cacheable(rt.args[0], signature, verbose)
+
+        elif rt.type.fullname in MUTABLE_CONTAINER_TYPES:
+            # Mutable containers are mutable regardless of their underlying type.
+            return False, None
+
+        elif "attrs" in rt.type.metadata:
+            # attrs classes are only cachable iff it is frozen (immutable itself)
+            # and all attributes are cachable.
+            frozen = rt.type.metadata["attrs"]["frozen"]
+            if frozen:
+                for attribute in rt.type.metadata["attrs"]["attributes"]:
+                    attribute_name = attribute["name"]
+                    symbol_node = rt.type.names[attribute_name].node
+                    assert isinstance(symbol_node, Var)
+                    assert symbol_node.type is not None
+                    ok, note = is_cacheable(symbol_node.type, signature, verbose)
+                    if not ok:
+                        return False, f"non-frozen attrs property: {attribute_name}"
+                # All attributes were frozen.
+                return True, None
+            else:
+                return False, "non-frozen attrs class"
+
+        else:
+            # Ensure we fail for unknown types, these generally means that the
+            # above code is not complete.
+            return (
+                False,
+                f"Don't know how to handle {rt.type.fullname} return type instance",
+            )
+
+    elif isinstance(rt, NoneType):
+        # None is cachable.
+        return True, None
+
+    elif isinstance(rt, (TupleType, UnionType)):
+        # Tuples and unions are cachable iff all their items are cachable.
+        for item in rt.items:
+            ok, note = is_cacheable(item, signature, verbose)
+            if not ok:
+                return False, note
+        # This discards notes but that's probably fine
+        return True, None
+
+    elif isinstance(rt, TypeAliasType):
+        # For a type alias, check if the underlying real type is cachable.
+        return is_cacheable(mypy.types.get_proper_type(rt), signature, verbose)
+
+    elif isinstance(rt, UninhabitedType) and rt.is_noreturn:
+        # There is no return value, just consider it cachable. This is only used
+        # in tests.
+        return True, None
+
+    else:
+        # Ensure we fail for unknown types, these generally means that the
+        # above code is not complete.
+        return False, f"Don't know how to handle {type(rt).__qualname__} return type"
+
+
 def plugin(version: str) -> Type[SynapsePlugin]:
     # This is the entry point of the plugin, and lets us deal with the fact
     # that the mypy plugin interface is *not* stable by looking at the version