mypy_synapse_plugin.py

# Copyright 2020 The Matrix.org Foundation C.I.C.
#
# 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.

"""This is a mypy plugin for Synpase to deal with some of the funky typing that
can crop up, e.g the cache descriptors.
"""

from typing import Callable, Optional, Tuple, Type, Union

import mypy.types
from mypy.erasetype import remove_instance_last_known_values
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 (
    AnyType,
    CallableType,
    Instance,
    NoneType,
    TupleType,
    TypeAliasType,
    UninhabitedType,
    UnionType,
)


class SynapsePlugin(Plugin):
    def get_method_signature_hook(
        self, fullname: str
    ) -> Optional[Callable[[MethodSigContext], CallableType]]:
        if fullname.startswith(
            (
                "synapse.util.caches.descriptors.CachedFunction.__call__",
                "synapse.util.caches.descriptors._LruCachedFunction.__call__",
            )
        ):
            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 returned 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.

    It already has *almost* the correct signature, except:

        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.
    """

    # 1. Mark this as a bound function signature.
    signature: CallableType = bind_self(ctx.default_signature)

    # 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
    # it's not the end of the world.
    context_arg_index = None
    for idx, name in enumerate(signature.arg_names):
        if name == "cache_context":
            context_arg_index = idx
            break

    arg_types = list(signature.arg_types)
    arg_names = list(signature.arg_names)
    arg_kinds = list(signature.arg_kinds)

    if context_arg_index:
        arg_types.pop(context_arg_index)
        arg_names.pop(context_arg_index)
        arg_kinds.pop(context_arg_index)

    # 3. Add an optional "on_invalidate" argument.
    #
    # This is a either
    # - a callable which accepts no input and returns nothing, or
    # - None.
    calltyp = UnionType(
        [
            NoneType(),
            CallableType(
                arg_types=[],
                arg_kinds=[],
                arg_names=[],
                ret_type=NoneType(),
                fallback=ctx.api.named_generic_type("builtins.function", []),
            ),
        ]
    )

    arg_types.append(calltyp)
    arg_names.append("on_invalidate")
    arg_kinds.append(ARG_NAMED_OPT)  # Arg is an optional kwarg.

    # 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,
        arg_names=arg_names,
        arg_kinds=arg_kinds,
        ret_type=ret_type,
    )

    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
    # string.
    #
    # However, since we pin the version of mypy Synapse uses in CI, we don't
    # really care.
    return SynapsePlugin