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// Copyright 2019 The MediaPipe Authors.
//
// 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.
//
// Helper macros and methods to return and propagate errors with
// `absl::Status`.
//
// The owners of mediapipe do not endorse use of these macros as a good
// programming practice, and would prefer that you write the equivalent C++
// directly. The macros are provided and supported for those that disagree,
// with the goal of having a single, consistent, and robust implementation.
#ifndef MEDIAPIPE_DEPS_STATUS_MACROS_H_
#define MEDIAPIPE_DEPS_STATUS_MACROS_H_
#include "mediapipe/framework/deps/status.h"
#include "mediapipe/framework/deps/status_builder.h"
// Evaluates an expression that produces a `absl::Status`. If the status
// is not ok, returns it from the current function.
//
// For example:
// absl::Status MultiStepFunction() {
// MP_RETURN_IF_ERROR(Function(args...));
// MP_RETURN_IF_ERROR(foo.Method(args...));
// return absl::OkStatus();
// }
//
// The macro ends with a `mediapipe::StatusBuilder` which allows the returned
// status to be extended with more details. Any chained expressions after the
// macro will not be evaluated unless there is an error.
//
// For example:
// absl::Status MultiStepFunction() {
// MP_RETURN_IF_ERROR(Function(args...)) << "in MultiStepFunction";
// MP_RETURN_IF_ERROR(foo.Method(args...)).Log(base_logging::ERROR)
// << "while processing query: " << query.DebugString();
// return absl::OkStatus();
// }
//
// `mediapipe::StatusBuilder` supports adapting the builder chain using a
// `With` method and a functor. This allows for powerful extensions to the
// macro.
//
// For example, teams can define local policies to use across their code:
//
// StatusBuilder TeamPolicy(StatusBuilder builder) {
// return std::move(builder.Log(base_logging::WARNING).Attach(...));
// }
//
// MP_RETURN_IF_ERROR(foo()).With(TeamPolicy);
// MP_RETURN_IF_ERROR(bar()).With(TeamPolicy);
//
// Changing the return type allows the macro to be used with Task and Rpc
// interfaces. See `mediapipe::TaskReturn` and `rpc::RpcSetStatus` for
// details.
//
// void Read(StringPiece name, mediapipe::Task* task) {
// int64 id;
// MP_RETURN_IF_ERROR(GetIdForName(name, &id)).With(TaskReturn(task));
// MP_RETURN_IF_ERROR(ReadForId(id)).With(TaskReturn(task));
// task->Return();
// }
//
// If using this macro inside a lambda, you need to annotate the return type
// to avoid confusion between a `mediapipe::StatusBuilder` and a
// `absl::Status` type. E.g.
//
// []() -> absl::Status {
// MP_RETURN_IF_ERROR(Function(args...));
// MP_RETURN_IF_ERROR(foo.Method(args...));
// return absl::OkStatus();
// }
#define MP_RETURN_IF_ERROR(expr) \
MP_STATUS_MACROS_IMPL_ELSE_BLOCKER_ \
if (mediapipe::status_macro_internal::StatusAdaptorForMacros \
status_macro_internal_adaptor = {(expr), MEDIAPIPE_LOC}) { \
} else /* NOLINT */ \
return status_macro_internal_adaptor.Consume()
#define MP_RAISE_IF_ERROR(expr, msg) \
MP_STATUS_MACROS_IMPL_ELSE_BLOCKER_\
if (mediapipe::status_macro_internal::StatusAdaptorForMacros \
status_macro_internal_adaptor = {(expr), MEDIAPIPE_LOC}) { \
} else /* NOLINT */ \
throw std::runtime_error(msg)
// Executes an expression `rexpr` that returns a `absl::StatusOr<T>`. On
// OK, extracts its value into the variable defined by `lhs`, otherwise returns
// from the current function. By default the error status is returned
// unchanged, but it may be modified by an `error_expression`. If there is an
// error, `lhs` is not evaluated; thus any side effects that `lhs` may have
// only occur in the success case.
//
// Interface:
//
// MP_ASSIGN_OR_RETURN(lhs, rexpr)
// MP_ASSIGN_OR_RETURN(lhs, rexpr, error_expression);
//
// WARNING: expands into multiple statements; it cannot be used in a single
// statement (e.g. as the body of an if statement without {})!
//
// Example: Declaring and initializing a new variable (ValueType can be anything
// that can be initialized with assignment, including references):
// MP_ASSIGN_OR_RETURN(ValueType value, MaybeGetValue(arg));
//
// Example: Assigning to an existing variable:
// ValueType value;
// MP_ASSIGN_OR_RETURN(value, MaybeGetValue(arg));
//
// Example: Assigning to an expression with side effects:
// MyProto data;
// MP_ASSIGN_OR_RETURN(*data.mutable_str(), MaybeGetValue(arg));
// // No field "str" is added on error.
//
// Example: Assigning to a std::unique_ptr.
// MP_ASSIGN_OR_RETURN(std::unique_ptr<T> ptr, MaybeGetPtr(arg));
//
// If passed, the `error_expression` is evaluated to produce the return
// value. The expression may reference any variable visible in scope, as
// well as a `mediapipe::StatusBuilder` object populated with the error and
// named by a single underscore `_`. The expression typically uses the
// builder to modify the status and is returned directly in manner similar
// to MP_RETURN_IF_ERROR. The expression may, however, evaluate to any type
// returnable by the function, including (void). For example:
//
// Example: Adjusting the error message.
// MP_ASSIGN_OR_RETURN(ValueType value, MaybeGetValue(query),
// _ << "while processing query " << query.DebugString());
//
// Example: Logging the error on failure.
// MP_ASSIGN_OR_RETURN(ValueType value, MaybeGetValue(query), _.LogError());
//
#define MP_ASSIGN_OR_RETURN(...) \
MP_STATUS_MACROS_IMPL_GET_VARIADIC_( \
(__VA_ARGS__, MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_3_, \
MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_2_)) \
(__VA_ARGS__)
#define MP_ASSIGN_OR_RAISE(lhs, var, rexpr, error_message) \
MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RAISE_3_(lhs, var, rexpr, error_message)
// =================================================================
// == Implementation details, do not rely on anything below here. ==
// =================================================================
// MSVC incorrectly expands variadic macros, splice together a macro call to
// work around the bug.
#define MP_STATUS_MACROS_IMPL_GET_VARIADIC_HELPER_(_1, _2, _3, NAME, ...) NAME
#define MP_STATUS_MACROS_IMPL_GET_VARIADIC_(args) \
MP_STATUS_MACROS_IMPL_GET_VARIADIC_HELPER_ args
#define MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_2_(lhs, rexpr) \
MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_( \
MP_STATUS_MACROS_IMPL_CONCAT_(_status_or_value, __LINE__), lhs, rexpr, \
return mediapipe::StatusBuilder( \
std::move(MP_STATUS_MACROS_IMPL_CONCAT_(_status_or_value, __LINE__)) \
.status(), \
MEDIAPIPE_LOC))
#define MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_3_(lhs, rexpr, \
error_expression) \
MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_( \
MP_STATUS_MACROS_IMPL_CONCAT_(_raise_or_value, __LINE__), lhs, rexpr, \
mediapipe::StatusBuilder _( \
std::move(MP_STATUS_MACROS_IMPL_CONCAT_(_raise_or_value, __LINE__)) \
.status(), \
MEDIAPIPE_LOC); \
(void)_; /* error_expression is allowed to not use this variable */ \
return (error_expression))
#define MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RETURN_(statusor, lhs, rexpr, \
error_expression) \
auto statusor = (rexpr); \
if (ABSL_PREDICT_FALSE(!statusor.ok())) { \
error_expression; \
} \
lhs = std::move(statusor).value()
#define MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RAISE_3_(lhs, var, rexpr, error_message) \
MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RAISE_( \
MP_STATUS_MACROS_IMPL_CONCAT_(_raise_or_value, __LINE__), lhs, var, rexpr, \
error_message)
#define MP_STATUS_MACROS_IMPL_MP_ASSIGN_OR_RAISE_(raiseor, lhs, var, rexpr, \
error_message) \
auto raiseor = (rexpr); \
if (ABSL_PREDICT_FALSE(!raiseor.ok())) { \
throw std::runtime_error(error_message); \
} \
var = new lhs(std::move(raiseor).value())
// Internal helper for concatenating macro values.
#define MP_STATUS_MACROS_IMPL_CONCAT_INNER_(x, y) x##y
#define MP_STATUS_MACROS_IMPL_CONCAT_(x, y) \
MP_STATUS_MACROS_IMPL_CONCAT_INNER_(x, y)
// The GNU compiler emits a warning for code like:
//
// if (foo)
// if (bar) { } else baz;
//
// because it thinks you might want the else to bind to the first if. This
// leads to problems with code like:
//
// if (do_expr) MP_RETURN_IF_ERROR(expr) << "Some message";
//
// The "switch (0) case 0:" idiom is used to suppress this.
#define MP_STATUS_MACROS_IMPL_ELSE_BLOCKER_ \
switch (0) \
case 0: \
default: // NOLINT
namespace mediapipe {
namespace status_macro_internal {
// Provides a conversion to bool so that it can be used inside an if statement
// that declares a variable.
class StatusAdaptorForMacros {
public:
StatusAdaptorForMacros(const absl::Status& status, source_location location)
: builder_(status, location) {}
StatusAdaptorForMacros(absl::Status&& status, source_location location)
: builder_(std::move(status), location) {}
StatusAdaptorForMacros(const StatusBuilder& builder,
source_location /*location*/)
: builder_(builder) {}
StatusAdaptorForMacros(StatusBuilder&& builder, source_location /*location*/)
: builder_(std::move(builder)) {}
StatusAdaptorForMacros(const StatusAdaptorForMacros&) = delete;
StatusAdaptorForMacros& operator=(const StatusAdaptorForMacros&) = delete;
explicit operator bool() const { return ABSL_PREDICT_TRUE(builder_.ok()); }
StatusBuilder&& Consume() { return std::move(builder_); }
private:
StatusBuilder builder_;
};
} // namespace status_macro_internal
} // namespace mediapipe
#endif // MEDIAPIPE_DEPS_STATUS_MACROS_H_