exceptions.hpp

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#pragma once
 
/// @file userver/server/handlers/exceptions.hpp
/// @brief Helpers for throwing exceptions
 
#include <stdexcept>
#include <string>
#include <unordered_map>
 
#include <userver/formats/json.hpp>
#include <userver/utils/assert.hpp>
#include <userver/utils/meta_light.hpp>
#include <userver/utils/str_icase.hpp>
#include <userver/utils/void_t.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace server::handlers {
 
/// Enumeration that defines protocol-agnostic handler error condition codes,
/// used by server::handlers::CustomHandlerException.
///
/// Specific error formats can derive various defaults from the this code, e.g.
/// HTTP status code, JSON service error code, and default error message texts.
///
/// For example, to provide an HTTP specific error code that is not presented
/// in this enum the HTTP code should be provided via
/// server::http::CustomHandlerException construction with the required
/// server::http::HttpStatus.
enum class HandlerErrorCode {
    kUnknownError,       //!< kUnknownError This value is to map possibly unknown codes
                         //!< to a description, shouldn't be used in client code
    kClientError,        //!< kInvalidRequest Invalid request data
    kRequestParseError,  //!< kRequestParseError
    kUnauthorized,       //!< kUnauthorized Client is not authorized to execute this
                         //!< handler
    kForbidden,          //!< kForbidden Requested action is forbidden
    kResourceNotFound,   //!< kResourceNoFound Requested resource doesn't exist
    kInvalidUsage,       //!< kInvalidUsage Invalid usage of the handler, e.g.
                         //!< unsupported HTTP method
    kNotAcceptable,      //!< kNotAcceptable The server cannot produce response,
                         //!< acceptable by the client
    kConflictState,      //!< kConflictState Request cannot be completed due to
                         //!< conflict resource state
    kPayloadTooLarge,    //!< kPayloadTooLarge The payload for the request exceeded
                         //!< handler's settings
    kTooManyRequests,    //!< kTooManyRequests Request limit exceeded
 
    // Client error codes are declared before the server side error to be
    // mapped correctly to a protocol-specific error code!
 
    kServerSideError,  //!< kServerSideError An error occurred while processing
                       //!< the request
    kBadGateway,       //!< kBadGateway An error occurred while passing the request to
                       //!< another service
 
    kGatewayTimeout,        //!< kGatewayTimeout A timeout occurred while passing the
                            //!< request to another service
    kUnsupportedMediaType,  //!< kUnsupportedMediaType Content-Encoding or
                            //!< Content-Type is not supported
 
    // Server error codes are declared after the client side error to be
    // mapped correctly to a protocol-specific error code!
};
// When adding enumerators here ^, please also add mappings to the
// implementation of:
// - server::handler::GetCodeDescription,
// - server::handler::GetFallbackServiceCode,
// - server::http::GetHttpStatus.
 
/// Hasher class for HandlerErrorCode
struct HandlerErrorCodeHash {
    std::size_t operator()(HandlerErrorCode c) const { return static_cast<std::size_t>(c); }
};
 
using Headers = std::unordered_map<std::string, std::string, utils::StrIcaseHash, utils::StrIcaseEqual>;
 
std::string_view GetCodeDescription(HandlerErrorCode);
 
std::string_view GetFallbackServiceCode(HandlerErrorCode);
 
struct ServiceErrorCode {
    std::string body;
};
 
struct InternalMessage {
    std::string body;
};
 
struct ExternalBody {
    std::string body;
};
 
struct ExtraHeaders {
    Headers headers;
};
 
namespace impl {
 
template <typename T>
using IsExternalBodyFormatted = std::bool_constant<T::kIsExternalBodyFormatted>;
 
template <typename T>
using HasServiceCode = decltype(std::declval<const T&>().GetServiceCode());
 
template <typename T>
using HasInternalMessage = decltype(std::declval<const T&>().GetInternalMessage());
 
template <typename T>
inline constexpr bool kHasInternalMessage = meta::kIsDetected<HasInternalMessage, T>;
 
template <typename T>
using HasExternalBody = decltype(std::declval<const T&>().GetExternalBody());
 
template <typename T>
inline constexpr bool kHasExternalBody = meta::kIsDetected<HasExternalBody, T>;
 
template <typename T>
inline constexpr bool kIsMessageBuilder = kHasExternalBody<T>;
 
template <typename T>
struct MessageExtractor {
    static_assert(
        meta::kIsDetected<HasExternalBody, T>,
        "Please use your message builder to build external body for "
        "your error. See server::handlers::CustomHandlerException "
        "for more info"
    );
 
    const T& builder;
 
    constexpr bool IsExternalBodyFormatted() const {
        return meta::DetectedOr<std::false_type, impl::IsExternalBodyFormatted, T>::value;
    }
 
    std::string GetServiceCode() const {
        if constexpr (meta::kIsDetected<HasServiceCode, T>) {
            return builder.GetServiceCode();
        } else {
            return std::string{};
        }
    }
 
    std::string GetExternalBody() const { return builder.GetExternalBody(); }
 
    std::string GetInternalMessage() const {
        if constexpr (kHasInternalMessage<T>) {
            return builder.GetInternalMessage();
        } else {
            return std::string{};
        }
    }
};
 
struct CustomHandlerExceptionData final {
    CustomHandlerExceptionData() = default;
    CustomHandlerExceptionData(const CustomHandlerExceptionData&) = default;
    CustomHandlerExceptionData(CustomHandlerExceptionData&&) noexcept = default;
 
    template <typename... Args>
    explicit CustomHandlerExceptionData(Args&&... args) {
        (Apply(std::forward<Args>(args)), ...);
    }
 
    bool is_external_body_formatted{false};
    HandlerErrorCode handler_code{HandlerErrorCode::kUnknownError};
    std::string service_code;
    std::string internal_message;
    std::string external_body;
    Headers headers;
    formats::json::Value details;
 
private:
    void Apply(HandlerErrorCode handler_code_) { handler_code = handler_code_; }
 
    void Apply(ServiceErrorCode service_code_) { service_code = std::move(service_code_.body); }
 
    void Apply(InternalMessage internal_message_) { internal_message = std::move(internal_message_.body); }
 
    void Apply(ExternalBody external_body_) { external_body = std::move(external_body_.body); }
 
    void Apply(ExtraHeaders headers_) { headers = std::move(headers_.headers); }
 
    void Apply(formats::json::Value details_) { details = std::move(details_); }
 
    template <typename MessageBuilder>
    void Apply(MessageBuilder&& builder) {
        impl::MessageExtractor<MessageBuilder> extractor{builder};
        is_external_body_formatted = extractor.IsExternalBodyFormatted();
        service_code = extractor.GetServiceCode();
        external_body = extractor.GetExternalBody();
        internal_message = extractor.GetInternalMessage();
    }
};
 
}  // namespace impl
 
/// @brief The generic base class for handler exceptions. Thrown exceptions
/// should typically derive from ExceptionWithCode instead.
class CustomHandlerException : public std::runtime_error {
public:
    // Type aliases for usage in descendent classes that are in other namespaces
    using HandlerErrorCode = handlers::HandlerErrorCode;
    using ServiceErrorCode = handlers::ServiceErrorCode;
    using InternalMessage = handlers::InternalMessage;
    using ExternalBody = handlers::ExternalBody;
    using ExtraHeaders = handlers::ExtraHeaders;
 
    /// @brief Construct manually from a set of (mostly optional) arguments, which
    /// describe the error details.
    ///
    /// ## Allowed arguments
    ///
    /// - HandlerErrorCode - defaults for other fields are derived from it
    /// - ServiceErrorCode - used e.g. in JSON error format
    /// - http::HttpStatus
    /// - InternalMessage - for logs
    /// - ExternalBody
    /// - ExtraHeaders
    /// - formats::json::Value - details for JSON error format
    /// - a message builder, see below
    ///
    /// Example:
    /// @snippet server/handlers/exceptions_test.cpp  Sample direct construction
    ///
    /// ## Message builders
    ///
    /// A message builder is a class that satisfies the following requirements:
    /// - provides a `GetExternalBody() const` function to form an external body
    /// - has an optional `kIsExternalBodyFormatted` set to true
    ///   to forbid changing the external body
    /// - has an optional `GetServiceCode() const` function to return machine
    ///   readable error code
    /// - has an optional `GetInternalMessage() const` function to form an message
    ///   for logging an error
    ///
    /// Some message builder data can be overridden by explicitly passed args, if
    /// these args go *after* the message builder.
    ///
    /// Example:
    /// @snippet server/handlers/exceptions_test.cpp  Sample custom error builder
    template <typename... Args>
    CustomHandlerException(HandlerErrorCode handler_code, Args&&... args)
        : CustomHandlerException(impl::CustomHandlerExceptionData{handler_code, std::forward<Args>(args)...}) {}
 
    /// @overload
    explicit CustomHandlerException(HandlerErrorCode handler_code)
        : CustomHandlerException(impl::CustomHandlerExceptionData{handler_code}) {}
 
    /// @deprecated Use the variadic constructor above instead.
    CustomHandlerException(
        ServiceErrorCode service_code,
        ExternalBody external_body,
        InternalMessage internal_message,
        HandlerErrorCode handler_code,
        ExtraHeaders headers = {},
        formats::json::Value details = {}
    )
        : CustomHandlerException(impl::CustomHandlerExceptionData{
              std::move(service_code),
              std::move(external_body),
              std::move(internal_message),
              handler_code,
              std::move(headers),
              std::move(details)}) {}
 
    /// @deprecated Use the variadic constructor above instead.
    template <typename MessageBuilder, typename = std::enable_if_t<impl::kIsMessageBuilder<MessageBuilder>>>
    CustomHandlerException(MessageBuilder&& builder, HandlerErrorCode handler_code)
        : CustomHandlerException(impl::CustomHandlerExceptionData{std::forward<MessageBuilder>(builder), handler_code}
          ) {}
 
    /// @cond
    explicit CustomHandlerException(impl::CustomHandlerExceptionData&& data)
        : runtime_error(
              data.internal_message.empty() ? std::string{GetCodeDescription(data.handler_code)} : data.internal_message
          ),
          data_(std::move(data)) {
        UASSERT_MSG(
            data_.details.IsNull() || data_.details.IsObject(),
            "The details JSON value must be either null or an object"
        );
    }
    /// @endcond
 
    HandlerErrorCode GetCode() const { return data_.handler_code; }
 
    const std::string& GetServiceCode() const { return data_.service_code; }
 
    bool IsExternalErrorBodyFormatted() const { return data_.is_external_body_formatted; }
 
    const std::string& GetExternalErrorBody() const { return data_.external_body; }
 
    const formats::json::Value& GetDetails() const { return data_.details; }
 
    const Headers& GetExtraHeaders() const { return data_.headers; }
 
private:
    impl::CustomHandlerExceptionData data_;
};
 
/// @brief Base class for handler exceptions. For common HandlerErrorCode values
/// you can use more specific exception classes below. For less common
/// HandlerErrorCode values, this class can be used directly.
template <HandlerErrorCode Code>
class ExceptionWithCode : public CustomHandlerException {
public:
    constexpr static HandlerErrorCode kDefaultCode = Code;
    using BaseType = ExceptionWithCode<kDefaultCode>;
 
    ExceptionWithCode(const ExceptionWithCode<Code>&) = default;
    ExceptionWithCode(ExceptionWithCode<Code>&&) noexcept = default;
 
    /// @see CustomHandlerException::CustomHandlerException for allowed args
    template <typename... Args>
    explicit ExceptionWithCode(Args&&... args) : CustomHandlerException(kDefaultCode, std::forward<Args>(args)...) {}
};
 
/// Exception class for situations when request preconditions have failed.
/// Corresponds to HTTP code 400.
class ClientError : public ExceptionWithCode<HandlerErrorCode::kClientError> {
public:
    using BaseType::BaseType;
};
 
/// Exception class for situations when a request cannot be processed
/// due to parsing errors. Corresponds to HTTP code 400.
class RequestParseError : public ExceptionWithCode<HandlerErrorCode::kRequestParseError> {
public:
    using BaseType::BaseType;
};
 
/// Exception class for situations when a request does not have valid
/// authentication credentials. Corresponds to HTTP code 401.
class Unauthorized : public ExceptionWithCode<HandlerErrorCode::kUnauthorized> {
public:
    using BaseType::BaseType;
};
 
/// Exception class for situations when some requested entity could not be
/// accessed, because it does not exist. Corresponds to HTTP code 404.
class ResourceNotFound : public ExceptionWithCode<HandlerErrorCode::kResourceNotFound> {
public:
    using BaseType::BaseType;
};
 
/// Exception class for situations when a conflict happens, e.g. the client
/// attempts to create an entity that already exists. Corresponds to
/// HTTP code 409.
class ConflictError : public server::handlers::ExceptionWithCode<server::handlers::HandlerErrorCode::kConflictState> {
public:
    using BaseType::BaseType;
};
 
/// Exception class for situations when an exception occurred while processing
/// the request. Corresponds to HTTP code 500.
class InternalServerError : public ExceptionWithCode<HandlerErrorCode::kServerSideError> {
public:
    using BaseType::BaseType;
};
 
}  // namespace server::handlers
 
USERVER_NAMESPACE_END