// Copyright 2016 Michal Witkowski. All Rights Reserved.
// See LICENSE for licensing terms.

package grpc_auth

import (
	"github.com/grpc-ecosystem/go-grpc-middleware"
	"golang.org/x/net/context"
	"google.golang.org/grpc"
)

// AuthFunc is the pluggable function that performs authentication.
//
// The passed in `Context` will contain the gRPC metadata.MD object (for header-based authentication) and
// the peer.Peer information that can contain transport-based credentials (e.g. `credentials.AuthInfo`).
//
// The returned context will be propagated to handlers, allowing user changes to `Context`. However,
// please make sure that the `Context` returned is a child `Context` of the one passed in.
//
// If error is returned, its `grpc.Code()` will be returned to the user as well as the verbatim message.
// Please make sure you use `codes.Unauthenticated` (lacking auth) and `codes.PermissionDenied`
// (authed, but lacking perms) appropriately.
type AuthFunc func(ctx context.Context) (context.Context, error)

// ServiceAuthFuncOverride allows a given gRPC service implementation to override the global `AuthFunc`.
//
// If a service implements the AuthFuncOverride method, it takes precedence over the `AuthFunc` method,
// and will be called instead of AuthFunc for all method invocations within that service.
type ServiceAuthFuncOverride interface {
	AuthFuncOverride(ctx context.Context, fullMethodName string) (context.Context, error)
}

// UnaryServerInterceptor returns a new unary server interceptors that performs per-request auth.
func UnaryServerInterceptor(authFunc AuthFunc) grpc.UnaryServerInterceptor {
	return func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
		var newCtx context.Context
		var err error
		if overrideSrv, ok := info.Server.(ServiceAuthFuncOverride); ok {
			newCtx, err = overrideSrv.AuthFuncOverride(ctx, info.FullMethod)
		} else {
			newCtx, err = authFunc(ctx)
		}
		if err != nil {
			return nil, err
		}
		return handler(newCtx, req)
	}
}

// StreamServerInterceptor returns a new unary server interceptors that performs per-request auth.
func StreamServerInterceptor(authFunc AuthFunc) grpc.StreamServerInterceptor {
	return func(srv interface{}, stream grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
		var newCtx context.Context
		var err error
		if overrideSrv, ok := srv.(ServiceAuthFuncOverride); ok {
			newCtx, err = overrideSrv.AuthFuncOverride(stream.Context(), info.FullMethod)
		} else {
			newCtx, err = authFunc(stream.Context())
		}
		if err != nil {
			return err
		}
		wrapped := grpc_middleware.WrapServerStream(stream)
		wrapped.WrappedContext = newCtx
		return handler(srv, wrapped)
	}
}
