server.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
	package birpc is a fork of the stdlib net/rpc which is frozen. It adds
	support for context.Context on the client and server, including
	propogating cancellation. See the README at
	https://github.com/cgrates/rpc for motivation why this exists.

	The API is exactly the same, except Client.Call takes a context.Context,
	and Server methods are expected to take a context.Context as the first
	argument. The following is the original rpc godoc updated to include
	context.Context. Additionally the wire protocol is unchanged, so is
	backwards compatible with net/rpc clients.

	package birpc provides access to the exported methods of an object across a
	network or other I/O connection.  A server registers an object, making it visible
	as a service with the name of the type of the object.  After registration, exported
	methods of the object will be accessible remotely.  A server may register multiple
	objects (services) of different types but it is an error to register multiple
	objects of the same type.

	Only methods that satisfy these criteria will be made available for remote access;
	other methods will be ignored:

		- the method's type is exported.
		- the method is exported.
		- the method has three arguments.
		- the method's first argument has type pointer to context.Context.
		- the method's last two arguments are exported (or builtin) types.
		- the method's third argument is a pointer.
		- the method has return type error.

	In effect, the method must look schematically like

		func (t *T) MethodName(ctx *context.Context, argType T1, replyType *T2) error

	where T1 and T2 can be marshaled by encoding/gob.
	These requirements apply even if a different codec is used.
	(In the future, these requirements may soften for custom codecs.)

	The method's second argument represents the arguments provided by the caller; the
	third argument represents the result parameters to be returned to the caller.
	The method's return value, if non-nil, is passed back as a string that the client
	sees as if created by errors.New.  If an error is returned, the reply parameter
	will not be sent back to the client.

	The server may handle requests on a single connection by calling ServeConn.  More
	typically it will create a network listener and call Accept or, for an HTTP
	listener, HandleHTTP and http.Serve.

	A client wishing to use the service establishes a connection and then invokes
	NewClient on the connection.  The convenience function Dial (DialHTTP) performs
	both steps for a raw network connection (an HTTP connection).  The resulting
	Client object has two methods, Call and Go, that specify the service and method to
	call, a pointer containing the arguments, and a pointer to receive the result
	parameters.

	The Call method waits for the remote call to complete while the Go method
	launches the call asynchronously and signals completion using the Call
	structure's Done channel.

	Unless an explicit codec is set up, package encoding/gob is used to
	transport the data.

	Here is a simple example.  A server wishes to export an object of type Arith:

		package server

		import "errors"

		type Args struct {
			A, B int
		}

		type Quotient struct {
			Quo, Rem int
		}

		type Arith int

		func (t *Arith) Multiply(ctx *context.Context, args *Args, reply *int) error {
			*reply = args.A * args.B
			return nil
		}

		func (t *Arith) Divide(ctx *context.Context, args *Args, quo *Quotient) error {
			if args.B == 0 {
				return errors.New("divide by zero")
			}
			quo.Quo = args.A / args.B
			quo.Rem = args.A % args.B
			return nil
		}

	The server calls (for HTTP service):

		arith := new(Arith)
		rpc.Register(arith)
		rpc.HandleHTTP()
		l, e := net.Listen("tcp", ":1234")
		if e != nil {
			log.Fatal("listen error:", e)
		}
		go http.Serve(l, nil)

	At this point, clients can see a service "Arith" with methods "Arith.Multiply" and
	"Arith.Divide".  To invoke one, a client first dials the server:

		client, err := rpc.DialHTTP("tcp", serverAddress + ":1234")
		if err != nil {
			log.Fatal("dialing:", err)
		}

	Then it can make a remote call:

		// Synchronous call
		args := &server.Args{7,8}
		var reply int
		err = client.Call(context.Background(), "Arith.Multiply", args, &reply)
		if err != nil {
			log.Fatal("arith error:", err)
		}
		fmt.Printf("Arith: %d*%d=%d", args.A, args.B, reply)

	or

		// Asynchronous call
		quotient := new(Quotient)
		divCall := client.Go("Arith.Divide", args, quotient, nil)
		replyCall := <-divCall.Done	// will be equal to divCall
		// check errors, print, etc.

	A server implementation will often provide a simple, type-safe wrapper for the
	client.

	The net/rpc package is frozen and is not accepting new features.
*/
package birpc

import (
	"errors"
	"io"
	"log"
	"net"
	"net/http"
	"reflect"
	"sync"

	"github.com/cgrates/birpc/context"
	"github.com/cgrates/birpc/internal/svc"
)

const (
	// Defaults used by HandleHTTP
	DefaultRPCPath = "/_goRPC_"
)

// Request is a header written before every RPC call. It is used internally
// but documented here as an aid to debugging, such as when analyzing
// network traffic.
type Request struct {
	ServiceMethod string   // format: "Service.Method"
	Seq           uint64   // sequence number chosen by client
	next          *Request // for free list in Server
}

// Response is a header written before every RPC return. It is used internally
// but documented here as an aid to debugging, such as when analyzing
// network traffic.
type Response struct {
	Seq   uint64    // echoes that of the request
	Error string    // error, if any.
	next  *Response // for free list in Server
}

// Server represents an RPC Server.
type Server struct {
	*basicServer
}

// NewServer returns a new Server.
func NewServer() *Server {
	return &Server{basicServer: newBasicServer()}
}

// DefaultServer is the default instance of *Server.
var DefaultServer = NewServer()

// ServeConn runs the server on a single connection.
// ServeConn blocks, serving the connection until the client hangs up.
// The caller typically invokes ServeConn in a go statement.
// ServeConn uses the gob wire format (see package gob) on the
// connection. To use an alternate codec, use ServeCodec.
// See NewClient's comment for information about concurrent access.
func (server *Server) ServeConn(conn io.ReadWriteCloser) {
	server.ServeCodec(NewServerCodec(conn))
}

// ServeCodec is like ServeConn but uses the specified codec to
// decode requests and encode responses.
func (server *Server) ServeCodec(codec ServerCodec) {
	sending := new(sync.Mutex)
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	pending := svc.NewPending(ctx)
	wg := new(sync.WaitGroup)
	for {
		service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)
		if err != nil {
			if err != io.EOF {
				debugln("rpc:", err)
			}
			if !keepReading {
				break
			}
			// send a response if we actually managed to read a header.
			if req != nil {
				server.sendResponse(sending, req, invalidRequest, codec, err.Error())
				server.freeRequest(req)
			}
			continue
		}
		wg.Add(1)
		go service.call(server.basicServer, sending, pending, wg, mtype, req, argv, replyv, codec)
	}
	// We've seen that there are no more requests.
	// Wait for responses to be sent before closing codec.
	wg.Wait()
	codec.Close()
}

// ServeRequest is like ServeCodec but synchronously serves a single request.
// It does not close the codec upon completion.
func (server *Server) ServeRequest(codec ServerCodec) error {
	return server.ServeRequestContext(context.Background(), codec)
}

// ServeRequest is like ServeCodec but synchronously serves a single request.
// It does not close the codec upon completion.
//
// Cancelling the context given here will propagate cancellation to the context
// of the called function.
func (server *Server) ServeRequestContext(ctx *context.Context, codec ServerCodec) error {
	sending := new(sync.Mutex)
	pending := svc.NewPending(ctx)
	service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)
	if err != nil {
		if !keepReading {
			return err
		}
		// send a response if we actually managed to read a header.
		if req != nil {
			server.sendResponse(sending, req, invalidRequest, codec, err.Error())
			server.freeRequest(req)
		}
		return err
	}
	service.call(server.basicServer, sending, pending, nil, mtype, req, argv, replyv, codec)
	return nil
}

func (server *Server) readRequest(codec ServerCodec) (service *Service, mtype *MethodType, req *Request, argv, replyv reflect.Value, keepReading bool, err error) {
	service, mtype, req, keepReading, err = server.readRequestHeader(codec)
	if err != nil {
		if !keepReading {
			return
		}
		// discard body
		codec.ReadRequestBody(nil)
		return
	}

	// Decode the argument value.
	var argIsValue bool // if true, need to indirect before calling.
	argv, argIsValue = getArgv(mtype)
	// argv guaranteed to be a pointer now.
	if err = codec.ReadRequestBody(argv.Interface()); err != nil {
		return
	}
	if argIsValue {
		argv = argv.Elem()
	}
	replyv = getReplyv(mtype)
	return
}

func (server *Server) readRequestHeader(codec ServerCodec) (svc *Service, mtype *MethodType, req *Request, keepReading bool, err error) {
	// Grab the request header.
	req = server.getRequest()
	err = codec.ReadRequestHeader(req)
	if err != nil {
		req = nil
		if err == io.EOF || err == io.ErrUnexpectedEOF {
			return
		}
		err = errors.New("rpc: server cannot decode request: " + err.Error())
		return
	}

	// We read the header successfully. If we see an error now,
	// we can still recover and move on to the next request.
	keepReading = true
	svc, mtype, err = server.getService(req)
	return
}

// Accept accepts connections on the listener and serves requests
// for each incoming connection. Accept blocks until the listener
// returns a non-nil error. The caller typically invokes Accept in a
// go statement.
func (server *Server) Accept(lis net.Listener) error {
	for {
		conn, err := lis.Accept()
		if err != nil {
			debugln("rpc.Serve: accept:", err.Error())
			return err
		}
		go server.ServeConn(conn)
	}
}

// Register publishes the receiver's methods in the DefaultServer.
func Register(rcvr interface{}) error { return DefaultServer.Register(rcvr) }

// RegisterName is like Register but uses the provided name for the type
// instead of the receiver's concrete type.
func RegisterName(name string, rcvr interface{}) error {
	return DefaultServer.RegisterName(name, rcvr)
}

// A ServerCodec implements reading of RPC requests and writing of
// RPC responses for the server side of an RPC session.
// The server calls ReadRequestHeader and ReadRequestBody in pairs
// to read requests from the connection, and it calls WriteResponse to
// write a response back. The server calls Close when finished with the
// connection. ReadRequestBody may be called with a nil
// argument to force the body of the request to be read and discarded.
// See NewClient's comment for information about concurrent access.
type ServerCodec interface {
	ReadRequestHeader(*Request) error
	ReadRequestBody(interface{}) error
	WriteResponse(*Response, interface{}) error

	// Close can be called multiple times and must be idempotent.
	Close() error
}

// ServeConn runs the DefaultServer on a single connection.
// ServeConn blocks, serving the connection until the client hangs up.
// The caller typically invokes ServeConn in a go statement.
// ServeConn uses the gob wire format (see package gob) on the
// connection. To use an alternate codec, use ServeCodec.
// See NewClient's comment for information about concurrent access.
func ServeConn(conn io.ReadWriteCloser) {
	DefaultServer.ServeConn(conn)
}

// ServeCodec is like ServeConn but uses the specified codec to
// decode requests and encode responses.
func ServeCodec(codec ServerCodec) {
	DefaultServer.ServeCodec(codec)
}

// ServeRequest is like ServeCodec but synchronously serves a single request.
// It does not close the codec upon completion.
func ServeRequest(codec ServerCodec) error {
	return ServeRequestContext(context.Background(), codec)
}

// ServeRequest is like ServeCodec but synchronously serves a single request.
// It does not close the codec upon completion.
//
// Cancelling the context given here will propagate cancellation to the context
// of the called function.
func ServeRequestContext(ctx *context.Context, codec ServerCodec) error {
	return DefaultServer.ServeRequestContext(ctx, codec)
}

// Accept accepts connections on the listener and serves requests
// to DefaultServer for each incoming connection.
// Accept blocks; the caller typically invokes it in a go statement.
func Accept(lis net.Listener) { DefaultServer.Accept(lis) }

// Can connect to RPC service using HTTP CONNECT to rpcPath.
var connected = "200 Connected to Go RPC"

// ServeHTTP implements an http.Handler that answers RPC requests.
func (server *Server) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if req.Method != "CONNECT" {
		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
		w.WriteHeader(http.StatusMethodNotAllowed)
		io.WriteString(w, "405 must CONNECT\n")
		return
	}
	conn, _, err := w.(http.Hijacker).Hijack()
	if err != nil {
		log.Print("rpc hijacking ", req.RemoteAddr, ": ", err.Error())
		return
	}
	io.WriteString(conn, "HTTP/1.0 "+connected+"\n\n")
	server.ServeConn(conn)
}

// HandleHTTP registers an HTTP handler for RPC messages on rpcPath,
// and a debugging handler on debugPath.
// It is still necessary to invoke http.Serve(), typically in a go statement.
func (server *Server) HandleHTTP(rpcPath string) {
	http.Handle(rpcPath, server)
}

// HandleHTTP registers an HTTP handler for RPC messages to DefaultServer
// on DefaultRPCPath and a debugging handler on DefaultDebugPath.
// It is still necessary to invoke http.Serve(), typically in a go statement.
func HandleHTTP() {
	DefaultServer.HandleHTTP(DefaultRPCPath)
}