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// 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 rpc 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 name is exported, that is, begins with an upper case letter.

                - the method receiver is exported or local (defined in the package

                  registering the service).

                - the method has two arguments, both exported or local types.

                - the method's second argument is a pointer.

                - the method has return type error.

        The method's first argument represents the arguments provided by the caller; the

        second 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.

        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.

        Call waits for the remote call to complete; Go launches the call asynchronously

        and returns a channel that will signal completion.

        Package "gob" is used to transport the data.

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

                package server

                type Args struct {

                        A, B int

                }

                type Quotient struct {

                        Quo, Rem int

                }

                type Arith int

                func (t *Arith) Multiply(args *Args, reply *int) error {

                        *reply = args.A * args.B

                        return nil

                }

                func (t *Arith) Divide(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("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, "ient, 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.

*/

package rpc

import (

        "bufio"

        "encoding/gob"

        "errors"

        "io"

        "log"

        "net"

        "net/http"

        "reflect"

        "strings"

        "sync"

        "unicode"

        "unicode/utf8"

)

const (

        // Defaults used by HandleHTTP

        DefaultRPCPath   = "/_goRPC_"

        DefaultDebugPath = "/debug/rpc"

)

// Precompute the reflect type for error.  Can't use error directly

// because Typeof takes an empty interface value.  This is annoying.

var typeOfError = reflect.TypeOf((*error)(nil)).Elem()

type methodType struct {

        sync.Mutex // protects counters

        method     reflect.Method

        ArgType    reflect.Type

        ReplyType  reflect.Type

        numCalls   uint

}

type service struct {

        name   string                 // name of service

        rcvr   reflect.Value          // receiver of methods for the service

        typ    reflect.Type           // type of the receiver

        method map[string]*methodType // registered methods

}

// 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 {

        ServiceMethod string    // echoes that of the Request

        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 {

        mu         sync.Mutex // protects the serviceMap

        serviceMap map[string]*service

        reqLock    sync.Mutex // protects freeReq

        freeReq    *Request

        respLock   sync.Mutex // protects freeResp

        freeResp   *Response

}

// NewServer returns a new Server.

func NewServer() *Server {

        return &Server{serviceMap: make(map[string]*service)}

}

// DefaultServer is the default instance of *Server.

var DefaultServer = NewServer()

// Is this an exported - upper case - name?

func isExported(name string) bool {

        rune, _ := utf8.DecodeRuneInString(name)

        return unicode.IsUpper(rune)

}

// Is this type exported or a builtin?

func isExportedOrBuiltinType(t reflect.Type) bool {

        for t.Kind() == reflect.Ptr {

                t = t.Elem()

        }

        // PkgPath will be non-empty even for an exported type,

        // so we need to check the type name as well.

        return isExported(t.Name()) || t.PkgPath() == ""

}

// Register publishes in the server the set of methods of the

// receiver value that satisfy the following conditions:

//      - exported method

//      - two arguments, both pointers to exported structs

//      - one return value, of type error

// It returns an error if the receiver is not an exported type or has no

// suitable methods.

// The client accesses each method using a string of the form "Type.Method",

// where Type is the receiver's concrete type.

func (server *Server) Register(rcvr interface{}) error {

        return server.register(rcvr, "", false)

}

// RegisterName is like Register but uses the provided name for the type

// instead of the receiver's concrete type.

func (server *Server) RegisterName(name string, rcvr interface{}) error {

        return server.register(rcvr, name, true)

}

func (server *Server) register(rcvr interface{}, name string, useName bool) error {

        server.mu.Lock()

        defer server.mu.Unlock()

        if server.serviceMap == nil {

                server.serviceMap = make(map[string]*service)

        }

        s := new(service)

        s.typ = reflect.TypeOf(rcvr)

        s.rcvr = reflect.ValueOf(rcvr)

        sname := reflect.Indirect(s.rcvr).Type().Name()

        if useName {

                sname = name

        }

        if sname == "" {

                log.Fatal("rpc: no service name for type", s.typ.String())

        }

        if !isExported(sname) && !useName {

                s := "rpc Register: type " + sname + " is not exported"

                log.Print(s)

                return errors.New(s)

        }

        if _, present := server.serviceMap[sname]; present {

                return errors.New("rpc: service already defined: " + sname)

        }

        s.name = sname

        s.method = make(map[string]*methodType)

        // Install the methods

        for m := 0; m < s.typ.NumMethod(); m++ {

                method := s.typ.Method(m)

                mtype := method.Type

                mname := method.Name

                if method.PkgPath != "" {

                        continue

                }

                // Method needs three ins: receiver, *args, *reply.

                if mtype.NumIn() != 3 {

                        log.Println("method", mname, "has wrong number of ins:", mtype.NumIn())

                        continue

                }

                // First arg need not be a pointer.

                argType := mtype.In(1)

                if !isExportedOrBuiltinType(argType) {

                        log.Println(mname, "argument type not exported or local:", argType)

                        continue

                }

                // Second arg must be a pointer.

                replyType := mtype.In(2)

                if replyType.Kind() != reflect.Ptr {

                        log.Println("method", mname, "reply type not a pointer:", replyType)

                        continue

                }

                if !isExportedOrBuiltinType(replyType) {

                        log.Println("method", mname, "reply type not exported or local:", replyType)

                        continue

                }

                // Method needs one out: error.

                if mtype.NumOut() != 1 {

                        log.Println("method", mname, "has wrong number of outs:", mtype.NumOut())

                        continue

                }

                if returnType := mtype.Out(0); returnType != typeOfError {

                        log.Println("method", mname, "returns", returnType.String(), "not error")

                        continue

                }

                s.method[mname] = &methodType{method: method, ArgType: argType, ReplyType: replyType}

        }

        if len(s.method) == 0 {

                s := "rpc Register: type " + sname + " has no exported methods of suitable type"

                log.Print(s)

                return errors.New(s)

        }

        server.serviceMap[s.name] = s

        return nil

}

// A value sent as a placeholder for the response when the server receives an invalid request.

type InvalidRequest struct{}

var invalidRequest = InvalidRequest{}

func (server *Server) sendResponse(sending *sync.Mutex, req *Request, reply interface{}, codec ServerCodec, errmsg string) {

        resp := server.getResponse()

        // Encode the response header

        resp.ServiceMethod = req.ServiceMethod

        if errmsg != "" {

                resp.Error = errmsg

                reply = invalidRequest

        }

        resp.Seq = req.Seq

        sending.Lock()

        err := codec.WriteResponse(resp, reply)

        if err != nil {

                log.Println("rpc: writing response:", err)

        }

        sending.Unlock()

        server.freeResponse(resp)

}

func (m *methodType) NumCalls() (n uint) {

        m.Lock()

        n = m.numCalls

        m.Unlock()

        return n

}

func (s *service) call(server *Server, sending *sync.Mutex, mtype *methodType, req *Request, argv, replyv reflect.Value, codec ServerCodec) {

        mtype.Lock()

        mtype.numCalls++

        mtype.Unlock()

        function := mtype.method.Func

        // Invoke the method, providing a new value for the reply.

        returnValues := function.Call([]reflect.Value{s.rcvr, argv, replyv})

        // The return value for the method is an error.

        errInter := returnValues[0].Interface()

        errmsg := ""

        if errInter != nil {

                errmsg = errInter.(error).Error()

        }

        server.sendResponse(sending, req, replyv.Interface(), codec, errmsg)

        server.freeRequest(req)

}

type gobServerCodec struct {

        rwc    io.ReadWriteCloser

        dec    *gob.Decoder

        enc    *gob.Encoder

        encBuf *bufio.Writer

}

func (c *gobServerCodec) ReadRequestHeader(r *Request) error {

        return c.dec.Decode(r)

}

func (c *gobServerCodec) ReadRequestBody(body interface{}) error {

        return c.dec.Decode(body)

}

func (c *gobServerCodec) WriteResponse(r *Response, body interface{}) (err error) {

        if err = c.enc.Encode(r); err != nil {

                return

        }

        if err = c.enc.Encode(body); err != nil {

                return

        }

        return c.encBuf.Flush()

}

func (c *gobServerCodec) Close() error {

        return c.rwc.Close()

}

// 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.

func (server *Server) ServeConn(conn io.ReadWriteCloser) {

        buf := bufio.NewWriter(conn)

        srv := &gobServerCodec{conn, gob.NewDecoder(conn), gob.NewEncoder(buf), buf}

        server.ServeCodec(srv)

}

// 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)

        for {

                service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)

                if err != nil {

                        if err != io.EOF {

                                log.Println("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

                }

                go service.call(server, sending, mtype, req, argv, replyv, codec)

        }

        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 {

        sending := new(sync.Mutex)

        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, sending, mtype, req, argv, replyv, codec)

        return nil

}

func (server *Server) getRequest() *Request {

        server.reqLock.Lock()

        req := server.freeReq

        if req == nil {

                req = new(Request)

        } else {

                server.freeReq = req.next

                *req = Request{}

        }

        server.reqLock.Unlock()

        return req

}

func (server *Server) freeRequest(req *Request) {

        server.reqLock.Lock()

        req.next = server.freeReq

        server.freeReq = req

        server.reqLock.Unlock()

}

func (server *Server) getResponse() *Response {

        server.respLock.Lock()

        resp := server.freeResp

        if resp == nil {

                resp = new(Response)

        } else {

                server.freeResp = resp.next

                *resp = Response{}

        }

        server.respLock.Unlock()

        return resp

}

func (server *Server) freeResponse(resp *Response) {

        server.respLock.Lock()

        resp.next = server.freeResp

        server.freeResp = resp

        server.respLock.Unlock()

}

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.

        argIsValue := false // if true, need to indirect before calling.

        if mtype.ArgType.Kind() == reflect.Ptr {

                argv = reflect.New(mtype.ArgType.Elem())

        } else {

                argv = reflect.New(mtype.ArgType)

                argIsValue = true

        }

        // argv guaranteed to be a pointer now.

        if err = codec.ReadRequestBody(argv.Interface()); err != nil {

                return

        }

        if argIsValue {

                argv = argv.Elem()

        }

        replyv = reflect.New(mtype.ReplyType.Elem())

        return

}

func (server *Server) readRequestHeader(codec ServerCodec) (service *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

        serviceMethod := strings.Split(req.ServiceMethod, ".")

        if len(serviceMethod) != 2 {

                err = errors.New("rpc: service/method request ill-formed: " + req.ServiceMethod)

                return

        }

        // Look up the request.

        server.mu.Lock()

        service = server.serviceMap[serviceMethod[0]]

        server.mu.Unlock()

        if service == nil {

                err = errors.New("rpc: can't find service " + req.ServiceMethod)

                return

        }

        mtype = service.method[serviceMethod[1]]

        if mtype == nil {

                err = errors.New("rpc: can't find method " + req.ServiceMethod)

        }

        return

}

// Accept accepts connections on the listener and serves requests

// for each incoming connection.  Accept blocks; the caller typically

// invokes it in a go statement.

func (server *Server) Accept(lis net.Listener) {

        for {

                conn, err := lis.Accept()

                if err != nil {

                        log.Fatal("rpc.Serve: accept:", err.Error()) // TODO(r): exit?

                }

                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.

type ServerCodec interface {

        ReadRequestHeader(*Request) error

        ReadRequestBody(interface{}) error

        WriteResponse(*Response, interface{}) error

        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.

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 DefaultServer.ServeRequest(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, debugPath string) {

        http.Handle(rpcPath, server)

        http.Handle(debugPath, debugHTTP{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, DefaultDebugPath)

}