// 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's type is exported.
- the method is exported.
- the method has two arguments, both exported (or builtin) types.
- the method's second argument is a pointer.
- the method has return type error.
In effect, the method must look schematically like
func (t *T) MethodName(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 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. 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(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, 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 rpc
import (
"bufio"
"encoding/gob"
"errors"
"go/token"
"io"
"log"
"net"
"net/http"
"reflect"
"strings"
"sync"
)
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 {
serviceMap sync.Map // 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{}
}
// DefaultServer is the default instance of *Server.
var DefaultServer = NewServer()
// 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 token.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 of exported type
// - two arguments, both of exported type
// - the second argument is a pointer
// - one return value, of type error
// It returns an error if the receiver is not an exported type or has
// no suitable methods. It also logs the error using package log.
// 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 {
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 == "" {
s := "rpc.Register: no service name for type " + s.typ.String()
log.Print(s)
return errors.New(s)
}
if !token.IsExported(sname) && !useName {
s := "rpc.Register: type " + sname + " is not exported"
log.Print(s)
return errors.New(s)
}
s.name = sname
// Install the methods
s.method = suitableMethods(s.typ, true)
if len(s.method) == 0 {
str := ""
// To help the user, see if a pointer receiver would work.
method := suitableMethods(reflect.PtrTo(s.typ), false)
if len(method) != 0 {
str = "rpc.Register: type " + sname + " has no exported methods of suitable type (hint: pass a pointer to value of that type)"
} else {
str = "rpc.Register: type " + sname + " has no exported methods of suitable type"
}
log.Print(str)
return errors.New(str)
}
if _, dup := server.serviceMap.LoadOrStore(sname, s); dup {
return errors.New("rpc: service already defined: " + sname)
}
return nil
}
// suitableMethods returns suitable Rpc methods of typ, it will report
// error using log if reportErr is true.
func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
methods := make(map[string]*methodType)
for m := 0; m < typ.NumMethod(); m++ {
method := typ.Method(m)
mtype := method.Type
mname := method.Name
// Method must be exported.
if method.PkgPath != "" {
continue
}
// Method needs three ins: receiver, *args, *reply.
if mtype.NumIn() != 3 {
if reportErr {
log.Printf("rpc.Register: method %q has %d input parameters; needs exactly three\n", mname, mtype.NumIn())
}
continue
}
// First arg need not be a pointer.
argType := mtype.In(1)
if !isExportedOrBuiltinType(argType) {
if reportErr {
log.Printf("rpc.Register: argument type of method %q is not exported: %q\n", mname, argType)
}
continue
}
// Second arg must be a pointer.
replyType := mtype.In(2)
if replyType.Kind() != reflect.Ptr {
if reportErr {
log.Printf("rpc.Register: reply type of method %q is not a pointer: %q\n", mname, replyType)
}
continue
}
// Reply type must be exported.
if !isExportedOrBuiltinType(replyType) {
if reportErr {
log.Printf("rpc.Register: reply type of method %q is not exported: %q\n", mname, replyType)
}
continue
}
// Method needs one out.
if mtype.NumOut() != 1 {
if reportErr {
log.Printf("rpc.Register: method %q has %d output parameters; needs exactly one\n", mname, mtype.NumOut())
}
continue
}
// The return type of the method must be error.
if returnType := mtype.Out(0); returnType != typeOfError {
if reportErr {
log.Printf("rpc.Register: return type of method %q is %q, must be error\n", mname, returnType)
}
continue
}
methods[mname] = &methodType{method: method, ArgType: argType, ReplyType: replyType}
}
return methods
}
// A value sent as a placeholder for the server's response value when the server
// receives an invalid request. It is never decoded by the client since the Response
// contains an error when it is used.
var invalidRequest = struct{}{}
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 debugLog && 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, wg *sync.WaitGroup, mtype *methodType, req *Request, argv, replyv reflect.Value, codec ServerCodec) {
if wg != nil {
defer wg.Done()
}
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
closed bool
}
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 {
if c.encBuf.Flush() == nil {
// Gob couldn't encode the header. Should not happen, so if it does,
// shut down the connection to signal that the connection is broken.
log.Println("rpc: gob error encoding response:", err)
c.Close()
}
return
}
if err = c.enc.Encode(body); err != nil {
if c.encBuf.Flush() == nil {
// Was a gob problem encoding the body but the header has been written.
// Shut down the connection to signal that the connection is broken.
log.Println("rpc: gob error encoding body:", err)
c.Close()
}
return
}
return c.encBuf.Flush()
}
func (c *gobServerCodec) Close() error {
if c.closed {
// Only call c.rwc.Close once; otherwise the semantics are undefined.
return nil
}
c.closed = true
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.
// See NewClient's comment for information about concurrent access.
func (server *Server) ServeConn(conn io.ReadWriteCloser) {
buf := bufio.NewWriter(conn)
srv := &gobServerCodec{
rwc: conn,
dec: gob.NewDecoder(conn),
enc: gob.NewEncoder(buf),
encBuf: 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)
wg := new(sync.WaitGroup)
for {
service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)
if err != nil {
if debugLog && 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
}
wg.Add(1)
go service.call(server, sending, 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 {
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, nil, 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())
switch mtype.ReplyType.Elem().Kind() {
case reflect.Map:
replyv.Elem().Set(reflect.MakeMap(mtype.ReplyType.Elem()))
case reflect.Slice:
replyv.Elem().Set(reflect.MakeSlice(mtype.ReplyType.Elem(), 0, 0))
}
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
dot := strings.LastIndex(req.ServiceMethod, ".")
if dot < 0 {
err = errors.New("rpc: service/method request ill-formed: " + req.ServiceMethod)
return
}
serviceName := req.ServiceMethod[:dot]
methodName := req.ServiceMethod[dot+1:]
// Look up the request.
svci, ok := server.serviceMap.Load(serviceName)
if !ok {
err = errors.New("rpc: can't find service " + req.ServiceMethod)
return
}
svc = svci.(*service)
mtype = svc.method[methodName]
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 until the listener
// returns a non-nil error. The caller typically invokes Accept in a
// go statement.
func (server *Server) Accept(lis net.Listener) {
for {
conn, err := lis.Accept()
if err != nil {
log.Print("rpc.Serve: accept:", err.Error())
return
}
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 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)
}
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