Does RPC have a timeout mechanism?

If RPC does not have a timeout mechanism, how do I "kill" an RPC call if it is trying to call an RPC method of a server that is closed?

You can use channels to implement a timeout pattern:

import "time"

c := make(chan error, 1)
go func() { c <- client.Call("Service", args, &result) } ()
select {
  case err := <-c:
    // use err and result
  case <-time.After(timeoutNanoseconds):
    // call timed out
}

The select will block until either client.Call returns or timeoutNanoseconds elapsed.

if you want to implement a timeout (to prevent a call from taking too long), then you'll want to change rpc.Dial for net.DialTimeout (notice they're separate packages: rpc vs net). Also be aware that the returned type isn't a client any more (as it is in the previous example); instead it is a 'connection'.

  conn, err := net.DialTimeout("tcp", "localhost:8080", time.Minute)
  if err != nil {
    log.Fatal("dialing:", err)
  }

  client := rpc.NewClient(conn)

It seems the only solution for net/rpc is to close the underlying connection when you notice stuck requests. Then the client should finish pending requests with "connection broken" errors.

An alternative way is to use https://github.com/valyala/gorpc , which supports timeout RPC calls out of the box.

func (client *Client) Call(serviceMethod string, args interface{}, reply interface{}) error

Call method may block goroutine forever

Change use Go method:

func (client *Client) Go(serviceMethod string, args interface{}, reply interface{}, done chan *Call) *Call

Client example:

call := rpcClient.Go(method, args, reply, make(chan *rpc.Call, 1))
	select {
	case <-time.After(timeout):
		log.Printf("[WARN] rpc call timeout(%v) %v => %v", timeout, rpcClient, s.RpcServer)
		rpcClient.Close()
		return errors.New("timeout")
	case resp := <-call.Done:
		if resp != nil && resp.Error != nil {
			rpcClient.Close()
			return resp.Error
		}

Or, anno now, someone might prefer to use context instead. This also takes care of returning a proper error when timed out. (context.DeadlineExceeded)

import (
	"context"
    "log"
	"net/rpc"
)

type Client struct {
	*rpc.Client
}

// CallEx is a context aware wrapper around rpc's Client.Call()
func (c *client) CallEx(ctx context.Context, serviceMethod string, args interface{}, reply interface{}) error {
	ec := make(chan error, 1)
	go func() {
		ec <- c.Call(serviceMethod, args, reply)
	}()
	select {
	case err := <-ec:
		return err
	case <-ctx.Done():
		return ctx.Err()
	}
}

Invoke this with a Deadlined context:

type Args struct {
   A, B int
}

func main(){
    rpc, err := rpc.DialHTTP("tcp", "host")
	if err != nil {
		t.Fatal(err)
	}
    c := client{rpc}

    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancel()
    
    var i int
    if err := c.CallEx(ctx, "Calc.Multiply", Args{2, 2}, &i); err != nil {
	    log.Fatal(err)
    }
}