Fix two race conditions in client.Do

* Common race condition is fixed by identifying that Client.respHandler
  can be completely removed since all respHandler operations (get, put
  and invocation) can be moved into the Client.processLoop goroutine,
  meaning that zero locking is required. This race condition resulted
  in a deadlock that was resolved by the response timeout at the
  end of client.Do, returning ErrLostConn

* Rare race condition is fixed by changing responseHandlerMap.get
  to .getAndRemove. This race condition resulted in the innerHandler
  for a new dtJobCreated assigned in client.Do overriding a stale older
  dtJobCreated request, and the newer innerHandler being removed by an
  older dtJobCreated in client.processLoop > client.handleInner. When
  the newer dtJobCreated response was received, the handler for it had
  already been deleted. This was resolved by the response timeout at the
  end of client.Do, returning ErrLostConn

client/client.go

@@ -19,7 +19,6 @@ type Client struct {
 	sync.Mutex
 
 	net, addr    string
-	respHandler  *responseHandlerMap
 	innerHandler *responseHandlerMap
 	in           chan *Response
 	conn         net.Conn
@@ -32,11 +31,16 @@ type Client struct {
 
 type responseHandlerMap struct {
 	sync.Mutex
-	holder map[string]ResponseHandler
+	holder map[string]handledResponse
+}
+
+type handledResponse struct {
+	internal ResponseHandler // internal handler, always non-nil
+	external ResponseHandler // handler passed in from (*Client).Do, sometimes nil
 }
 
 func newResponseHandlerMap() *responseHandlerMap {
-	return &responseHandlerMap{holder: make(map[string]ResponseHandler, queueSize)}
+	return &responseHandlerMap{holder: make(map[string]handledResponse, queueSize)}
 }
 
 func (r *responseHandlerMap) remove(key string) {
@@ -45,21 +49,22 @@ func (r *responseHandlerMap) remove(key string) {
 	r.Unlock()
 }
 
-func (r *responseHandlerMap) get(key string) (ResponseHandler, bool) {
+func (r *responseHandlerMap) getAndRemove(key string) (handledResponse, bool) {
 	r.Lock()
 	rh, b := r.holder[key]
+	delete(r.holder, key)
 	r.Unlock()
 	return rh, b
 }
 
-func (r *responseHandlerMap) put(key string, rh ResponseHandler) {
+func (r *responseHandlerMap) putWithExternalHandler(key string, internal, external ResponseHandler) {
 	r.Lock()
-	r.holder[key] = rh
+	r.holder[key] = handledResponse{internal: internal, external: external}
 	r.Unlock()
 }
 
-func (r *responseHandlerMap) putNoLock(key string, rh ResponseHandler) {
+func (r *responseHandlerMap) put(key string, rh ResponseHandler) {
-	r.holder[key] = rh
+	r.putWithExternalHandler(key, rh, nil)
 }
 
 // New returns a client.
@@ -67,7 +72,6 @@ func New(network, addr string) (client *Client, err error) {
 	client = &Client{
 		net:             network,
 		addr:            addr,
-		respHandler:     newResponseHandlerMap(),
 		innerHandler:    newResponseHandlerMap(),
 		in:              make(chan *Response, queueSize),
 		ResponseTimeout: DefaultTimeout,
@@ -168,21 +172,26 @@ ReadLoop:
 }
 
 func (client *Client) processLoop() {
+	rhandlers := map[string]ResponseHandler{}
 	for resp := range client.in {
 		switch resp.DataType {
 		case dtError:
 			client.err(getError(resp.Data))
 		case dtStatusRes:
-			resp = client.handleInner("s"+resp.Handle, resp)
+			client.handleInner("s"+resp.Handle, resp, nil)
 		case dtJobCreated:
-			resp = client.handleInner("c", resp)
+			client.handleInner("c", resp, rhandlers)
 		case dtEchoRes:
-			resp = client.handleInner("e", resp)
+			client.handleInner("e", resp, nil)
 		case dtWorkData, dtWorkWarning, dtWorkStatus:
-			resp = client.handleResponse(resp.Handle, resp)
+			if cb := rhandlers[resp.Handle]; cb != nil {
+				cb(resp)
+			}
 		case dtWorkComplete, dtWorkFail, dtWorkException:
-			client.handleResponse(resp.Handle, resp)
+			if cb := rhandlers[resp.Handle]; cb != nil {
-			client.respHandler.remove(resp.Handle)
+				cb(resp)
+				delete(rhandlers, resp.Handle)
+			}
 		}
 	}
 }
@@ -193,21 +202,13 @@ func (client *Client) err(e error) {
 	}
 }
 
-func (client *Client) handleResponse(key string, resp *Response) *Response {
+func (client *Client) handleInner(key string, resp *Response, rhandlers map[string]ResponseHandler) {
-	if h, ok := client.respHandler.get(key); ok {
+	if h, ok := client.innerHandler.getAndRemove(key); ok {
-		h(resp)
+		if h.external != nil && resp.Handle != "" {
-		return nil
+			rhandlers[resp.Handle] = h.external
 		}
-	return resp
+		h.internal(resp)
-}
-
-func (client *Client) handleInner(key string, resp *Response) *Response {
-	if h, ok := client.innerHandler.get(key); ok {
-		h(resp)
-		client.innerHandler.remove(key)
-		return nil
 	}
-	return resp
 }
 
 type handleOrError struct {
@@ -216,14 +217,14 @@ type handleOrError struct {
 }
 
 func (client *Client) do(funcname string, data []byte,
-	flag uint32) (handle string, err error) {
+	flag uint32, h ResponseHandler) (handle string, err error) {
 	if client.conn == nil {
 		return "", ErrLostConn
 	}
 	var result = make(chan handleOrError, 1)
 	client.Lock()
 	defer client.Unlock()
-	client.innerHandler.put("c", func(resp *Response) {
+	client.innerHandler.putWithExternalHandler("c", func(resp *Response) {
 		if resp.DataType == dtError {
 			err = getError(resp.Data)
 			result <- handleOrError{"", err}
@@ -231,7 +232,7 @@ func (client *Client) do(funcname string, data []byte,
 		}
 		handle = resp.Handle
 		result <- handleOrError{handle, nil}
-	})
+	}, h)
 	id := IdGen.Id()
 	req := getJob(id, []byte(funcname), data)
 	req.DataType = flag
@@ -264,12 +265,7 @@ func (client *Client) Do(funcname string, data []byte,
 		datatype = dtSubmitJob
 	}
 
-	client.respHandler.Lock()
+	handle, err = client.do(funcname, data, datatype, h)
-	defer client.respHandler.Unlock()
-	handle, err = client.do(funcname, data, datatype)
-	if err == nil && h != nil {
-		client.respHandler.putNoLock(handle, h)
-	}
 	return
 }
 
@@ -289,7 +285,7 @@ func (client *Client) DoBg(funcname string, data []byte,
 	default:
 		datatype = dtSubmitJobBg
 	}
-	handle, err = client.do(funcname, data, datatype)
+	handle, err = client.do(funcname, data, datatype, nil)
 	return
 }
 

client/client_test.go

@@ -1,9 +1,11 @@
 package client
 
 import (
+	"errors"
 	"flag"
 	"os"
 	"testing"
+	"time"
 )
 
 const (
@@ -17,6 +19,7 @@ var (
 
 func TestMain(m *testing.M) {
 	integrationsTestFlag := flag.Bool("integration", false, "Run the integration tests (in addition to the unit tests)")
+	flag.Parse()
 	if integrationsTestFlag != nil {
 		runIntegrationTests = *integrationsTestFlag
 	}
@@ -95,6 +98,66 @@ func TestClientDo(t *testing.T) {
 	}
 }
 
+func TestClientMultiDo(t *testing.T) {
+	if !runIntegrationTests {
+		t.Skip("To run this test, use: go test -integration")
+	}
+
+	// This integration test requires that examples/pl/worker_multi.pl be running.
+	//
+	// Test invocation is:
+	//    go test -integration -timeout 10s -run '^TestClient(AddServer|MultiDo)$'
+	//
+	// Send 1000 requests to go through all race conditions
+	const nreqs = 1000
+	errCh := make(chan error)
+	gotCh := make(chan string, nreqs)
+
+	olderrh := client.ErrorHandler
+	client.ErrorHandler = func(e error) { errCh <- e }
+	client.ResponseTimeout = 5 * time.Second
+	defer func() { client.ErrorHandler = olderrh }()
+
+	nextJobCh := make(chan struct{})
+	defer close(nextJobCh)
+	go func() {
+		for range nextJobCh {
+			start := time.Now()
+			handle, err := client.Do("PerlToUpper", []byte("abcdef"), JobNormal, func(r *Response) { gotCh <- string(r.Data) })
+			if err == ErrLostConn && time.Since(start) > client.ResponseTimeout {
+				errCh <- errors.New("Impossible 'lost conn', deadlock bug detected")
+			} else if err != nil {
+				errCh <- err
+			}
+			if handle == "" {
+				errCh <- errors.New("Handle is empty.")
+			}
+		}
+	}()
+
+	for i := 0; i < nreqs; i++ {
+		select {
+		case err := <-errCh:
+			t.Fatal(err)
+		case nextJobCh <- struct{}{}:
+		}
+	}
+
+	remaining := nreqs
+	for remaining > 0 {
+		select {
+		case err := <-errCh:
+			t.Fatal(err)
+		case got := <-gotCh:
+			if got != "ABCDEF" {
+				t.Error("Unexpected response from PerlDoUpper: ", got)
+			}
+			remaining--
+			t.Logf("%d response remaining", remaining)
+		}
+	}
+}
+
 func TestClientStatus(t *testing.T) {
 	if !runIntegrationTests {
 		t.Skip("To run this test, use: go test -integration")

example/pl/worker_multi.pl

@@ -0,0 +1,33 @@
+#!/usr/bin/perl
+
+# Runs 20 children that expose "PerlToUpper" before returning the result.
+
+use strict; use warnings;
+use constant CHILDREN => 20;
+use Time::HiRes qw(usleep);
+use Gearman::Worker;
+
+$|++;
+my @child_pids;
+for (1 .. CHILDREN) {
+  if (my $pid = fork) {
+    push @child_pids, $pid;
+    next;
+  }
+  eval {
+    my $w = Gearman::Worker->new(job_servers => '127.0.0.1:4730');
+    $w->register_function(PerlToUpper => sub { print "."; uc $_[0]->arg });
+    $w->work while 1;
+  };
+  warn $@ if $@;
+  exit 0;
+}
+
+$SIG{INT} = $SIG{HUP} = sub {
+  kill 9, @child_pids;
+  print "\nChildren shut down, gracefully exiting\n";
+  exit 0;
+};
+
+printf "Forked %d children, serving 'PerlToUpper' function to gearman\n", CHILDREN;
+sleep;