From d6c3d4bf571bcbd9fffd02d5965216e658333bba Mon Sep 17 00:00:00 2001
From: mapogolions <ivanovpavelalex45@gmail.com>
Date: Sun, 2 Feb 2025 15:15:21 +0500
Subject: [PATCH 1/3] reproduce actor blocking on duplicate Respond call

---
 actor/engine_test.go | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/actor/engine_test.go b/actor/engine_test.go
index a58f36e9..3ec05c94 100644
--- a/actor/engine_test.go
+++ b/actor/engine_test.go
@@ -480,3 +480,25 @@ func TestMultipleStops(t *testing.T) {
 		<-done
 	}
 }
+
+func TestShouldNotBlockActorOnAccidentalDuplicateRespond(t *testing.T) {
+	e, err := NewEngine(NewEngineConfig())
+	require.NoError(t, err)
+	pid := e.SpawnFunc(func(ctx *Context) {
+		msg := ctx.Message()
+		if s, ok := msg.(string); ok {
+			if s == "foo" {
+				ctx.Respond(len(s))
+				// missing `return` statement, causing an unintended second response
+			}
+			ctx.Respond(len(s)) // sends a duplicate response
+		}
+	}, "str", WithID("len"))
+
+	_ = e.Request(pid, "foo", 2*time.Second) // response will be consumed later
+	resp := e.Request(pid, "barbaz", 2*time.Second)
+
+	r, err := resp.Result()
+	require.NoError(t, err)
+	require.Equal(t, 6, r)
+}

From 1c7caf3851f21fa37dd43504f673ea2e93bfc15a Mon Sep 17 00:00:00 2001
From: mapogolions <ivanovpavelalex45@gmail.com>
Date: Sun, 2 Feb 2025 15:22:06 +0500
Subject: [PATCH 2/3] reproduce receiving accidentally sent second result

---
 actor/engine_test.go | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/actor/engine_test.go b/actor/engine_test.go
index 3ec05c94..698bb37e 100644
--- a/actor/engine_test.go
+++ b/actor/engine_test.go
@@ -502,3 +502,46 @@ func TestShouldNotBlockActorOnAccidentalDuplicateRespond(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 6, r)
 }
+
+func TestShouldNotReceiveAccidentallySentSecondResult(t *testing.T) {
+	e, err := NewEngine(NewEngineConfig())
+	require.NoError(t, err)
+	done := make(chan struct{})
+	pid := e.SpawnFunc(func(ctx *Context) {
+		msg := ctx.Message()
+		if s, ok := msg.(string); ok && s == "foo" {
+			defer close(done)
+			if s == "foo" {
+				ctx.Respond(1)
+				// missing `return` statement, causing an unintended second response
+			}
+			select { // sends a duplicate response
+			case <-time.After(100 * time.Millisecond):
+			case <-runAsync(func() {
+				ctx.Respond(2)
+			}):
+			}
+		}
+	}, "kind")
+
+	resp := e.Request(pid, "foo", 200*time.Millisecond)
+	<-done
+
+	r, err := resp.Result()
+	require.NoError(t, err)
+	require.Equal(t, 1, r)
+	require.Nil(t, e.Registry.get(resp.pid))
+
+	r, err = resp.Result()
+	require.Error(t, err)
+	require.Nil(t, r)
+}
+
+func runAsync(f func()) <-chan struct{} {
+	ch := make(chan struct{})
+	go func() {
+		defer close(ch)
+		f()
+	}()
+	return ch
+}

From 47b938c26076c5c31ec296b3b63aa3ce1f788b1d Mon Sep 17 00:00:00 2001
From: mapogolions <ivanovpavelalex45@gmail.com>
Date: Sun, 2 Feb 2025 15:25:36 +0500
Subject: [PATCH 3/3] fix issues caused by duplicate response

---
 actor/registry.go | 10 ++++++++++
 actor/response.go |  6 +++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/actor/registry.go b/actor/registry.go
index 8f578b81..f5418773 100644
--- a/actor/registry.go
+++ b/actor/registry.go
@@ -69,3 +69,13 @@ func (r *Registry) add(proc Processer) {
 	r.mu.Unlock()
 	proc.Start()
 }
+
+func (r *Registry) remove(pid *PID) bool {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	_, ok := r.lookup[pid.ID]
+	if ok {
+		delete(r.lookup, pid.ID)
+	}
+	return ok
+}
diff --git a/actor/response.go b/actor/response.go
index c6493f79..68d4aff3 100644
--- a/actor/response.go
+++ b/actor/response.go
@@ -40,7 +40,11 @@ func (r *Response) Result() (any, error) {
 }
 
 func (r *Response) Send(_ *PID, msg any, _ *PID) {
-	r.result <- msg
+	// Under normal conditions, the method is expected to be called only once.
+	// To prevent accidental duplicate responses, we promptly remove the process from the registry
+	if r.engine.Registry.remove(r.pid) {
+		r.result <- msg
+	}
 }
 
 func (r *Response) PID() *PID         { return r.pid }