From 128d3ff95fc101863d233f543e5dda41a1b7b53c Mon Sep 17 00:00:00 2001
From: Andrew Yuan <Andrew.Yuan@temporal.io>
Date: Wed, 10 Dec 2025 11:36:18 -0500
Subject: [PATCH] fix race where multiple workers' callbacks overwrite shared
 c.recValue field

---
 internal/internal_flags.go                   |  2 +-
 internal/internal_workflow.go                |  8 ++-
 internal/internal_workflow_testsuite_test.go | 62 ++++++++++++++++++++
 3 files changed, 69 insertions(+), 3 deletions(-)

diff --git a/internal/internal_flags.go b/internal/internal_flags.go
index 456a7fe81..756ab2e09 100644
--- a/internal/internal_flags.go
+++ b/internal/internal_flags.go
@@ -34,7 +34,7 @@ const (
 
 // unblockSelectorSignal exists to allow us to configure the default behavior of
 // SDKFlagBlockedSelectorSignalReceive. This is primarily useful with tests.
-var unblockSelectorSignal = os.Getenv("UNBLOCK_SIGNAL_SELECTOR") != ""
+var unblockSelectorSignal = os.Getenv("UNBLOCK_SIGNAL_SELECTOR") != "false"
 
 func sdkFlagFromUint(value uint32) sdkFlag {
 	switch value {
diff --git a/internal/internal_workflow.go b/internal/internal_workflow.go
index eca3cdda3..f71aa2fe4 100644
--- a/internal/internal_workflow.go
+++ b/internal/internal_workflow.go
@@ -1401,6 +1401,7 @@ func (s *selectorImpl) Select(ctx Context) {
 		if pair.receiveFunc != nil {
 			f := *pair.receiveFunc
 			c := pair.channel
+			hasDefault := s.defaultFunc != nil
 			callback := &receiveCallback{
 				fn: func(v interface{}, more bool) bool {
 					if readyBranch != nil {
@@ -1416,12 +1417,15 @@ func (s *selectorImpl) Select(ctx Context) {
 						dropSignalFlag = env.GetFlag(SDKFlagBlockedSelectorSignalReceive)
 					}
 
-					if dropSignalFlag {
+					// Only store value immediately when default branch exists,
+					// otherwise store in readyBranch to avoid race when multiple
+					// selectors blocked on same channel
+					if dropSignalFlag && hasDefault {
 						c.recValue = &v
 					}
 
 					readyBranch = func() {
-						if !dropSignalFlag {
+						if !dropSignalFlag || !hasDefault {
 							c.recValue = &v
 						}
 						f(c, more)
diff --git a/internal/internal_workflow_testsuite_test.go b/internal/internal_workflow_testsuite_test.go
index 6bf136647..de13d4f8c 100644
--- a/internal/internal_workflow_testsuite_test.go
+++ b/internal/internal_workflow_testsuite_test.go
@@ -4250,3 +4250,65 @@ func (s *WorkflowTestSuiteUnitTest) Test_SignalNotLost() {
 	err := env.GetWorkflowError()
 	s.NoError(err)
 }
+
+func (s *WorkflowTestSuiteUnitTest) TestChannelWorkerPattern() {
+	previousFlag := unblockSelectorSignal
+	unblockSelectorSignal = true
+	defer func() { unblockSelectorSignal = previousFlag }()
+
+	require := s.Require()
+
+	// Two workers listening on the same channel with multiple items sent quickly.
+	// Without the fix, callbacks overwrite c.recValue causing items to be lost.
+	workflowFn := func(ctx Context) error {
+		ch := NewChannel(ctx)
+		var received []int
+
+		Go(ctx, func(ctx Context) {
+			ch.Send(ctx, 1)
+			ch.Send(ctx, 2)
+			ch.Send(ctx, 3)
+			ch.Close()
+		})
+
+		// Two workers both selecting on the same channel
+		wg := NewWaitGroup(ctx)
+		wg.Add(2)
+		for i := 0; i < 2; i++ {
+			Go(ctx, func(ctx Context) {
+				defer wg.Done()
+				for {
+					selector := NewSelector(ctx)
+					done := false
+					selector.AddReceive(ch, func(c ReceiveChannel, more bool) {
+						if more {
+							var v int
+							c.Receive(ctx, &v)
+							received = append(received, v)
+						} else {
+							done = true
+						}
+					})
+					selector.Select(ctx)
+					if done {
+						break
+					}
+				}
+			})
+		}
+
+		wg.Wait(ctx)
+
+		// Without the fix: callbacks overwrite c.recValue, causing items to be lost
+		// With the fix: each callback's value is stored in its own readyBranch closure
+		require.Len(received, 3, "Expected 3 items to be received")
+		require.ElementsMatch([]int{1, 2, 3}, received, "Expected all items to be received exactly once")
+		return nil
+	}
+
+	env := s.NewTestWorkflowEnvironment()
+	env.ExecuteWorkflow(workflowFn)
+
+	require.True(env.IsWorkflowCompleted())
+	require.NoError(env.GetWorkflowError())
+}