From 5e5b4d7054ca913499b82e7dc5742d6c66970655 Mon Sep 17 00:00:00 2001
From: Grant Zvolsky <gzvolsky-gf@thoughtmachine.net>
Date: Thu, 3 Sep 2026 08:18:46 +0000
Subject: [PATCH] make cycleDetector.stopped atomic

Fixes the flaky build-darwin job and adds a regression test for the race
with `go state.checkForCycles()` in `src/core/state.go`. The main
goroutine can call Stop while the async check is in flight.
---
 src/core/cycle_detector.go      | 11 ++++++-----
 src/core/cycle_detector_test.go | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/src/core/cycle_detector.go b/src/core/cycle_detector.go
index 39dc99549e..af808cb2bb 100644
--- a/src/core/cycle_detector.go
+++ b/src/core/cycle_detector.go
@@ -3,17 +3,18 @@ package core
 import (
 	"fmt"
 	"strings"
+	"sync/atomic"
 )
 
 type cycleDetector struct {
 	graph   *BuildGraph
-	stopped bool
+	stopped atomic.Bool
 }
 
 // Check runs a single check of the build graph to see if any cycles can be detected.
 // If it finds one an errCycle is returned.
 func (c *cycleDetector) Check() *errCycle {
-	if c.stopped {
+	if c.stopped.Load() {
 		return nil
 	}
 	log.Debug("Running cycle detection...")
@@ -27,7 +28,7 @@ func (c *cycleDetector) Check() *errCycle {
 	// cycle is complete or not (if not the caller will need to add its node to it as well).
 	var visit func(target *BuildTarget) ([]*BuildTarget, bool)
 	visit = func(target *BuildTarget) ([]*BuildTarget, bool) {
-		if c.stopped {
+		if c.stopped.Load() {
 			return nil, false
 		} else if _, present := complete[target]; present {
 			return nil, false
@@ -49,7 +50,7 @@ func (c *cycleDetector) Check() *errCycle {
 	}
 
 	for _, target := range c.graph.AllTargets() {
-		if c.stopped {
+		if c.stopped.Load() {
 			log.Debug("Cycle detection terminated")
 			return nil
 		}
@@ -66,7 +67,7 @@ func (c *cycleDetector) Check() *errCycle {
 
 // Stop stops any existing run of the cycle detector.
 func (c *cycleDetector) Stop() {
-	c.stopped = true
+	c.stopped.Store(true)
 }
 
 // An errCycle is emitted when a graph cycle is detected.
diff --git a/src/core/cycle_detector_test.go b/src/core/cycle_detector_test.go
index 70506f59b7..241c373f2c 100644
--- a/src/core/cycle_detector_test.go
+++ b/src/core/cycle_detector_test.go
@@ -1,6 +1,7 @@
 package core
 
 import (
+	"fmt"
 	"testing"
 	"time"
 
@@ -71,4 +72,22 @@ func TestCycleDetector(t *testing.T) {
 		log.Warning("%s", err)
 		assert.Equal(t, []*BuildTarget{g, e, f}, err.Cycle)
 	})
+
+	// This is a regression test for a race with `go state.checkForCycles()`
+	// in `src/core/state.go`. The main goroutine can call Stop while the
+	// async check is in flight.
+	t.Run("StopDuringCheck", func(t *testing.T) {
+		state := NewDefaultBuildState()
+		for i := 0; i < 100; i++ {
+			state.Graph.AddTarget(NewBuildTarget(ParseBuildLabel(fmt.Sprintf("//src:t%d", i), "")))
+		}
+		detector := cycleDetector{graph: state.Graph}
+		done := make(chan struct{})
+		go func() {
+			defer close(done)
+			assert.Nil(t, detector.Check())
+		}()
+		detector.Stop()
+		<-done
+	})
 }