Fix race in CloseResults that drops test failure events

src/core/state.go

@@ -12,6 +12,7 @@ import (
 	iofs "io/fs"
 	"iter"
 	"path/filepath"
+	"runtime"
 	"runtime/pprof"
 	"sort"
 	"strings"
@@ -424,12 +425,27 @@ func (state *BuildState) Stop() {
 // CloseResults closes the result channels.
 func (state *BuildState) CloseResults() {
 	state.progress.cycleDetector.Stop()
+	// Drain any remaining results from the internal channel before closing.
+	// This prevents a race where forwardResults hasn't yet forwarded a result
+	// that was sent before all tasks completed.
+	state.drainResults()
 	state.progress.mutex.Lock()
 	defer state.progress.mutex.Unlock()
 	if state.progress.results != nil {
 		state.progress.resultOnce.Do(func() {
 			close(state.progress.results)
 		})
+		// Set to nil so forwardResults won't attempt to send on the closed
+		// channel if it reads a late result from internalResults.
+		state.progress.results = nil
+	}
+}
+
+// drainResults waits until all pending results in the internal channel have
+// been forwarded to the external results channel.
+func (state *BuildState) drainResults() {
+	for len(state.progress.internalResults) > 0 {
+		runtime.Gosched()
 	}
 }