syz-manager: don't report all pkg/repro errors on shutdown

Otherwise we're getting "repro failed: all VMs failed to boot" pkg/repro
errors if a sykaller instance is shutting down.

pkg/repro/repro.go

@@ -543,12 +543,14 @@ func (ctx *context) testWithInstance(callback func(execInterface) (rep *instance
 	return true, nil
 }
 
+var ErrNoVMs = errors.New("all VMs failed to boot")
+
 // A helper method for testWithInstance.
 func (ctx *context) runOnInstance(callback func(execInterface) (rep *instance.RunResult,
 	err error)) (*instance.RunResult, error) {
 	inst := <-ctx.instances
 	if inst == nil {
-		return nil, fmt.Errorf("all VMs failed to boot")
+		return nil, ErrNoVMs
 	}
 	defer ctx.returnInstance(inst)
 	return callback(inst.execProg)

syz-manager/manager.go

@@ -469,14 +469,26 @@ func (mgr *Manager) vmLoop() {
 }
 
 func reportReproError(err error) {
+	shutdown := false
+	select {
+	case <-vm.Shutdown:
+		shutdown = true
+	default:
+	}
+
 	switch err {
 	case repro.ErrNoPrograms:
 		// This is not extraordinary as programs are collected via SSH.
 		log.Logf(0, "repro failed: %v", err)
-	default:
-		// Report everything else as errors.
-		log.Errorf("repro failed: %v", err)
+		return
+	case repro.ErrNoVMs:
+		// This error is to be expected if we're shutting down.
+		if shutdown {
+			return
+		}
 	}
+	// Report everything else as errors.
+	log.Errorf("repro failed: %v", err)
 }
 
 func (mgr *Manager) runRepro(crash *Crash, vmIndexes []int, putInstances func(...int)) *ReproResult {