Fail loudly on a touch read, stop the workers before their sockets close, and file down the small edges

main.go

@@ -640,6 +640,20 @@ func run(aName, bName string, nsPerM float64) error {
 		defer wg.Done()
 		samp.run(&done, startTx)
 	}()
+	// Every return from here on stops the workers before the deferred closes
+	// pull their sockets out from under them: otherwise the sampler panics on a
+	// closed fd and can mask the error that actually ended the run. An error
+	// before the gate opens closes it here, or the wait would hang on
+	// goroutines still parked at startTx.
+	defer func() {
+		done.Store(true)
+		select {
+		case <-startTx:
+		default:
+			close(startTx)
+		}
+		wg.Wait()
+	}()
 	rxReady.Wait()
 
 	sig := make(chan os.Signal, 1)
@@ -674,8 +688,6 @@ func run(aName, bName string, nsPerM float64) error {
 	for {
 		select {
 		case <-sig:
-			done.Store(true)
-			wg.Wait()
 			return nil
 		case <-space:
 			start = resetAll(dirs, stats)