Fix spurious Upgrader test failures

TestUpgraderOnOS is susceptible to a race: if the child process exits
too quickly after calling Ready(), the parent may see the exit before
the readiness notification. Wait for the parent to exit before exiting
the test.

Author: lmb

upgrader_test.go

@@ -121,10 +121,23 @@ func childProcess(upg *Upgrader) error {
 		file.Close()
 	}
 
+	rExit, err := upg.Fds.File("rExit")
+	if err != nil {
+		return err
+	}
+
+	// Ready closes all inherited but unused files.
 	if err := upg.Ready(); err != nil {
 		return fmt.Errorf("can't signal ready: %s", err)
 	}
 
+	// Block until the parent is done with us. Returning an
+	// error here won't make the parent fail, so don't bother.
+	if rExit != nil {
+		var b [1]byte
+		rExit.Read(b[:])
+	}
+
 	return nil
 }
 
@@ -159,6 +172,11 @@ func TestUpgraderOnOS(t *testing.T) {
 
 	addPipe("wState", wState)
 
+	rExit, wExit := pipe()
+	defer wExit.Close()
+
+	addPipe("rExit", rExit)
+
 	var readers []*os.File
 	defer func() {
 		for _, r := range readers {
@@ -188,6 +206,9 @@ func TestUpgraderOnOS(t *testing.T) {
 		}
 	}
 
+	// Tell child it's OK to exit now.
+	wExit.Close()
+
 	// Close copies of write pipes, so that
 	// reads below return EOF.
 	u.Stop()