Help rewrite function to be compatible with synctest

Getting Help
1

I have a function whose behavior depends on the type of input it receives. Of which there are two types:

  1. Close message
  2. Everything else

Calls with [2] can run concurrently with each other. But [1] must have exclusive access because it sets a flag to prevent further processing.

package main

import (
	"sync"
)

type Session struct {
	writeMu sync.RWMutex
    closed bool
}

func (s *Session) Write(input str) {
	if s.closed {
		return
	}

	isClose := input == "close"

	var locker sync.Locker
	if isClose {
		locker = &s.writeMu
	} else {
		locker = s.writeMu.RLocker()
	}
	locker.Lock()
	defer locker.Unlock()

    if s.closed {
		return
	}

    if isClose {
        s.closed = true
        return
    } else {
        // ...
        // Process input
        // ...
    } 
}

This works well but is not compatible with synctest. Which is unfortunate because in my tests I verify application behavior when processing takes extended periods of time and a bunch of timeouts are involved.

Can you suggest a rewrite that maintains performance characteristics while being compatible with synctest?