RWMutex in Go is not working as expected - caching

Question

I have written following sample program using sync.RWMutex.
package main
import (
"fmt"
"sync"
"time"
)
// SessionData : capture session id and cc-request-number
type SessionData struct {
id string
reqNo string
}
// SessionCache : cache for the SessionData
type SessionCache struct {
sess map[SessionData]bool
sync.RWMutex
}
// InitSessionCache : Init for SessionCache
func InitSessionCache() SessionCache {
return SessionCache{sess: make(map[SessionData]bool)}
}
// Read : read value from session cache
func (s *SessionCache) Read(sd SessionData) bool {
s.RLock()
defer s.RUnlock()
_, found := s.sess[sd]
return found
}
func (s *SessionCache) Write(sd SessionData) {
s.Lock()
defer s.Unlock()
fmt.Println("Entry not found for ", sd.id, sd.reqNo, "Creating the entry now")
s.sess[sd] = true
}
func (s *SessionCache) chkDuplicate(sessionID string, Reqno string) bool {
sd := SessionData{
id: sessionID,
reqNo: Reqno,
}
found := s.Read(sd)
if !found {
s.Write(sd)
return found
}
return found
}
func main() {
mySessionData := InitSessionCache()
for i := 0; i < 10; i++ {
go mySessionData.chkDuplicate("session1", "1")
go mySessionData.chkDuplicate("session1", "1")
go mySessionData.chkDuplicate("session1", "2")
go mySessionData.chkDuplicate("session1", "2")
go mySessionData.chkDuplicate("session1", "4")
go mySessionData.chkDuplicate("session1", "2")
}
time.Sleep(300)
fmt.Println(mySessionData)
}
when I run this program in playground https://play.golang.org/p/g93UtVxZ2dl
I see that it is working correctly as the write happens only 3 times for the unique values.
Entry not found for session1 1 Creating the entry now
Entry not found for session1 2 Creating the entry now
Entry not found for session1 4 Creating the entry now
{map[{session1 1}:true {session1 2}:true {session1 4}:true] {{0 0} 0 0 0 0}}
however when I run the same program from my windows 10 machine (on VS Code) I see following output.
Entry not found for session1 1 Creating the entry now
Entry not found for session1 2 Creating the entry now
Entry not found for session1 2 Creating the entry now
Entry not found for session1 2 Creating the entry now
Entry not found for session1 4 Creating the entry now
{map[{session1 1}:true {session1 2}:true {session1 4}:true] {{0 0} 0 0 0 0}}
Am I doing something wrong?
Why does this behaves differently on my Machine and Playground?

Answer 1

There is no syncronisation between the call to Read and Write. All your goroutines are running concurrently, imagine if they all run up to this line and then yield to another goroutine:
found := s.Read(sd)
They will all return false because none of the goroutines have moved past this point. Now they all move on to the next line and believe that found == false, so all perform the s.Write(sd).
You need to perform the Read and Write without unlocking. Maybe something like:
func (s *SessionCache) TryWrite(sd SessionData) err {
s.Lock()
defer s.Unlock()
if _, found := s.sess[sd]; found {
return fmt.Errorf("Entry already exists")
}
s.sess[sd] = true
}