MM-30863: Fix race in LRU (#16382)

* MM-30863: Fix race in LRU

After shortening the critical section, we missed out the fact
that the byte slice is still accessible after the element is returned.
So the lock needs to be active until the byte slice is fully read and
unmarshaled

https://mattermost.atlassian.net/browse/MM-30863

```release-note
NONE
```

* incorporate suggestions

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

services/cache/lru.go

@@ -185,14 +185,14 @@ func (l *LRU) set(key string, value interface{}, ttl time.Duration) error {
 }
 
 func (l *LRU) get(key string, value interface{}) error {
-	e, err := l.getItem(key)
+	val, err := l.getItem(key)
 	if err != nil {
 		return err
 	}
 
 	// We use a fast path for hot structs.
 	if msgpVal, ok := value.(msgp.Unmarshaler); ok {
-		_, err := msgpVal.UnmarshalMsg(e.value)
+		_, err := msgpVal.UnmarshalMsg(val)
 		return err
 	}
 
@@ -207,26 +207,26 @@ func (l *LRU) get(key string, value interface{}) error {
 	switch v := value.(type) {
 	case **model.User:
 		var u model.User
-		_, err := u.UnmarshalMsg(e.value)
+		_, err := u.UnmarshalMsg(val)
 		*v = &u
 		return err
 	case **model.Session:
 		var s model.Session
-		_, err := s.UnmarshalMsg(e.value)
+		_, err := s.UnmarshalMsg(val)
 		*v = &s
 		return err
 	case *map[string]*model.User:
 		var u model.UserMap
-		_, err := u.UnmarshalMsg(e.value)
+		_, err := u.UnmarshalMsg(val)
 		*v = u
 		return err
 	}
 
 	// Slow path for other structs.
-	return msgpack.Unmarshal(e.value, value)
+	return msgpack.Unmarshal(val, value)
 }
 
-func (l *LRU) getItem(key string) (*entry, error) {
+func (l *LRU) getItem(key string) ([]byte, error) {
 	l.lock.Lock()
 	defer l.lock.Unlock()
 
@@ -240,7 +240,7 @@ func (l *LRU) getItem(key string) (*entry, error) {
 		return nil, ErrKeyNotFound
 	}
 	l.evictList.MoveToFront(ent)
-	return e, nil
+	return e.value, nil
 }
 
 func (l *LRU) removeElement(e *list.Element) {

services/cache/lru_test.go

@@ -5,6 +5,7 @@ package cache
 
 import (
 	"fmt"
+	"sync"
 	"testing"
 	"time"
 
@@ -618,3 +619,32 @@ func BenchmarkLRU(b *testing.B) {
 		}
 	})
 }
+
+func TestLRURace(t *testing.T) {
+	l2 := NewLRU(&LRUOptions{
+		Size:                   1,
+		DefaultExpiry:          0,
+		InvalidateClusterEvent: "",
+	})
+	var wg sync.WaitGroup
+	l2.Set("test", "value1")
+
+	wg.Add(2)
+
+	go func() {
+		defer wg.Done()
+		value1 := "simplestring"
+		err := l2.Set("test", value1)
+		require.Nil(t, err)
+	}()
+
+	go func() {
+		defer wg.Done()
+
+		var val string
+		err := l2.Get("test", &val)
+		require.Nil(t, err)
+	}()
+
+	wg.Wait()
+}