Event persistence for MQTT (#7268)

- The events will be persisted in queueStore if `queueDir` is set.
- Else, if queueDir is not set events persist in memory.

The events are replayed back when the mqtt broker is back online.

pkg/event/target/memorystore.go

@@ -0,0 +1,104 @@
+/*
+ * Minio Cloud Storage, (C) 2019 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package target
+
+import (
+	"sync"
+
+	"github.com/minio/minio/pkg/event"
+)
+
+const (
+	maxStoreLimit = 10000
+)
+
+// MemoryStore persists events in memory.
+type MemoryStore struct {
+	sync.RWMutex
+	events map[string]event.Event
+	eC     uint16
+	limit  uint16
+}
+
+// NewMemoryStore creates a memory store instance.
+func NewMemoryStore(limit uint16) *MemoryStore {
+	if limit == 0 || limit > maxStoreLimit {
+		limit = maxStoreLimit
+	}
+	memoryStore := &MemoryStore{
+		events: make(map[string]event.Event),
+		limit:  limit,
+	}
+	return memoryStore
+}
+
+// Open is in-effective here.
+// Implemented for interface compatibility.
+func (store *MemoryStore) Open() error {
+	return nil
+}
+
+// Put - puts the event in store.
+func (store *MemoryStore) Put(e event.Event) error {
+	store.Lock()
+	defer store.Unlock()
+	if store.eC == store.limit {
+		return ErrLimitExceeded
+	}
+	key, kErr := getNewUUID()
+	if kErr != nil {
+		return kErr
+	}
+	store.events[key] = e
+	store.eC++
+	return nil
+}
+
+// Get - retrieves the event from store.
+func (store *MemoryStore) Get(key string) (event.Event, error) {
+	store.RLock()
+	defer store.RUnlock()
+
+	if event, exist := store.events[key]; exist {
+		return event, nil
+	}
+
+	return event.Event{}, ErrNoSuchKey
+}
+
+// Del - deletes the event from store.
+func (store *MemoryStore) Del(key string) {
+	store.Lock()
+	defer store.Unlock()
+
+	delete(store.events, key)
+
+	store.eC--
+}
+
+// ListAll - lists all the keys in the store.
+func (store *MemoryStore) ListAll() []string {
+	store.RLock()
+	defer store.RUnlock()
+
+	keys := []string{}
+	for k := range store.events {
+		keys = append(keys, k)
+	}
+
+	return keys
+}