Add admin API to send trace notifications to registered (#7128)

Remove current functionality to log trace to file
using MINIO_HTTP_TRACE env, and replace it with
mc admin trace command on mc client.

pkg/pubsub/pubsub.go

@@ -0,0 +1,83 @@
+/*
+ * 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 pubsub
+
+import (
+	"sync"
+)
+
+// PubSub holds publishers and subscribers
+type PubSub struct {
+	subs  []chan interface{}
+	pub   chan interface{}
+	mutex sync.Mutex
+}
+
+// process item to subscribers.
+func (ps *PubSub) process() {
+	for item := range ps.pub {
+		ps.mutex.Lock()
+		for _, sub := range ps.subs {
+			go func(s chan interface{}) {
+				s <- item
+			}(sub)
+		}
+		ps.mutex.Unlock()
+	}
+}
+
+// Publish message to pubsub system
+func (ps *PubSub) Publish(item interface{}) {
+	ps.pub <- item
+}
+
+// Subscribe - Adds a subscriber to pubsub system
+func (ps *PubSub) Subscribe() chan interface{} {
+	ps.mutex.Lock()
+	defer ps.mutex.Unlock()
+	ch := make(chan interface{})
+	ps.subs = append(ps.subs, ch)
+	return ch
+}
+
+// Unsubscribe removes current subscriber
+func (ps *PubSub) Unsubscribe(ch chan interface{}) {
+	ps.mutex.Lock()
+	defer ps.mutex.Unlock()
+
+	for i, sub := range ps.subs {
+		if sub == ch {
+			close(ch)
+			ps.subs = append(ps.subs[:i], ps.subs[i+1:]...)
+		}
+	}
+}
+
+// HasSubscribers returns true if pubsub system has subscribers
+func (ps *PubSub) HasSubscribers() bool {
+	ps.mutex.Lock()
+	defer ps.mutex.Unlock()
+	return len(ps.subs) > 0
+}
+
+// New inits a PubSub system
+func New() *PubSub {
+	ps := &PubSub{}
+	ps.pub = make(chan interface{})
+	go ps.process()
+	return ps
+}