Add healthcheck endpoints (#5543)

This PR adds readiness and liveness endpoints to probe Minio server
instance health. Endpoints can only be accessed without authentication
and the paths are /minio/health/live and /minio/health/ready for
liveness and readiness respectively.

The new healthcheck liveness endpoint is used for Docker healthcheck
now.

Fixes #5357
Fixes #5514

cmd/healthcheck-handler_test.go

@@ -0,0 +1,44 @@
+/*
+ * Minio Cloud Storage, (C) 2018 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 cmd
+
+import (
+	"testing"
+	"time"
+)
+
+func TestGoroutineCountCheck(t *testing.T) {
+	tests := []struct {
+		threshold int
+		wantErr   bool
+	}{
+		{5000, false},
+		{5, true},
+		{6, true},
+	}
+	for _, tt := range tests {
+		// Make goroutines -- to make sure number of go-routines is higher than threshold
+		if tt.threshold == 5 || tt.threshold == 6 {
+			for i := 0; i < 6; i++ {
+				go time.Sleep(5)
+			}
+		}
+		if err := goroutineCountCheck(tt.threshold); (err != nil) != tt.wantErr {
+			t.Errorf("goroutineCountCheck() error = %v, wantErr %v", err, tt.wantErr)
+		}
+	}
+}