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-router.go

@@ -0,0 +1,43 @@
+/*
+ * 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 (
+	"net/http"
+
+	router "github.com/gorilla/mux"
+)
+
+const (
+	healthCheckPath          = "/health"
+	healthCheckLivenessPath  = "/live"
+	healthCheckReadinessPath = "/ready"
+	healthCheckPathPrefix    = minioReservedBucketPath + healthCheckPath
+)
+
+// registerHealthCheckRouter - add handler functions for liveness and readiness routes.
+func registerHealthCheckRouter(mux *router.Router) {
+
+	// Healthcheck router
+	healthRouter := mux.NewRoute().PathPrefix(healthCheckPathPrefix).Subrouter()
+
+	// Liveness handler
+	healthRouter.Methods(http.MethodGet).Path(healthCheckLivenessPath).HandlerFunc(LivenessCheckHandler)
+
+	// Readiness handler
+	healthRouter.Methods(http.MethodGet).Path(healthCheckReadinessPath).HandlerFunc(ReadinessCheckHandler)
+}