config: Add api requests max & deadline configs (#9273)

Add two new configuration entries, api.requests-max and
api.requests-deadline which have the same role of
MINIO_API_REQUESTS_MAX and MINIO_API_REQUESTS_DEADLINE.

cmd/throttling.go

@@ -0,0 +1,80 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 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"
+	"sync"
+	"time"
+)
+
+type apiThrottling struct {
+	mu      sync.RWMutex
+	enabled bool
+
+	requestsDeadline time.Duration
+	requestsPool     chan struct{}
+}
+
+func (t *apiThrottling) init(max int, deadline time.Duration) {
+	if max <= 0 {
+		return
+	}
+
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	t.requestsPool = make(chan struct{}, max)
+	t.requestsDeadline = deadline
+	t.enabled = true
+}
+
+func (t *apiThrottling) get() (chan struct{}, <-chan time.Time) {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+
+	if !t.enabled {
+		return nil, nil
+	}
+
+	return t.requestsPool, time.NewTimer(t.requestsDeadline).C
+}
+
+// maxClients throttles the S3 API calls
+func maxClients(f http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		pool, deadlineTimer := globalAPIThrottling.get()
+		if pool == nil {
+			f.ServeHTTP(w, r)
+			return
+		}
+
+		select {
+		case pool <- struct{}{}:
+			defer func() { <-pool }()
+			f.ServeHTTP(w, r)
+		case <-deadlineTimer:
+			// Send a http timeout message
+			writeErrorResponse(r.Context(), w,
+				errorCodes.ToAPIErr(ErrOperationMaxedOut),
+				r.URL, guessIsBrowserReq(r))
+			return
+		case <-r.Context().Done():
+			return
+		}
+	}
+}