Rate limit is working and supports limited waiting clients (#2295)

rate-limit-handler.go

@@ -17,36 +17,64 @@
 package main
 
 import (
+	"errors"
 	"net/http"
 	"sync"
 )
 
+var errTooManyRequests = errors.New("Too many clients in the waiting list")
+
 // rateLimit - represents datatype of the functionality implemented to
 // limit the number of concurrent http requests.
 type rateLimit struct {
-	handler     http.Handler
-	rqueue      chan struct{}
-	releaseOnce sync.Once
+	handler   http.Handler
+	lock      sync.Mutex
+	workQueue chan struct{}
+	waitQueue chan struct{}
 }
 
 // acquire and release implement a way to send and receive from the
 // channel this is in-turn used to rate limit incoming connections in
 // ServeHTTP() http.Handler method.
-func (c *rateLimit) acquire() { c.rqueue <- struct{}{} }
-func (c *rateLimit) release() { <-c.rqueue }
+func (c *rateLimit) acquire() error {
+	// Kick out clients when it is really crowded
+	if len(c.waitQueue) == cap(c.waitQueue) {
+		return errTooManyRequests
+	}
+
+	// Add new element in waitQueue to keep track of clients
+	// wanting to process their requests
+	c.waitQueue <- struct{}{}
+
+	// Moving from wait to work queue is protected by a mutex
+	// to avoid draining waitQueue with multiple simultaneous clients.
+	c.lock.Lock()
+	c.workQueue <- <-c.waitQueue
+	c.lock.Unlock()
+	return nil
+}
+
+// Release one element from workQueue to serve a new client
+// in the waiting list
+func (c *rateLimit) release() {
+	<-c.workQueue
+}
 
 // ServeHTTP is an http.Handler ServeHTTP method, implemented to rate
 // limit incoming HTTP requests.
 func (c *rateLimit) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// Acquire the connection if queue is not full, otherwise
 	// code path waits here until the previous case is true.
-	c.acquire()
+	if err := c.acquire(); err != nil {
+		w.WriteHeader(http.StatusTooManyRequests)
+		return
+	}
 
 	// Serves the request.
 	c.handler.ServeHTTP(w, r)
 
-	// Release by draining the channel once.
-	c.releaseOnce.Do(c.release)
+	// Release
+	c.release()
 }
 
 // setRateLimitHandler limits the number of concurrent http requests based on MINIO_MAXCONN.
@@ -57,7 +85,8 @@ func setRateLimitHandler(handler http.Handler) http.Handler {
 
 	// For max connection limit of > '0' we initialize rate limit handler.
 	return &rateLimit{
-		handler: handler,
-		rqueue:  make(chan struct{}, globalMaxConn),
+		handler:   handler,
+		workQueue: make(chan struct{}, globalMaxConn),
+		waitQueue: make(chan struct{}, globalMaxConn*4),
 	}
 }