limit number of open connections

Refs #1740

also, expose limit option for number concurrent streams for gRPC
(unlimited by default)

rpc/lib/server/http_server_test.go

@@ -0,0 +1,62 @@
+package rpcserver
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/tendermint/tmlibs/log"
+)
+
+func TestMaxOpenConnections(t *testing.T) {
+	const max = 5 // max simultaneous connections
+
+	// Start the server.
+	var open int32
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if n := atomic.AddInt32(&open, 1); n > int32(max) {
+			t.Errorf("%d open connections, want <= %d", n, max)
+		}
+		defer atomic.AddInt32(&open, -1)
+		time.Sleep(10 * time.Millisecond)
+		fmt.Fprint(w, "some body")
+	})
+	l, err := StartHTTPServer("tcp://127.0.0.1:0", mux, log.TestingLogger(), Config{MaxOpenConnections: max})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer l.Close()
+
+	// Make N GET calls to the server.
+	attempts := max * 2
+	var wg sync.WaitGroup
+	var failed int32
+	for i := 0; i < attempts; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			c := http.Client{Timeout: 3 * time.Second}
+			r, err := c.Get("http://" + l.Addr().String())
+			if err != nil {
+				t.Log(err)
+				atomic.AddInt32(&failed, 1)
+				return
+			}
+			defer r.Body.Close()
+			io.Copy(ioutil.Discard, r.Body)
+		}()
+	}
+	wg.Wait()
+
+	// We expect some Gets to fail as the server's accept queue is filled,
+	// but most should succeed.
+	if int(failed) >= attempts/2 {
+		t.Errorf("%d requests failed within %d attempts", failed, attempts)
+	}
+}