Add initial load balancer (#487)

* lb: library for creation of load balancer

* lb: library for creation of load balancer

* Add balance subcommand to fn

* make fnlb its own command

* Update Changelogg

* Add Makefile for fnlb

lb/lb.go

@@ -0,0 +1,52 @@
+package lb
+
+import (
+	"bytes"
+	"context"
+	"net/http"
+	"net/http/httputil"
+	"strconv"
+	"sync"
+	"sync/atomic"
+
+	"github.com/golang/groupcache/consistenthash"
+)
+
+// ConsistentHashReverseProxy returns a new ReverseProxy that routes
+// URLs to the scheme, host, and base path provided in by a consistent hash
+// algorithm. If the target's path is "/base" and the incoming request was for
+// "/dir", the target request will be for /base/dir.
+// ConsistentHashReverseProxy does not rewrite the Host header.
+func ConsistentHashReverseProxy(ctx context.Context, nodes []string) *httputil.ReverseProxy {
+	ch := consistenthash.New(len(nodes), nil)
+	ch.Add(nodes...)
+
+	bufPool := sync.Pool{
+		New: func() interface{} {
+			return new(bytes.Buffer)
+		},
+	}
+
+	var i int64
+	director := func(req *http.Request) {
+		buf := bufPool.Get().(*bytes.Buffer)
+		defer bufPool.Put(buf)
+		buf.Reset()
+		buf.WriteString(req.URL.Path)
+		buf.WriteString("??")
+		b := strconv.AppendInt(buf.Bytes(), atomic.AddInt64(&i, 1), 10)
+
+		target := ch.Get(string(b))
+		req.URL.Scheme = "http"
+		req.URL.Host = target
+		if _, ok := req.Header["User-Agent"]; !ok {
+			// explicitly disable User-Agent so it's not set to default value
+			req.Header.Set("User-Agent", "")
+		}
+	}
+
+	return &httputil.ReverseProxy{
+		Director:  director,
+		Transport: NewRoundTripper(ctx, nodes),
+	}
+}