Add roundrobin picker.

balancer/picker/roundrobin.go

@@ -0,0 +1,54 @@
+// Copyright 2023 Buf Technologies, 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 picker
+
+import (
+	"math/rand"
+	"net/http"
+	"sync/atomic"
+
+	"github.com/bufbuild/go-http-balancer/balancer/conn"
+)
+
+type roundRobinFactory struct{}
+
+type roundRobin struct {
+	conns []conn.Conn
+	// +checkatomic
+	counter atomic.Uint64
+}
+
+// NewRoundRobinFactory creates a picker factory that picks connections in a
+// "round-robin" fashion, that is to say, in sequential order. In order to
+// mitigate the risk of a "thundering herd" scenario, the order of connections
+// is randomized each time the list of hosts changes.
+func NewRoundRobinFactory() Factory {
+	return &roundRobinFactory{}
+}
+
+func (f roundRobinFactory) New(_ Picker, allConns conn.Connections) Picker {
+	numConns := allConns.Len()
+	conns := make([]conn.Conn, numConns)
+	for i := 0; i < numConns; i++ {
+		j := rand.Intn(i + 1) //nolint:gosec
+		conns[i] = conns[j]
+		conns[j] = allConns.Get(i)
+	}
+	return &roundRobin{conns: conns}
+}
+
+func (r *roundRobin) Pick(_ *http.Request) (conn conn.Conn, whenDone func(), err error) {
+	return r.conns[r.counter.Add(1)%uint64(len(r.conns))], nil, nil
+}

balancer/picker/roundrobin_test.go

@@ -0,0 +1,42 @@
+// Copyright 2023 Buf Technologies, 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 picker_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/bufbuild/go-http-balancer/balancer/conn"
+	"github.com/bufbuild/go-http-balancer/balancer/picker"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRoundRobinPicker(t *testing.T) {
+	t.Parallel()
+
+	// TODO: when possible, it'd be good to test multiple connections to verify
+	// that shuffling works and that we get round-robin behavior.
+
+	dummyConn := conn.Conn(nil)
+	allConns := dummyConns{[]conn.Conn{
+		dummyConn,
+	}}
+
+	factory := picker.NewRoundRobinFactory()
+	picker := factory.New(nil, allConns)
+	conn, _, err := picker.Pick(&http.Request{})
+	assert.NoError(t, err)
+	assert.Equal(t, dummyConn, conn)
+}