client: add fuzzy timer

For randomizing update check intervals and backoff delays to reduce
chance of DoSing the server if lots of clients start together.

omaha/client/fuzzytime.go

@@ -0,0 +1,53 @@
+// Copyright 2017 CoreOS, 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 client
+
+import (
+	"math/rand"
+	"time"
+)
+
+func init() {
+	// Ensure seeding the prng is never forgotten, that would defeat
+	// the whole point of using fuzzy timers to guard against a DoS.
+	rand.Seed(time.Now().UnixNano())
+}
+
+// FuzzyDuration randomizes the duration d within the range specified
+// by fuzz. Specifically the value range is: [d-(fuzz/2), d+(fuzz/2)]
+// The result will never be negative.
+func FuzzyDuration(d, fuzz time.Duration) time.Duration {
+	if fuzz < 0 {
+		return d
+	}
+	// apply range [-fuzz/2, fuzz/2]
+	d += time.Duration(rand.Int63n(int64(fuzz)+1) - (int64(fuzz) / 2))
+	if d < 0 {
+		return 0
+	}
+	return d
+}
+
+// FuzzyAfter waits for the fuzzy duration to elapse and then sends the
+// current time on the returned channel. See FuzzyDuration.
+func FuzzyAfter(d, fuzz time.Duration) <-chan time.Time {
+	return time.After(FuzzyDuration(d, fuzz))
+}
+
+// FuzzySleep pauses the current goroutine for the fuzzy duration d.
+// See FuzzyDuration.
+func FuzzySleep(d, fuzz time.Duration) {
+	time.Sleep(FuzzyDuration(d, fuzz))
+}

omaha/client/fuzzytime_test.go

@@ -0,0 +1,32 @@
+// Copyright 2017 CoreOS, 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 client
+
+import (
+	"testing"
+	"time"
+)
+
+func TestFuzzyDuration(t *testing.T) {
+	const d = time.Minute
+	for i := 0; i < 1000; i++ {
+		f := FuzzyDuration(d, d)
+		if f < d/2 {
+			t.Errorf("%d < %d", f, d/2)
+		} else if f > d+d/2 {
+			t.Errorf("%d > %d", f, d+d/2)
+		}
+	}
+}