Vendor dependencies for GCS

vendor/cloud.google.com/go/spanner/backoff.go

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+/*
+Copyright 2017 Google Inc. All Rights Reserved.
+
+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 spanner
+
+import (
+	"math/rand"
+	"time"
+)
+
+const (
+	// minBackoff is the minimum backoff used by default.
+	minBackoff = 1 * time.Second
+	// maxBackoff is the maximum backoff used by default.
+	maxBackoff = 32 * time.Second
+	// jitter is the jitter factor.
+	jitter = 0.4
+	// rate is the rate of exponential increase in the backoff.
+	rate = 1.3
+)
+
+var defaultBackoff = exponentialBackoff{minBackoff, maxBackoff}
+
+type exponentialBackoff struct {
+	min, max time.Duration
+}
+
+// delay calculates the delay that should happen at n-th
+// exponential backoff in a series.
+func (b exponentialBackoff) delay(retries int) time.Duration {
+	min, max := float64(b.min), float64(b.max)
+	delay := min
+	for delay < max && retries > 0 {
+		delay *= rate
+		retries--
+	}
+	if delay > max {
+		delay = max
+	}
+	delay -= delay * jitter * rand.Float64()
+	if delay < min {
+		delay = min
+	}
+	return time.Duration(delay)
+}