chore(clbot): Refactor backoff utility into a separate package.

It'll be reused by the IRC side of things too.

Change-Id: I3d84f3fd5fca6a6d948f331143b14f096d10675d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/342
Reviewed-by: tazjin <mail@tazj.in>

fun/clbot/backoffutil/backoffutil.go

@@ -0,0 +1,43 @@
+// Package backoffutil provides useful utilities for backoff.
+package backoffutil
+
+import (
+	"time"
+
+	backoff "github.com/cenkalti/backoff/v4"
+)
+
+// ZeroStartingBackOff is a backoff.BackOff that returns "0" as the first Duration after a reset.
+// This is useful for constructing loops and just enforcing a backoff duration on every loop, rather than incorporating this logic into the loop directly.
+type ZeroStartingBackOff struct {
+	bo      backoff.BackOff
+	initial bool
+}
+
+// NewZeroStartingBackOff creates a new ZeroStartingBackOff.
+func NewZeroStartingBackOff(bo backoff.BackOff) *ZeroStartingBackOff {
+	return &ZeroStartingBackOff{bo: bo, initial: true}
+}
+
+// NewDefaultBackOff creates a sensibly configured BackOff that starts at zero.
+func NewDefaultBackOff() backoff.BackOff {
+	ebo := backoff.NewExponentialBackOff()
+	ebo.MaxElapsedTime = 0
+	return NewZeroStartingBackOff(ebo)
+}
+
+// NextBackOff returns the next back off duration to use.
+// For the first call after a call to Reset(), this is 0. For each subsequent duration, the underlying BackOff is consulted.
+func (bo *ZeroStartingBackOff) NextBackOff() time.Duration {
+	if bo.initial == true {
+		bo.initial = false
+		return 0
+	}
+	return bo.bo.NextBackOff()
+}
+
+// Reset resets to the initial state, and also passes a Reset through to the underlying BackOff.
+func (bo *ZeroStartingBackOff) Reset() {
+	bo.initial = true
+	bo.bo.Reset()
+}