backoff: allow configuration of backoff strategy

The backoff function and parameters have been pulled up into an
interface `backoffStrategy`. The default parameters are now part of a
package variable `DefaultBackoffConfig`. The strategy is then plumbed
through `dialOptions`. As a result of this PR, the
maximum backoff delay can now be set using the `WithBackoffConfig` dial
option.

While the addition of strategy may seem premature, this allows one to
simply export `BackoffStrategy` and `WithBackoff` to allow arbirarily
configurable backoff strategies.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

rpc_util.go

@@ -41,9 +41,7 @@ import (
 	"io"
 	"io/ioutil"
 	"math"
-	"math/rand"
 	"os"
-	"time"
 
 	"github.com/golang/protobuf/proto"
 	"golang.org/x/net/context"
@@ -411,38 +409,6 @@ func convertCode(err error) codes.Code {
 	return codes.Unknown
 }
 
-const (
-	// how long to wait after the first failure before retrying
-	baseDelay = 1.0 * time.Second
-	// upper bound of backoff delay
-	maxDelay = 120 * time.Second
-	// backoff increases by this factor on each retry
-	backoffFactor = 1.6
-	// backoff is randomized downwards by this factor
-	backoffJitter = 0.2
-)
-
-func backoff(retries int) (t time.Duration) {
-	if retries == 0 {
-		return baseDelay
-	}
-	backoff, max := float64(baseDelay), float64(maxDelay)
-	for backoff < max && retries > 0 {
-		backoff *= backoffFactor
-		retries--
-	}
-	if backoff > max {
-		backoff = max
-	}
-	// Randomize backoff delays so that if a cluster of requests start at
-	// the same time, they won't operate in lockstep.
-	backoff *= 1 + backoffJitter*(rand.Float64()*2-1)
-	if backoff < 0 {
-		return 0
-	}
-	return time.Duration(backoff)
-}
-
 // SupportPackageIsVersion1 is referenced from generated protocol buffer files
 // to assert that that code is compatible with this version of the grpc package.
 //