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>

clientconn.go

@@ -75,6 +75,7 @@ type dialOptions struct {
 	codec    Codec
 	cp       Compressor
 	dc       Decompressor
+	bs       backoffStrategy
 	picker   Picker
 	block    bool
 	insecure bool
@@ -114,6 +115,22 @@ func WithPicker(p Picker) DialOption {
 	}
 }
 
+// WithBackoffConfig configures the dialer to use the provided backoff
+// parameters after connection failures.
+func WithBackoffConfig(b *BackoffConfig) DialOption {
+	return withBackoff(b)
+}
+
+// withBackoff sets the backoff strategy used for retries after a
+// failed connection attempt.
+//
+// This can be exported if arbitrary backoff strategies are allowed by GRPC.
+func withBackoff(bs backoffStrategy) DialOption {
+	return func(o *dialOptions) {
+		o.bs = bs
+	}
+}
+
 // WithBlock returns a DialOption which makes caller of Dial blocks until the underlying
 // connection is up. Without this, Dial returns immediately and connecting the server
 // happens in background.
@@ -180,6 +197,11 @@ func Dial(target string, opts ...DialOption) (*ClientConn, error) {
 		// Set the default codec.
 		cc.dopts.codec = protoCodec{}
 	}
+
+	if cc.dopts.bs == nil {
+		cc.dopts.bs = DefaultBackoffConfig
+	}
+
 	if cc.dopts.picker == nil {
 		cc.dopts.picker = &unicastPicker{
 			target: target,
@@ -416,7 +438,7 @@ func (cc *Conn) resetTransport(closeTransport bool) error {
 				return ErrClientConnTimeout
 			}
 		}
-		sleepTime := backoff(retries)
+		sleepTime := cc.dopts.bs.backoff(retries)
 		timeout := sleepTime
 		if timeout < minConnectTimeout {
 			timeout = minConnectTimeout