backoff: set default values on BackoffConfig

Because most of the fields on `BackoffConfig` are unexported, correctly
using the config requires copying from the default. This sets the
defaults appropriately and falls back to a default if MaxDelay is
negative or zero.

Tests are added to ensure that the backoff is set correctly in common
use cases.

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

clientconn_test.go

@@ -80,3 +80,42 @@ func TestCredentialsMisuse(t *testing.T) {
 		t.Fatalf("Dial(_, _) = _, %v, want _, %v", err, ErrCredentialsMisuse)
 	}
 }
+
+func TestWithBackoffConfigDefault(t *testing.T) {
+	testBackoffConfigSet(t, DefaultBackoffConfig)
+}
+
+func TestWithBackoffConfig(t *testing.T) {
+	b := BackoffConfig{MaxDelay: DefaultBackoffConfig.MaxDelay / 2}
+	expected := b
+	expected.setDefaults() // defaults should be set
+	testBackoffConfigSet(t, &expected, WithBackoffConfig(&b))
+}
+
+func TestWithBackoffMaxDelay(t *testing.T) {
+	md := DefaultBackoffConfig.MaxDelay / 2
+	expected := BackoffConfig{MaxDelay: md}
+	expected.setDefaults()
+	testBackoffConfigSet(t, &expected, WithBackoffMaxDelay(md))
+}
+
+func testBackoffConfigSet(t *testing.T, expected *BackoffConfig, opts ...DialOption) {
+	opts = append(opts, WithInsecure())
+	conn, err := Dial("foo:80", opts...)
+	if err != nil {
+		t.Fatalf("unexpected error dialing connection: %v", err)
+	}
+
+	if conn.dopts.bs == nil {
+		t.Fatalf("backoff config not set")
+	}
+
+	actual, ok := conn.dopts.bs.(*BackoffConfig)
+	if !ok {
+		t.Fatalf("unexpected type of backoff config: %v", conn.dopts.bs)
+	}
+
+	if *actual != *expected {
+		t.Fatalf("unexpected backoff config on connection: %v, want %v", actual, expected)
+	}
+}