internal/retry: Never return nil from GRPCStatus()

An error's GRPCStatus() method should never returns nil, which maps to a status with
Code.OK (https://github.com/grpc/grpc-go/blob/642dd63a85275a96d172f446911fd04111e2c74c/internal/status/status.go#L71-L76). Instead
it should return an actual error code, such as Status.Unknown.

This addresses #8127.

internal/retry.go

@@ -17,6 +17,7 @@ package internal
 import (
 	"context"
 	"fmt"
+	"google.golang.org/grpc/codes"
 	"time"
 
 	gax "github.com/googleapis/gax-go/v2"
@@ -81,5 +82,5 @@ func (e wrappedCallErr) GRPCStatus() *status.Status {
 	if s, ok := status.FromError(e.wrappedErr); ok {
 		return s
 	}
-	return nil
+	return status.New(codes.Unknown, e.Error())
 }

internal/retry_test.go

@@ -93,3 +93,29 @@ func TestRetryPreserveError(t *testing.T) {
 		t.Errorf("got message %q, want %q", g, w)
 	}
 }
+
+func TestRetryWrapsErrorWithStatusUnknown(t *testing.T) {
+	// When retrying on an error that is not a grpc error, make sure to return
+	// a valid gRPC status.
+	err := retry(context.Background(), gax.Backoff{},
+		func() (bool, error) {
+			return false, errors.New("test error")
+		},
+		func(context.Context, time.Duration) error {
+			return context.DeadlineExceeded
+		})
+	if err == nil {
+		t.Fatalf("unexpectedly got nil error")
+	}
+	wantError := "retry failed with context deadline exceeded; last error: test error"
+	if g, w := err.Error(), wantError; g != w {
+		t.Errorf("got error %q, want %q", g, w)
+	}
+	got, ok := status.FromError(err)
+	if !ok {
+		t.Fatal("expected error to implement a gRPC status")
+	}
+	if g, w := got.Code(), codes.Unknown; g != w {
+		t.Errorf("got code %v, want %v", g, w)
+	}
+}