feat: Async refresh dry run in parallel with sync refresh

internal/settings.go

@@ -70,8 +70,9 @@ type DialSettings struct {
 	RequestReason string
 
 	// New Auth library Options
-	AuthCredentials      *auth.Credentials
-	EnableNewAuthLibrary bool
+	AuthCredentials          *auth.Credentials
+	EnableNewAuthLibrary     bool
+	EnableAsyncRefreshDryRun func()
 }
 
 // GetScopes returns the user-provided scopes, if set, or else falls back to the

option/internaloption/internaloption.go

@@ -209,6 +209,27 @@ func (w enableNewAuthLibrary) Apply(o *internal.DialSettings) {
 	o.EnableNewAuthLibrary = bool(w)
 }
 
+// EnableAsyncRefreshDryRun  returns a ClientOption that specifies if libraries in this
+// module should asynchronously refresh auth token in parallel to sync refresh
+//
+// errHandler function will be called when there is an error while refreshing
+// the token asynchronously
+//
+// This is an experimental option and will be removed in the future
+func EnableAsyncRefreshDryRun(errHandler func()) option.ClientOption {
+	return enableAsyncRefreshDryRun{
+		errHandler: errHandler,
+	}
+}
+
+type enableAsyncRefreshDryRun struct {
+	errHandler func()
+}
+
+func (w enableAsyncRefreshDryRun) Apply(o *internal.DialSettings) {
+	o.EnableAsyncRefreshDryRun = w.errHandler
+}
+
 // EmbeddableAdapter is a no-op option.ClientOption that allow libraries to
 // create their own client options by embedding this type into their own
 // client-specific option wrapper. See example for usage.

transport/grpc/dial.go

@@ -262,6 +262,37 @@ func prepareDialOptsNewAuth(ds *internal.DialSettings) []grpc.DialOption {
 	return append(opts, ds.GRPCDialOpts...)
 }
 
+// dryRunAsync is a wrapper for oauth2.TokenSource that performs a sync refresh
+// after an async refresh. Token generated by async refresh is not used
+//
+// This is an experimental feature and may be removed or changed in the future.
+// It is a temporary struct to determine if the async refresh
+// is working properly.
+type dryRunAsync struct {
+	asyncTokenSource oauth2.TokenSource
+	syncTokenSource  oauth2.TokenSource
+	errHandler       func()
+}
+
+func newDryRunAsync(ts oauth2.TokenSource, errHandler func()) dryRunAsync {
+	tp := auth.NewCachedTokenProvider(oauth2adapt.TokenProviderFromTokenSource(ts), nil)
+	asyncTs := oauth2adapt.TokenSourceFromTokenProvider(tp)
+	return dryRunAsync{
+		syncTokenSource:  ts,
+		asyncTokenSource: asyncTs,
+		errHandler:       errHandler,
+	}
+}
+
+// Token returns a token or an error.
+func (async dryRunAsync) Token() (*oauth2.Token, error) {
+	_, err := async.asyncTokenSource.Token()
+	if err != nil {
+		async.errHandler()
+	}
+	return async.syncTokenSource.Token()
+}
+
 func dial(ctx context.Context, insecure bool, o *internal.DialSettings) (*grpc.ClientConn, error) {
 	if o.HTTPClient != nil {
 		return nil, errors.New("unsupported HTTP client specified")
@@ -298,8 +329,13 @@ func dial(ctx context.Context, insecure bool, o *internal.DialSettings) (*grpc.C
 			if err != nil {
 				return nil, err
 			}
+
+			ts := creds.TokenSource
+			if o.EnableAsyncRefreshDryRun != nil {
+				ts = newDryRunAsync(ts, o.EnableAsyncRefreshDryRun)
+			}
 			grpcOpts = append(grpcOpts, grpc.WithPerRPCCredentials(grpcTokenSource{
-				TokenSource:   oauth.TokenSource{TokenSource: creds.TokenSource},
+				TokenSource:   oauth.TokenSource{TokenSource: ts},
 				quotaProject:  internal.GetQuotaProject(creds, o.QuotaProject),
 				requestReason: o.RequestReason,
 			}))