Revert "Remove unused code path in maybeFetchToken (#209)"

This reverts commit cab4a1d.

Author: petea

GoogleSignIn/Sources/GIDSignIn.m

@@ -629,7 +629,7 @@ - (void)processAuthorizationResponse:(OIDAuthorizationResponse *)authorizationRe
       authFlow.authState = [[OIDAuthState alloc]
           initWithAuthorizationResponse:authorizationResponse];
       // perform auth code exchange
-      [self maybeFetchToken:authFlow];
+      [self maybeFetchToken:authFlow fallback:nil];
     } else {
       // There was a failure, convert to appropriate error code.
       NSString *errorString;
@@ -701,14 +701,17 @@ - (void)authenticateWithOptions:(GIDSignInInternalOptions *)options {
   // Complete the auth flow using saved auth in keychain.
   GIDAuthFlow *authFlow = [[GIDAuthFlow alloc] init];
   authFlow.authState = authState;
-  [self maybeFetchToken:authFlow];
+  [self maybeFetchToken:authFlow fallback:options.interactive ? ^() {
+    [self authenticateInteractivelyWithOptions:options];
+  } : nil];
   [self addDecodeIdTokenCallback:authFlow];
   [self addSaveAuthCallback:authFlow];
   [self addCompletionCallback:authFlow];
 }
 
-// Fetches the access token if necessary as part of the auth flow.
-- (void)maybeFetchToken:(GIDAuthFlow *)authFlow {
+// Fetches the access token if necessary as part of the auth flow. If |fallback|
+// is provided, call it instead of continuing the auth flow in case of error.
+- (void)maybeFetchToken:(GIDAuthFlow *)authFlow fallback:(nullable void (^)(void))fallback {
   OIDAuthState *authState = authFlow.authState;
   // Do nothing if we have an auth flow error or a restored access token that isn't near expiration.
   if (authFlow.error ||
@@ -755,6 +758,14 @@ - (void)maybeFetchToken:(GIDAuthFlow *)authFlow {
     [authState updateWithTokenResponse:tokenResponse error:error];
     authFlow.error = error;
 
+    if (!tokenResponse.accessToken || error) {
+      if (fallback) {
+        [authFlow reset];
+        fallback();
+        return;
+      }
+    }
+
 #if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
     if (authFlow.emmSupport) {
       [GIDAuthentication handleTokenFetchEMMError:error completion:^(NSError *error) {