Issue #3248928: Redirect or provide link to apply form for user who started update if accessing start form

src/Form/UpdaterForm.php

@@ -114,8 +114,10 @@ class UpdaterForm extends FormBase {
   public function buildForm(array $form, FormStateInterface $form_state) {
     $this->messenger()->addWarning($this->t('This is an experimental Automatic Updater using Composer. Use at your own risk.'));
 
-    $stage_exists = FALSE;
-    if (!$this->updater->isAvailable()) {
+    if ($this->updater->isAvailable()) {
+      $stage_exists = FALSE;
+    }
+    else {
       $stage_exists = TRUE;
 
       // If there's a stage ID stored in the session, try to claim the stage
@@ -130,17 +132,11 @@ class UpdaterForm extends FormBase {
           ]);
         }
         catch (StageException $e) {
-          // We already know a staging exists, even if it's not ours, so we
-          // don't have to do anything else here.
+          // We already know a stage exists, even if it's not ours, so we don't
+          // have to do anything else here.
         }
       }
     }
-    // If the form has been submitted, do not display this error message because
-    // ::deleteExistingUpdate() may run on submit. The message will still be
-    // displayed on form build if needed.
-    if ($stage_exists && !$form_state->getUserInput()) {
-      $this->messenger()->addError($this->t('Cannot begin an update because another Composer operation is currently in progress.'));
-    }
 
     $form['last_check'] = [
       '#theme' => 'update_last_check',
@@ -252,6 +248,12 @@ class UpdaterForm extends FormBase {
     $this->displayResults($results, $this->messenger());
 
     if ($stage_exists) {
+      // If the form has been submitted, do not display this error message
+      // because ::deleteExistingUpdate() may run on submit. The message will
+      // still be displayed on form build if needed.
+      if (!$form_state->getUserInput()) {
+        $this->messenger()->addError($this->t('Cannot begin an update because another Composer operation is currently in progress.'));
+      }
       $form['actions']['delete'] = [
         '#type' => 'submit',
         '#value' => $this->t('Delete existing update'),