Dynamic redirects are no longer possible in the batch API (breaks updating existing modules or themes with the Update Manager)

Demoting doesn't really make sense to me; this is critical for at least one (and possibly two) reasons at https://www.drupal.org/node/45111 - what does the other issue have to do with issue priorities?

If this functionality were moved to contrib, it would be an equally critical bug there. In the meantime, demoting it just means developers will be less likely to work on fixing it (though I don't think that will be a problem in practice here).

Anyway, let's see if we can fix it...

Here is how I would fix it (entirely within the batch API)... It seems like the ability to control the redirect from a batch 'finished' callback is a useful feature that worked in Drupal 7 but has gone missing in Drupal 8?

This patch works for me when applied on top of the one from #2042447: Install a module user interface does not install modules (or themes). (The code added in that issue should in theory allow tests to be written for this one too.)

I'll take a look and see if I can figure out the failing tests.

In the meantime, moving this to the batch API queue since the code to fix this is entirely in the batch API - it needs fixing and review there regardless of the Update Manager. (We should still write an Update Manager test for this once #2042447: Install a module user interface does not install modules (or themes) is committed, though.)

This should do it, I think. Expecting two failures from the tests-only patch.

No interdiff from #10 because I also moved the new test to its own method, and cleaned up the commented-out code, etc. But here are the two essential changes:

     // Displaying the page triggers batch 1.
     $this->drupalGet('batch-test/finish-redirect');
     $this->assertBatchMessages($this->_resultMessages('batch_1'), 'Batch for step 2 performed successfully.');
     $this->assertEqual(batch_test_stack(), $this->_resultStack('batch_1'), 'Execution order was correct.');
-    $this->assertText('Redirection successful.', 'Redirection after batch execution is correct.');
+    $this->assertText('Test page text.', 'Custom redirection after batch execution displays the correct page.');
     $this->assertUrl(Url::fromRoute('test_page_test.test_page'));

This was just asserting the wrong thing. We are trying to force a redirect to the custom "test page" in this case, so we need to assert the text that appears on that page.

 function _batch_test_finished_1_finished($success, $results, $operations) {
   _batch_test_finished_helper(1, $success, $results, $operations);
-  return new RedirectResponse(Url::fromRoute('test_page_test.test_page')->toString());
+  return new RedirectResponse(Url::fromRoute('test_page_test.test_page', [], ['absolute' => TRUE])->toString());
 }

This I don't entirely understand why it's necessary, but absolute URLs seem to be the way to go here to make the tests pass. (And interestingly, the Update Manager itself is already using an absolute URL in its batch finished callback - that's how I got the idea to try it here.)

Since this was committed before #2042447: Install a module user interface does not install modules (or themes) it went in without tests for the Update Manager. I created a new issue with a patch for the tests at #2548511: Write tests for the Update Manager downloading and applying updates for a module now (seems pretty important to get that in so this problem can't regress).