Varnish cache server errors on some admin pages

[danhgov]

As noted in #508, some admin pages are getting a Varnish error:

I've seen it on these two pages:

• Structure... Pages
• Configuration... Development... Default Content Deploy... Export

On Dev and Prod, I saw this error only for the latter of those two items -- on Test, it's for both of them.

I haven't looked hard -- there may be lots more instances of this.

A clear-cache didn't fix it.

[NicoledeGreef]

The Varnish cache error is now manifesting in each environment when a user tries to download a data dictionary from a metadata record. Reported by a user in Production so will be prioritized accordingly.

Logs say:

#0 /var/www/html/modules/custom/bc_dc/src/Controller/BcDcCreateFileController.php(127): PhpOffice\PhpSpreadsheet\Spreadsheet->addSheet(Object(PhpOffice\PhpSpreadsheet\Worksheet\Worksheet))
#1 [internal function]: Drupal\bc_dc\Controller\BcDcCreateFileController->createFile(Object(Drupal\node\Entity\Node), 'xlsx')
#2 /var/www/html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#3 /var/www/html/core/lib/Drupal/Core/Render/Renderer.php(638): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber{closure}()
#4 /var/www/html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#5 /var/www/html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#6 /var/www/vendor/symfony/http-kernel/HttpKernel.php(181): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber{closure}()
#7 /var/www/vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#8 /var/www/html/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#9 /var/www/html/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#10 /var/www/html/core/lib/Drupal/Core/StackMiddleware/ContentLength.php(28): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#11 /var/www/html/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\ContentLength->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#12 /var/www/html/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#13 /var/www/html/modules/contrib/shield/src/ShieldMiddleware.php(263): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#14 /var/www/html/modules/contrib/shield/src/ShieldMiddleware.php(130): Drupal\shield\ShieldMiddleware->bypass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#15 /var/www/html/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\shield\ShieldMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#16 /var/www/html/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#17 /var/www/html/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(36): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /var/www/html/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\AjaxPageState->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /var/www/html/core/lib/Drupal/Core/DrupalKernel.php(741): Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /var/www/html/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#21 {main}