Coerce bubbled max-age to a floor or ceiling

Neat idea patch looks good to me.

1. +++ b/src/EventSubscriber/CacheControlOverrideSubscriber.php
   @@ -43,10 +61,40 @@ class CacheControlOverrideSubscriber implements EventSubscriberInterface {
   +        if (isset($minimum)) {
   +          $max_age = max($minimum, $max_age);
   ...
   +        if (isset($maximum)) {
   +          $max_age = min($maximum, $max_age);
   

   Let's add some comments here.

2. +++ b/src/EventSubscriber/CacheControlOverrideSubscriber.php
   @@ -43,10 +61,40 @@ class CacheControlOverrideSubscriber implements EventSubscriberInterface {
   +  protected function getMaxAgeMinimum() {
   ...
   +  protected function getMaxAgeMaximum() {
   

   Why not getMinimumMaxAge() and getMaximumMaxAge()?

We need to add config/install/cache_control_override.settings.ymland a hook_update_N to add it to active storage.

Here you go.

Added schema test.

Fixed the failing test.

Nice work! This is 👍 from me. I have found just some nitpicks, feel free to disregard if you like.

1. +++ b/cache_control_override.install
   @@ -0,0 +1,17 @@
   +function cache_control_override_update_8101() {
   +  \Drupal::configFactory()
   +    ->getEditable('cache_control_override.settings')
   +    ->set('max_age.minimum', 0)
   +    ->set('max_age.maximum', -1)
   +    ->save(TRUE);
   
   +++ b/config/install/cache_control_override.settings.yml
   @@ -0,0 +1,3 @@
   +max_age:
   +  minimum: 0
   +  maximum: -1
   

   I'm confused why these default values are necessary, especially due to these descriptions:

   Minimum max age, if max age is greater than 0. Leave empty to disable.

   Maximum max age, if max age is greater than 0. Leave empty to disable.

   Wouldn't a "no limit by default" be reasonable so that we don't need the install hook and initial values?

   (also, in case we do want to leave them, maybe using Cache::PERMANENT instead of -1 would make it more self-explanatory?
   (It's a bit unfortunate #2951046: Allow parsing and writing PHP class constants and enums in YAML files is not in yet so we could use it in the yml file, but at least in the update hook it could make sense? :) )

2. +++ b/src/EventSubscriber/CacheControlOverrideSubscriber.php
   @@ -43,10 +61,44 @@ class CacheControlOverrideSubscriber implements EventSubscriberInterface {
   +        // Force maximum max-age if configured.
   +        $maximum = $this->getMaxAgeMaximum();
   +        if (isset($maximum) && $maximum != Cache::PERMANENT) {
   +          $max_age = min($maximum, $max_age);
   +        }
   

   I guess we could enforce $maximum here to also be > 0, even though I admit this is borderline paranoia, since the value can only be defined in settings.php... :)