[12.x] Feature/cache flexible forever

[giuspe]

Problem

As a developer I would like to be able to use Cache::forever with a flexible approach to trigger background refreshes.

By definition, Cache::forever never automatically refreshes the value unless manually handled via application logic.

Proposal

We add a flexibleForever utility method to cache Repository which can serve stale values while triggering deferred refreshing (leveraging the new Cache::flexible method).

The new method exposes the same signature as Cache::remember while implementing a flexible version of Cache::forever

public function flexibleForever($key, $refreshTtl, $callback, $lock = null, $alwaysDefer = false)
{
   return $this->flexible($key, [$refreshTtl, Carbon::now()->addYears(1000)], $callback, $lock, $alwaysDefer);
}

Of course the same result could be achieved at application level via registering a macro in AppServiceProvider

class AppServiceProvider extends ServiceProvider
{
    public function boot()
    {
        // [...]

        Cache::macro('flexibleForever', function (string $key, int $ttl, callable $generator) {
            return Cache::flexible($key, [$ttl, Carbon::now()->addYears(1000)], $generator);
        });
    }
}

or via simply calling Cache::flexible with a fixed, high upper ttl.

[ahinkle]

I believe the flexible pattern idea was about graceful degradation of stale data - serving cached content while refreshing it in the background. "Forever" implies content that never goes stale, which seems to contradict the pattern's purpose.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!

[giuspe]

Thank you @ahinkle and @taylorotwell for your feedback!