Add support for Flysystem S3

Thanks for reporting the issue and the patch! Attached one that checks the flysystem driver settings.

$flysystemSettings casing is incorrect.

Also, what module defines the following setting?

Settings::get('flysystem', []);

Let's try this one. Updated dependency injection for settings. The flysystem & flysystem_s3 module. Custom settings are added to settings.php. Here is an example from the readme:

Note the key 's3' is default in the example, but that can be any string for the scheme.

$schemes = [
  's3' => [
    'driver' => 's3',
    'config' => [
      'key'    => '[your key]',      // 'key' and 'secret' do not need to be
      'secret' => '[your secret]',   // provided if using IAM roles.
      'region' => '[aws-region-id]',
      'bucket' => '[bucket-name]',
      // Optional configuration settings.
      // ...
    ],

    'cache' => TRUE, // Creates a metadata cache to speed up lookups.
  ],
];

$settings['flysystem'] = $schemes;

Added a code comment to help describe what the settings are for.
// Check if Flysystem settings exist and if there are any S3 scheme's.

I don't agree with the case change since the module and library are both spelled "Flysystem", but I'm happy to change that if needed.

I don't agree with the case change since the module and library are both spelled "Flysystem", but I'm happy to change that if needed.

Well the issue here is Drupal coding standards, only class variables should be cased like this. Otherwise from within methods, the variable should be named like this (snake case):
$flysystem_settings

Snake case! I misunderstood the note.

I ran a few manual tests on a fresh install. I think it has to be a TrustedRedirectResponse if an external URL is used. The 'public' => TRUE option for flysystem_s3 uses the external S3 bucket URL. The patch with TrustedRedirectResponse worked for me. With 'public' => FALSE it uses an internal URL and I get the error:

The controller result claims to be providing relevant cache metadata, but leaked metadata was detected.

Additional notes:

Removing the entire S3 condition block temporarily solves this issue

I'll try to test a bit more, but I can confirm that both seem to work without the if 's3' code.

If we still want to support the external S3 URL this patch needs to check if the public option is also TRUE before returning the TrustedRedirectResponse.

Note that I am testing on 8.9.20 with everything as up to date as it can be.

Someone should probably test the s3fs module (I'll try to sometime soon).

Side note: the download counter is not updating with S3.

Same results on D9 - everything up to date.

I'm not familiar with all of the s3fs settings, but initial testing with a basic setup works as expected ($scheme is 's3' and redirects to the S3 URL).

Updated the patch to keep the TrustedRedirectResponse and check the flysystem config public setting.

Changing to a feature request to add support for Flysystem S3.

Updated issues summary to include example settings and the full error message.

If flysytem settings uses the default 's3' scheme the same error can occur if public is not true...

This should work:
if (($scheme === 's3' && !isset($flysystem_settings[$scheme])) || ...

Okay, final patch from me for now!

Updated the if statement to handle flysystem with 's3' scheme name.

Committed!