Dependency on composer_manager makes it impossible to install when using composer_autoloader

When I check the "No Composer Manager" checkbox, the site immediately becomes unavailable with the following watchdog error:

Error: Class "Aws\S3\StreamWrapper" not found in include_once() (line 26 of /path/to/module/s3fs/S3fsStreamWrapper.inc).

It seems like, for reasons that I can't figure out, that class is being invoked before /vendor/autoload.php has run. I can eliminate the error by adding this at the top of the file:

if (!class_exists('Aws\Sdk')) include DRUPAL_ROOT . '/../vendor/autoload.php';

I haven't added that to the current patch because it seems like there must be a better way to handle this—what might be causing that to be invoked before everything has been autoloaded?

In case anyone comes across this and runs into the same issues I had: I think I was getting a class not found error when using S3 from the vendor directory because "Use S3 for private:// files" was checked—that would explain why other people haven't run into the issue, and it seems plausible that Drupal 7 would bootstrap the file paths before /vendor/autoload.php runs. I worked around the issue by adding this just above the "use" statements at the top of S3fsStreamWrapper.inc:

 if (!class_exists('Aws\Sdk')) {
  require_once DRUPAL_ROOT . '/../vendor/autoload.php';
} 

I'm not sure this is a smart solution, and this code specifically expects the web root in /web (which isn't common for D7), but I put a patch in a Gist in case it's useful to anyone. Remove the opening "/.." if the web root is the install root.

One additional little thing: I couldn't get the s3fs_sdk_autoloaded variable to stick on a fresh install because the settings aren't saved if the SDK isn't found (and the SDK can't be found until the settings are saved). The variable can be set using Drush, then everything else works:

drush vset s3fs_sdk_autoloaded 1

Drupal 7 end-of-life triage:
Drupal 7 will reach end of life on January 5th.

The 7.x branches of S3FS do not have any additional planned releases.

the 8.x-3.x and newer releases require composer managed installs and do not experience the issues described here.