Acquia Search uses a Solarium adapter that conflicts with the remote_stream_wrapper module

TL;DR: Solarium, as used by the acquia_search module, relies on some very specific handling for HTTP headers in PHP's file_get_contents(), but when there's a stream wrapper available for 'http://' streams, headers are treated differently. This means that the remote_stream_wrapper module (8.x-1.0 and 8.x-1.x-dev@d1f54fc6) breaks acquia_search. This can be solved by tweaking the Solarium usage in this module to use the Guzzle adapter; I've attached a patch.

The longer version:

The SearchApiSolrAcquiaConnector class specifies using Solarium's Solarium\Core\Client\Adapter\Http connection adapter instead of the default Curl adapter. This adapter class uses PHP's file_get_contents() function to fetch the contents of a URI; it looks for the HTTP headers to be populated by file_get_contents() in the $http_response_header variable.

However, when the remote_stream_wrapper module is enabled, file_get_contents() uses the Drupal\remote_stream_wrapper\StreamWrapper\HttpStreamWrapper class to read from the URI instead; this stream wrapper doesn't set the weird/ugly/vestigial $http_response_header variable to match PHP's core behavior. This means that Solarium no longer has access to the response headers, so SearchApiSolrAcquiaConnector can't tell whether it has reached the Acquia Solr server or authenticate against it.

Switching from Solarium's Http adapter to the Guzzle adapter removes the dependency on this quirky PHP behavior.