I have found that routes with dynamic paths through path processing are breaking under pathologic. For example, image styles.
The way these work is that a route is registered on a subfolder the public files directory path, then a path processor translates the trailing directory/filename into a query parameter which is in turn grabbed by the route controller. This is so that the subdirectory can have unlimited depth in the tree and still work.
For example:
My image style route is registered at /sites/default/files/styles/{image_style}/{scheme}
My image style url is /sites/default/files/styles/thumbnail/public/2016-05/myimage.jpg
When pathologic translates this url, the line:
$url = Url::fromUri('internal:/' . $url_params['path'], $url_params['options'])->toString();
returns:
/sites/default/files/styles/thumbnail/public
I'm assuming this is because that is the path that the actual route is registered on.
An easy solution would be to allow pathologic configuration to ignore paths. I actually found this bug when implementing a similar mechanism for caching external images.
Needs review
2.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Added MR !17 on 2.0.x branch with the changes from #25 (without language piece)