How Do I change the Document Root Per Request

[David E. Wheeler]

Hi,

I want to set the document root for a request to map to the basic auth 
username. I tried this in a PerlFixupHandler:

    sub handler {
        my $r = shift;

        # We only want to do this once per request.
        return DECLINED unless $r->is_initial_req;

        # Get the username.
        my $user = $r->user or return HTTP_UNAUTHORIZED;

        # Return forbidden if the username subdiectory does not exist.
        my $doc_root = File::Spec->catdir($r->document_root, $user);
        return HTTP_FORBIDDEN unless -d $doc_root;

        # Set the document root for the duration of this request and return.
        $r->document_root($doc_root);
        return DECLINED;
    }

But alas, it still serves the original document root. How can I get it to 
change the document root on a per-request basis? If I cant, should I change the 
URI or the filename, instead?

Thnks,

David

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