Adding CORS response headers

To anyone who comes in afterwards and wants to find a 'final' solution, here's what we ended up with (which functions perfectly, at least for us):

when HTTP_REQUEST priority 200 {
    unset cors_origin -nocomplain
    if { [HTTP::header Origin] ends_with ".example.com" } {
        if { ( [HTTP::method] equals "OPTIONS" ) and ( [HTTP::header exists "Access-Control-Request-Method"] ) } {
             CORS preflight request - return response immediately
            HTTP::respond 200 "Access-Control-Allow-Origin" [HTTP::header "Origin"] \
                              "Access-Control-Allow-Methods" "POST, GET, OPTIONS" \
                              "Access-Control-Allow-Headers" [HTTP::header "Access-Control-Request-Headers"] \
                              "Access-Control-Max-Age" "86400"
        } else {
             CORS GET/POST requests - set cors_origin variable
            set cors_origin [HTTP::header "Origin"]
        }
    }
    ...
    ...
    ...
    other irules
    ...
    ...
    ...
}
when HTTP_RESPONSE {
     CORS GET/POST response - check cors_origin variable set in request
    if { [info exists cors_origin] } {
        HTTP::header insert "Access-Control-Allow-Origin" $cors_origin
        HTTP::header insert "Access-Control-Allow-Credentials" "true"
        HTTP::header insert "Vary" "Origin"
    }
}

If you have any comments about this, please do so. And, of course, feel free to use it yourself.