varnish 4 - abandon, vcl_synth, and restarting requests with new headers - varnish

Question

I've already asked and received an awesome answer about how to get an error from the backend to force serving from stale cache (grace) found here: varnish 4: serve graced object if beresp.status is an error?
but now that logic needs an extra step: i include the following code currently
sub vcl_backend_fetch {
if (bereq.retries == 0) {
unset bereq.http.X-Varnish-Backend-5xx;
unset bereq.http.X-Varnish-Backend-206;
} else {
if (bereq.http.X-Varnish-Backend-5xx) {
return (abandon);
}
if (bereq.http.X-Varnish-Backend-206) {
return (abandon);
}
}
}
sub vcl_synth {
if (resp.status == 503 &&
req.method != "POST" &&
!req.http.X-Varnish-Restarted-5xx) {
set req.http.X-Varnish-Restarted-5xx = "1";
return (restart);
}
if (resp.status == 503 &&
req.method != "POST" &&
!req.http.X-Varnish-Restarted-206) {
set req.http.X-Varnish-Restarted-206 = "1";
return (restart);
}
}
obviously the second if statement in the vcl_synth is virtually identical to the first one, with the exception of the header it's looking for. I need to differentiate the 206 to restart with a different request header, but I am not sure how. the issue is that, if the backend returns a 206, the rest of the logic abandons the backend fetch (which hands off to vcl_synth with a 503), and vcl_synth restarts the request to force serving graced cached objects. however, if there's no graced cache object to return to the user, then the user sees a 503 instead of a 206.
Before realizing that this line of thinking was not possible, i tried to have vcl_backend_fetch return a synth(206), so that vcl_synth could use resp.status to differentiate, and add a different request header before restarting the request. then i would be able to look for that header in vcl_miss, and if it's there, i could restart the whole request a second time, and force it to serve the 206 from the usual backend.
TL;DR: how do I differentiate two different cases in a vcl_backend_fetch abandon, to get vcl_synth to restart the request with two different headers?

Answer 1

I'm pretty sure it's not possible to do anything in vcl_backend_fetch in order to abandon the request and transfer some kind of flag to vcl_synth.
I'm not sure if I've completely understood your use case, but it seems that would need something like:
sub vcl_synth {
if (resp.status == 503 &&
req.method != "POST" &&
!req.http.X-Varnish-Restarted-5xx) {
# Let's restart the request for the first time and try to
# serve grace content. This could select an always-sick
# backend during vcl_recv.
set req.http.X-Varnish-Restarted-5xx = "1";
return (restart);
}
if (resp.status == 503 &&
req.method != "POST" &&
req.http.X-Varnish-Restarted-5xx &&
!req.http.X-Varnish-Restarted-206) {
# Grace content was not available after the first restart. Let's
# restart again the request and try to serve content using some
# other backend.
set req.http.X-Varnish-Restarted-206 = "1";
return (restart);
}
}