Add support for esi onerror flags

[tripodsan]

the esi specification allows to specify a onerror attribute, which controls how the system should behave in case the resource doesn't exist:

If an ESI Processor can fetch neither the src nor the alt, it returns a HTTP status code greater than 400 with an error message, unless the onerror attribute is present. If it is, and onerror="continue" is specified, ESI Processors will delete the include element silently.

so the default behaviour is to respond with an error unless the attribute is onerror="continue" is present.

the current esi in the simulator implementation ignores errors when fetching the include content failed; where as the esi processor in fastly includes the response.body from the 404.

[trieloff]

I can only speak for Fastly, which does not implement onerror and includes the 404 error page as is.

[tripodsan]

I can only speak for Fastly, which does not implement onerror and includes the 404 error page as is.

yes, I saw this...this is a bit a bummer.

[kptdobe]

this is a bit a bummer.

I think this is much more: this makes ESI almost useless in our case. Since any request could return a 404 because corresponding content may not exist, having the "404 error page" inside the caller page makes it unusable.

[tripodsan]

Since any request could return a 404 because corresponding content may not exist, having the "404 error page" inside the caller page makes it unusable.

we could tag the esi includes with ?x-esi=true or something and then ensure that fastly strips out any body for a 404. or we talk to fastly about this...