RFC how to handle failures at esi_level > 0

[nigoroll]

I am going to merge #3242 now because I think it qualifies for "trivial fix", I had opened it during the freeze and also it has one approval.

There is an important aspect to retain from that ticket: Can we / do we want to improve error reporting for the case that an ESI subrequest fails?

Right now, we just drop that segment, such that parts of the ESI-generated page become missing, which I think is the worst of all options.

Suggestions from #3242 are:

• Would it make sense to - literally - insert a (canned) red flag into the markup? If it was markup, users of the web page could provide feedback, if it wasn't, the flag would most likely render whatever is being rendered invalid. That canned red flag would have a plain and a gzip variant, depending on the compression context, and would update the parent's CRC etc.
• Can we interrupt the delivery? Sudden close of the session or RST of the stream?
• Can we push the VDPs earlier and not find out about the problem in the middle of the delivery?

[bsdphk]

We hashed this at bugwash today, and various things came up:

There are two distinct cases:

A) The subrequest have pushed bytes into VDP but fails before it's done.

This only happens if the subrequest is streaming and the backend fails.

This can only be be avoided by not streaming the subrequest.

Not sure what our error handling is today, or indeed what it should be. Probably ought to abort the entire delivery, as there may be strange partial content in the delivery.

B) The subrequest has not yet pushed any bytes into VDP.

In this case the parent could have exeception handling.

Currently we just ignore and move on.

We could implement ESI's alt= attribute.

Using alt= a red flag could be delivered by vcl_synth{}

See also: ESI's onerror=

[bsdphk]

Next step is when somebody writes some code.

[nigoroll]

I'd like to briefly talk about this ticket again during bugwash: are we happy with the onerror= change from #3767 or do we want alt= also, or even something else which we did not mention so far?