Original text:
Create an ordered list of the field values of each instance of the field in the message, in the order that they occur (or will occur) <del> in the message.  If necessary, separate individual values found in a field instance. </del>  Strip leading and trailing whitespace from each item in the list. Note that since HTTP field values are not allowed to contain

Reviewer comments:
URL This does not work. It's impossible to split multiple values (be it a list-based field or not) without knowledge of the field value's grammar. For example, what is the canonicalized value for: ?
The value would be Since it was sent as two instances, you combine the instances with a comma and space. The language to split the field values was added late based on several conversations about list type fields and how they can be combined, but we can remove this. If someone needs additional armoring they can use the "sf" flag on a list formatted field.
Ok, but what the spec currently says would result in: URL that sentence really needs to go.
No, it would not result in that, because as you point out those are not separate field values that can be separated.
So what does: mean then? When is it "necessary"? How does the code know that for any given field name?
I think what is meant is that you take a list of the values of a given field (one member of the list for each header line) and then concatenate them by separating them with ", ". In scala this is or just to make it explicit
Thanks NAME - I get that, but that doesn't address the question I asked...
Ok, I see the problem is: I would not know how to do that, whereas I do know how to seperate the fields in the list.
We need to be very clear with the terminology. What do you mean by "seperate the fields in the list"? Do you have an example?
If you are talking to me, I meant what the code above showed: the list is the list of header values (for a given header name). And I meant seperating the values of that list. I don't know how to write that up well in the spec though...
Ok, AFAIU: just drop "If necessary, separate individual values found in a field instance." from the spec.
URL I don't quite get the reasoning for the second "MUST". Why does it matter? What would break if the client uses a single comma, which is also allowed by HTTP? IMHO the exact delimiter sequence here does not matter at all, as long as it is allowed per HTTP. Note that HTTP allows combining field values, it does not allow splitting them afterwards.
This isn't combination at the HTTP level by the client or intermediary, this is combination by either the signer or verifier when making the signature base. It's a MUST here because there needs to be only one way to do it at that step. If the lines are already combined then leave them combined.
hm. ok. So one confusing thing here is that HTTP 5.2 is already only about combining field values. So the "Specifically" is a distraction. Also, 5.2 does not mandate using COMMA SP. The section doing that is 5.3.
Also...: I think I misread that as a recommendation to parse individual field values into a list, before recombining everything. Maybe "parsing out individual field values" needs to be rephrased.
You would only parse out individual values if you: 1: Know the overall syntax/grammar of the field 2: Know that it's a list separated by commas 3: Can't send everything as a single field line 4: Know that an intermediary is going to mess with things sent as multiple lines in a way that you don't want The recommendation is really only for that one corner case, not for every value.
I agree with that, but the spec really needs to be clearer here.
(but I'll note that github's way of displaying diffs in long lines makes it extrremely hard to find out what actually changed)

Revised text:
Create an ordered list of the field values of each instance of the field in the message, in the order that they occur (or will occur)  <ins> in the message. </ins> Strip leading and trailing whitespace from each item in the list. Note that since HTTP field values are not allowed to contain