[quicwg/base-drafts] PRIORITY frame on control stream referencing unopened request stream (#2502)

[Robin Marx <[e-mail]>]

There is a specific prioritization situation which to my interpretation isn't fully clear in the draft-18 text.

Context: trying to re-create the concept of exclusive prioritization (which was removed from h3 a while ago). 
Approach: simulate exclusivity by sending new PRIORITY frames on the control stream for all the streams we want to be dependent on a new request stream.

Example:
```
A                      B                         C
1                      1                         1   
|                     |  \                       |
2                     2   3                      3
                                                 |
                                                 2     
```
Step A: stream 1 and 2 are opened in sequence with priority frame as first frame of their stream
Step B: stream 3 is opened with priority frame as first frame
Step C: we have sent a priority frame on the control stream for stream 2, updating its dependency to stream 3

(In http/2, step B and C would be the same due to exclusivity, in h3 we need step C to simulate this).

The question:

**What happens if the priority frame from step C arrives before stream 3 is opened? We then have a priority update for stream 2, making it dependent on stream 3, which does not exist yet (at least at the receiving endpoint).**

There are various points in the text that might lead to an interpretation of this, but none really reference this exact situation (afaict).

e.g., 

> A PRIORITY frame that references a non-existent Push ID, a
   Placeholder ID greater than the server's limit, or a Stream ID the
   client is not yet permitted to open MUST be treated as an
   HTTP_LIMIT_EXCEEDED error.

Here, stream 3 is certainly "permitted to open"

>  A reference to an element which
   is no longer in the tree is treated as a reference to the root of the
   tree.

"no longer" != "not yet"
If this is the current intended behavior here though, I would suggest to change it to allow the use case above.

> Due to reordering between streams, an element can also be prioritized
   which is not yet in the tree.  Such elements are added to the tree
   with the requested priority.

This one reads a bit weird to me... I assume it is talking about the case where you don't send an initial priority frame on the request stream itself, but rather only on the control stream, and that can arrive before the request stream itself. In this case I assume the priority frame is buffered until the stream is opened. 

This *could* be read to also apply to my situation above, but if that's the case I would make it a bit more explicit. If that's not the case, I would change it to something like "Due to reordering between the control stream and request streams, ..."

___

If I'm not missing something and this case is indeed not yet handled in the text, we might add something like "If a priority frame references a request stream that is permitted to be opened but which has not yet been opened, the priority frame MUST be applied once the referenced request stream becomes opened."




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