2051477 - Align WebTransport datagrams-writable algorithm state with the spec's internal-slot model

Status: NEW

(Core :: Networking, task, P3)

Description

[Randell Jesup [:jesup] (needinfo me)]

Follow-up from review of D286306 (Bug 2007165). In the WebTransport spec, the datagrams writable's algorithms are stateless actions that read internal slots on the stream object itself ([[SendGroup]], [[SendOrder]], [[OutgoingDatagramsQueue]]). Our implementation instead models them as a refcounted OutgoingDatagramStreamAlgorithms object that carries its own copy of that state, which diverges from the spec and duplicates state across two objects.

Current shape (dom/webtransport/api/WebTransportDatagramsWritable.{h,cpp},
dom/webtransport/api/WebTransportDatagramDuplexStream.h):

• WebTransportDatagramsWritable holds mSendGroup / mSendOrder, and keeps a
  separate mAlgorithms back-pointer to the OutgoingDatagramStreamAlgorithms.
• OutgoingDatagramStreamAlgorithms independently owns mSendGroup, mSendOrder,
  mChild, mDatagrams, and mPendingDatagramPromises.
• SetSendGroup / SetSendOrder write the stream's members and forward to the
  algorithms object's copies, keeping two sources of truth in sync by hand.

Desired shape (spec-aligned):

• Move the per-writable state (sendGroup, sendOrder, child, outgoing-datagram queue, pending-datagram promise map) to live only on WebTransportDatagramsWritable, mirroring the spec's stream internal slots.
• Make OutgoingDatagramStreamAlgorithms stateless: its writeDatagrams action reads those slots from the stream it was created for.
• Drop the redundant mAlgorithms member (the WritableStreamDefaultController already owns the algorithms; reach them via Controller()->GetAlgorithms() only where needed - see comment 2 on D286306).
• Collapse SetSendGroup / SetSendOrder to a single slot write, removing the hand-maintained forwarding.

No observable behavior change; makes the code track the spec algorithm structure and removes the state duplication.