drpcstream: per-stream receive window and read-path grant wiring#72
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Add recvWindow, the per-stream receive side of flow control. It tracks buffered bytes (in-progress reassembly plus completed, not-yet-consumed data) and decides when to return credit to the sender: grants are withheld while buffered is at or above the high-water mark, and the accrued returnable credit is otherwise released once it reaches the threshold, coalescing many frames into a single grant. Consuming reopens a gate that the high-water mark had closed. dispatched/consumed return the credit delta the caller should emit as a KindWindowUpdate. Emitting the frame, wiring into the read path (HandleFrame/MsgRecv), and the connection-level receive budget come in later commits. Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>
…ad path
Install a per-stream recvWindow and drive it from the read/recv path:
- HandleFrame intercepts an incoming KindWindowUpdate before the
assembler and applies it to the send window. Intercepting early keeps
a grant that interleaves with an in-progress message (grants are
emitted off the write lock) from disturbing reassembly.
- A dispatched KindMessage frame returns credit via recvWindow, emitting
a KindWindowUpdate once the accrued amount crosses the threshold and
the high-water gate is open.
- RawRecv/MsgRecv call recvWindow after dequeuing; consuming can reopen a
gate the high-water mark had closed and flush the accrued grant.
Grants are control frames, appended without blocking, so emitting them
from the reader goroutine is safe. The window is opt-in: with no recvw
installed no grants are emitted, and an incoming grant with no send
window is ignored, so default behavior is unchanged.
Stream-level only; the connection-level receive budget and the enablement
path that installs the windows come in later commits.
The packet assembler can silently drop an unfinished message when a
higher message id supersedes it (a legitimate sequence: a send preempted
mid-message, then a cancel). Those bytes were counted into the receive
window at dispatch and would inflate buffered forever, wedging the
high-water gate shut and deadlocking the sender once its credit ran
out. The assembler now records the drop (TakeDiscarded) and HandleFrame
releases the bytes as consumed, letting their credit flow back. The
release runs last (deferred past the replacement frame's accounting and
handlePacket), so the grant decision sees the final buffered state
rather than the transient dip, and a terminal packet that supersedes a
partial suppresses the grant via termination.
emitGrant is also a no-op once the stream has terminated: the ring
drains queued messages after close, and returning credit behind the
terminal frame would invite the peer to send more doomed data.
Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>
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1. Per-stream receive-window grant policy (
recvWindow).Tracks buffered bytes (in-progress reassembly + completed, not-yet-consumed) and decides when to return credit: withholds grants while buffered is at/above the high-water mark, otherwise releases accrued returnable credit once it reaches the threshold.
dispatched/consumedreturn the credit delta to emit; consuming reopens a gate the high-water mark had closed.2. Read-path wiring.
HandleFrameintercepts an incomingKindWindowUpdatebefore the assembler and applies it to the send window. Intercepting early keeps a grant that interleaves with an in-progress message (grants are emitted off the write lock) from disturbing reassembly.bufferedforever, causing the high-water gate to shut. So, the assembler now reports the drop (TakeDiscarded) andHandleFramereleases the bytes as consumed, so their accrued credit flows back.