drpcmanager: cap concurrent streams with MaxStreams#74
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Add Options.MaxStreams, a per-connection cap on how many concurrent streams a peer may open against this manager. When the cap is reached an inbound stream is refused in the read path -- before any per-stream state is allocated -- with a stream-level KindError, leaving the connection up. 0 means unlimited. This bounds the per-stream bookkeeping (goroutines, stream-table entries, window counters) that the byte-based flow-control windows do not, giving DoS resistance against a peer that opens many zero-byte streams. It is the third flow-control bound alongside the per-stream and connection windows, and is independent of the credit path. Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>
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Caps the number of concurrent admitted streams a peer may open against this manager.
Options.MaxStreams(0 = unlimited). An inbound stream beyond the cap is refused in the read path with a stream-levelKindError, without admitting it or tearing down the connection. The reject is a normal (non-control) frame so it respects write-buffer backpressure: over-cap refusals are unbounded (a peer can flood over-cap invokes), so bypassing the buffer via the control bit would let them append pastMaximumBufferSize. A full buffer parks the reader, throttling the flooder.Merge safety: default 0 = unlimited, so a dependency bump changes nothing. Independent of flow control — a standalone gRPC-style
MaxConcurrentStreamsknob.Tests: refuses-excess-inbound; slot-frees-after-close; zero-unlimited (default admits many). All under
-race.Deferred to follow-up (parked on
sujatha/flow-control-setup-bounds, tracked on CRDB-65356): counting streams in setup against the cap, a control-payload-size bound, and a setup timeout. These bound half-open-stream memory that the admitted-stream cap alone does not; they're being landed separately to keep this PR to the concurrent-stream cap.