feat: allow empty HTTP/2 DATA frames to be throttled

[pjfanning]

Motivation

A DATA frame carrying no payload has sizeInWindow == 0, so it consumes no flow-control window. The number of such frames is therefore not bounded by flow control at all, and on an open stream a peer can send them continuously — each one costing a pass through the stream state machine, a buffer ++= empty, and a flow-control recompute.

The frame-type-throttle mechanism could not be pointed at them. It maps an alias to a frame type name:

case "reset" => Some("RstStreamFrame")
...
case _       => None

and had no alias for DATA, so DATA frames were unthrottleable by configuration.

Adding a plain "data" alias would not have helped. Throttling every DATA frame at the configured rate throttles legitimate throughput along with it, so nobody could realistically enable it for this purpose — which is presumably why the config documents the throttle as being for "non-data frame types". Only the empty frames need it, because data-carrying ones are already bounded by flow control.

Modification

Add an "empty-data" alias that charges DATA frames whose payload is empty. It resolves to a name that is deliberately not a real frameTypeName, which frameCost recognises and applies to empty DATA frames only:

case d: DataFrame if d.payload.isEmpty && frameTypesForThrottle.contains(EmptyDataFrameThrottleName) => 1

Off by default, like every throttle target other than "reset".

Result

An operator can throttle a flood of empty DATA frames without throttling data-carrying ones.

Tests

• sbt "http-core/testOnly org.apache.pekko.http.impl.engine.http2.Http2BlueprintSpec" — pass (11 tests), with a new case for the alias.
• sbt "http2-tests/testOnly ...Http2ServerEmptyDataThrottleSpec ...Http2ServerEnableFrameTypeThrottleSpec ...Http2ServerDisableFrameTypeThrottleSpec" — pass (3 tests). The new spec floods empty DATA frames with the throttle configured for them and requires the connection to be torn down. Verified it fails when the alias resolves to None, which is the behaviour before this change.
• sbt http-core/mimaReportBinaryIssues — pass.
• Header on the new file generated with sbt headerCreateAll; native scalafmt clean.

References

Refs #332 - extends the frame type throttle to empty DATA frames

🤖 Generated with Claude Code