Wait for positive HTTP/2 flow-control credit when stream window goes negative#935
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Kludex merged 5 commits intoMay 15, 2026
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`_wait_for_outgoing_flow` previously waited only while the available
flow was exactly zero. Per RFC 7540 §6.9.2, when a peer sends a
SETTINGS frame that reduces INITIAL_WINDOW_SIZE, every existing
stream send window is adjusted by the delta and may become negative.
With the `while flow == 0` guard, a negative window passed straight
through and h2 raised `LocalProtocolError("Cannot send N bytes, flow
control window is -M")` on the next send_data call.
Switching the guard to `while flow <= 0` keeps the stream parked
until WINDOW_UPDATE frames restore positive credit.
Refs: encode/httpcore#1082
Refs: encode/httpx#3601
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
…ow-control-window
Kludex
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Summary
_wait_for_outgoing_flowinhttpcore2's HTTP/2 implementation to wait while the available flow is non-positive, not just exactly zero.INITIAL_WINDOW_SIZE, existing stream send windows are adjusted by the delta and may become negative. With the previouswhile flow == 0guard, a negative value would pass straight through and h2 would raiseLocalProtocolError("Cannot send N bytes, flow control window is -M")on the nextsend_datacall.while flow <= 0keeps the stream parked untilWINDOW_UPDATEframes restore positive credit.Refs: encode/httpcore#1082, encode/httpx#3601.
Test plan
test_http2_connection_with_negative_flow_control_windowadded in bothtests/httpcore2/_async/test_http2.pyandtests/httpcore2/_sync/test_http2.py. Reproduces the bug via a mock stream that sends a window-reducing SETTINGS frame after the default 65535-byte window is exhausted, then providesWINDOW_UPDATEcredit to finish the upload.tests/httpcore2/_async/test_http2.pyandtests/httpcore2/_sync/test_http2.pypass — 33/33.scripts/checkpasses (format / mypy / ruff).🤖 Generated with Claude Code