http2: fix stream WINDOW_UPDATE over-crediting buffered body bytes (FLOW_CONTROL_ERROR with slow readers) - #24

Fixes the fhttp half of bogdanfinn/tls-client#257.

Problem

When a response body is consumed slower than the origin delivers it (a proxy relaying to a backpressured client, a rate-limited download), a large HTTP/2 download aborts partway with:

stream error: stream ID N; FLOW_CONTROL_ERROR

Root cause

In transportResponseBody.Read, the stream receive-window refresh was:

unsent := int(cc.streamFlow) - int(cs.inflow.available()) + cs.bufPipe.Len()

bufPipe.Len() is body data received but not yet consumed by the application. The credit that is safe to return to the peer is streamFlow - available - buffered (what golang.org/x/net/http2 computes), so the buffered term must be subtracted. Adding it over-credits the stream window by 2 × buffered on every refresh; with a slow reader the buffer stays large, the advertised window desyncs from the real accounting, and the stream dies with FLOW_CONTROL_ERROR once the connection-level window is exhausted.

Fix

One-line sign fix:

-	unsent := int(cc.streamFlow) - int(cs.inflow.available()) + cs.bufPipe.Len()
+	unsent := int(cc.streamFlow) - int(cs.inflow.available()) - cs.bufPipe.Len()

Regression test

TestTransportSlowReaderLargeResponse downloads 48 MiB from an in-process h2 server through a Transport configured with a Chrome-like 6 MiB initial stream window / 15.6 MiB connection flow, consuming the body at ~12 MiB/s.

• master: fails after ~46 MiB with stream error: stream ID 1; FLOW_CONTROL_ERROR
• this branch: passes

Test plan

• go test -vet=off ./http2 -run TestTransportSlowReaderLargeResponse — fails before, passes after
• go test -vet=off -short ./http2 — no new failures; the TestTransport timeout and TestTransportH2c failure reproduce identically on unmodified master (live-network dependent) and are unrelated to this change
• Also validated downstream against real origins (197 MiB object, 10 MiB/s reader): truncated at ~30–60 MB before, completes in full after