http2: port upstream inbound flow control fixes to stop window leaks

[hiroTamada]

Summary

Ports two golang.org/x/net inbound flow-control fixes the fork predates. Together they stop pooled HTTP/2 connections from strangling to zero window when response bodies are closed mid-download — the failure mode where every later stream on a connection hangs with zero bytes until the origin resets it after ~5 minutes.

What was wrong

Two independent leaks of connection-level flow control credit, both triggered by aborting response bodies:

1. Read-credit stranding (deterministic). The Read path refreshed the connection window only once available dropped below half of cc.connFlow, and Close() refunds only unread bytes. Bytes that were read on a stream that then closed before the half-window threshold was crossed were never refunded. Successive aborted downloads accumulate the deficit; once the window hits zero no read can ever happen again, so no refresh can ever fire — a terminal wedge. The refresh ceiling was also cc.connFlow while the advertised window is cc.connFlow + 65535, permanently stranding the protocol-default slack.
2. Close/data race (probabilistic). processData checked cs.didReset under cc.mu but wrote to the body pipe after releasing it, while Close() refunded buffered bytes and only then broke the pipe. Data written in between was silently discarded by the broken pipe (pipe.Write succeeded on a broken pipe) and its credit was never returned.

What this ports

• 7805fdc3 "http2: rewrite inbound flow control tracking" (client side): the inflow accounting type. Every consumed byte is credited exactly once; wire updates are batched (≥4 KiB, or enough to at least double the peer's remaining window). Replaces the threshold refresh and the adaptive stream-window logic, both of which are subsumed. Fixes x/net/http2: flow control desync when receiving data for canceled stream golang/go#56558, x/net/http2: WINDOW_UPDATE sent rate too high and can't be configured golang/go#28732.
• 9f24bb44 "http2: properly discard data received after request/response body is closed": break the pipe before refunding unread credit, fail pipe writes after a break, refund connection credit for DATA that loses the race with Close, plus the server-side handler-closed-body refund. Fixes x/net/http2: TestTransportReturnsUnusedFlowControlMultipleWrites failures golang/go#57578.

Not ported: the server-side half of the inflow rewrite (out of scope; the server keeps the old flow accounting plus the 9f24bb44 refund fix).

Testing

• 新建 TestTransportBodyCloseRaceRefundsConnFlow: drives 200 aborted-mid-flood downloads over one pooled connection against a frame-level fake server that strictly obeys the client's advertised windows, then asserts a full-body canary request completes. Wedges on master (canary starves with zero bytes, window ledger decays to 0) and passes with this change, including -race -count=3.
• TestTransportSlowReaderLargeResponse (the existing large-download regression test) still passes — the inflow accounting also covers the bug it guards.
• Full ./http2 suite compared against master: identical failure set (20 pre-existing failures, e.g. TestHeaderOrder3, TestTransportH2c, and TestTransport hangs on both); no new failures. Suite run with -vet=off because pre-existing vet errors in fhttp_test.go block the test build.

Behavior notes for review

• WINDOW_UPDATE cadence changes: updates are now batched at ≥4 KiB granularity (or window-doubling) instead of refresh-to-full below half-window. SETTINGS values, the connection-preface WINDOW_UPDATE increment, and header/pseudo-header ordering — the fingerprint-relevant surface — are untouched.
• pipe.Write on a broken pipe now returns errClosedPipeWrite instead of silently discarding; both transport and server callers handle it by refunding.

Release

This branch is based on the v0.6.8-kernel.1 lineage (master + kernel patches + module rename), so merging is a maintainer tagging v0.6.8-kernel.2 from it and bumping github.com/kernel/fhttp in consumers.

---

Note

Medium Risk
HTTP/2 transport flow-control and body-close ordering changed in core connection pooling paths; module rename breaks consumers until they bump imports. SETTINGS/fingerprint surface is unchanged per PR notes.

Overview
Renames the Go module from github.com/bogdanfinn/fhttp to github.com/kernel/fhttp and updates imports, go:generate bundle paths, and tests accordingly.

The functional work ports upstream golang.org/x/net HTTP/2 inbound flow-control fixes so pooled connections no longer wedge when response bodies are aborted mid-transfer:

• Introduces an inflow type that credits every consumed byte and batches WINDOW_UPDATE frames (≥4 KiB or window-doubling), replacing half-window refresh and the fork’s adaptive small-window stream logic on the client transport.
• transportResponseBody.Close breaks the body pipe before refunding unread credit; pipe.Write after a break returns errClosedPipeWrite instead of silently accepting data.
• processData enforces connection + stream windows with takeInflows, refunds credit when DATA loses the race with a closed body, and the server stops treating handler-closed request bodies as a stream error while returning connection-level flow for discarded DATA.

Adds TestTransportSlowReaderLargeResponse and TestTransportBodyCloseRaceRefundsConnFlow to guard WINDOW_UPDATE desync and connection-credit leaks.

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[hiroTamada]

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