Upstream Sync Report 2026-09-02: Transport.CancelRequest becomes a no-op

Upstream Change

Go stdlib commit a0f3b6a (2026-08-31):

net/http: make Transport.CancelRequest into a no-op

Fixes golang/go#79040.

Affected Code

req inlines src/net/http/transport.go as root transport.go, which still contains the legacy cancellation machinery:

• Transport.reqMu / Transport.reqCanceler fields
• prepareTransportCancel (registers every in-flight request in a per-Transport map)
• Transport.CancelRequest implementation
• roundTrip wiring that installs the wrapper cancel func

What the upstream change does

• Removes reqMu / reqCanceler and prepareTransportCancel
• Turns the deprecated Transport.CancelRequest into a no-op
• Drops the per-request mutex + map bookkeeping from the roundTrip hot path

Impact

• Behavior change: after syncing, the deprecated Transport.CancelRequest stops working. Users should migrate to context cancellation (NewRequestWithContext), which req already supports.
• Minor performance benefit: no per-request registration under reqMu.
• Not a security fix.

Other commits since the last baseline (no impact)

• 0d6e66c net/http/internal/http2: per-Server error channel pool — server.go is not inlined by req
• ef00dcf httputil/reverseproxy: preserve original header on 1xx — reverseproxy is not inlined
• f50b1aa net/http: enable more HTTP/3 tests — test-only
• 56fb6d1 net/http: return non-nil Response from ClientConn ping requests — clientconn.go bridge is not inlined (req uses its own legacy ClientConn)

Suggested action

Sync a0f3b6a into root transport.go (keeping req customizations) via the usual merge upstream net/http procedure.

Start by comparing root transport.go with Go stdlib commit a0f3b6a and review the usual “merge upstream net/http” procedure. Inspect req’s customizations around Transport.reqMu, Transport.reqCanceler, prepareTransportCancel, CancelRequest, and roundTrip. Done means the upstream cancellation changes are synced without losing req customizations, with context cancellation still supported and CancelRequest becoming a no-op.