Upstream Sync Report 2026-08-27: net/http idle-pool race and HTTP/2 IDNA consistency

[imroc]

Upstream changes

• golang/go@88b2ccb — net/http: add multiplexed connections to the idle pool prior to delivery

  • Adds HTTP/2 and HTTP/3 connections to the idle pool before delivering them to the waiting request.
  • Fixes a race where CloseIdleConnections can run before a multiplexed connection is visible in the idle pool.

• golang/go@2117d7c — net/http: use consistent IDNA processing for h1 and h2

  • Aligns net/http and the bundled HTTP/2 implementation on idna.Lookup.ToASCII with an ASCII fast path.
  • Prevents HTTP/2 pool lookups from using a different host normalization than net/http.

Impact on req

• transport.go still follows the old ordering in dialConnFor: it delivers an alternate connection to the waiting request first and only puts it in the idle pool afterward. This leaves the same CloseIdleConnections race present for HTTP/2 and HTTP/3.
• http.go already uses the aligned idna.Lookup.ToASCII behavior with an ASCII fast path.
• internal/http2/transport.go still uses idna.ToASCII in authorityAddr without the ASCII fast path, so HTTP/2 connection-pool keys can diverge from the root transport's normalized address for IDN hosts.

Suggested action

• Port 88b2ccb into transport.go, preserving req's custom transport behavior.
• Align internal/http2/transport.go with the IDNA normalization used by http.go.
• Add regression coverage for closing idle connections while an alternate connection is being delivered, and for IDN-based HTTP/2 connection reuse.

[imroc]

Two additional upstream commits extend the IDNA sync scope (same code paths as above):

• golang/go@9eac81b — net/http: reject IDNA conversion to the empty string

  • Follow-up to 2117d7c: rejects conversions that produce an empty string.
  • src/net/http/transport.go changes idnaASCIIFromURL to if v, err := idnaASCII(addr); err == nil && v != "". req's root transport.go still has the old form without the v != "" check, so this needs to be ported.
  • src/net/http/internal/http2/transport.go applies the same a != "" check in authorityAddr.

• golang/net@55577aa — http2: use IDNA Lookup profile, not raw Punycode translation

  • x/net/http2 counterpart of the stdlib IDNA alignment: switches authorityAddr to idna.Lookup.ToASCII with an ASCII fast path (plus the a != "" rejection).
  • Upstream implements this across http2/ascii.go (isASCII) and http2/transport_common.go; in req the corresponding code lives in internal/http2/transport.go (authorityAddr).

56480a2 (net/http: bind the HTTP/3 test client to the server's address) is test-only and requires no sync.