[aelnahas]

Motivation/summary

Experiment branched off #21721 to check whether the http2legacy build tag unblocks the Go 1.27 upgrade.

Go 1.27 moved the HTTP/2 implementation into net/http. With x/net >= v0.55.0, http2.ConfigureServer registers that internal implementation and net/http serves h2 connections itself, never calling the TLSNextProto handler gmux relies on to multiplex gRPC and HTTP/2. That is the cause of the TestHTTP2OverTLSFramingConsistency failure tracked in #21924.

Building with -tags http2legacy selects x/net's original implementation, which registers TLSNextProto["h2"], so net/http takes its retained legacy ALPN path and gmux works unchanged. This PR appends the tag to DEFAULT_TAGS in the Makefile, which flows into every build, test, and system-test target.

This is a stopgap: x/net only backports critical and security fixes to the legacy implementation. The durable fix is in gmux.

How to test these changes

CI system tests, in particular TestHTTP2OverTLSFramingConsistency and the gRPC/OTLP system tests.

Related issues

• Go 1.27 upgrade blocker due to gmux #21924
• feat: bump go to 1.27 #21721

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[mergify]

This pull request does not have a backport label. Could you fix it @aelnahas? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8.19 is the label to automatically backport to the 8.19 branch.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[mergify]

This pull request is now in conflicts. Could you fix it @aelnahas? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b feat/go1.27-http2legacy upstream/feat/go1.27-http2legacy
git merge upstream/main
git push upstream feat/go1.27-http2legacy

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