fix: expose ALPN in TLS handshake

[howardjohn]

New versions of gRPC-go are enforcing the h2 ALPN to be presented during the TLS handshake. See
https://pkg.go.dev/google.golang.org/grpc/internal/envconfig#pkg-variables GRPC_ENFORCE_ALPN_ENABLED. The TLS server here isn't automatically getting this set due to usage of GetConfigForClient.

This properly sets it.

Without this, istio-csr will be incompatible with Istio 1.24, which upgrades the gRPC version. Note this can be worked around by setting GRPC_ENFORCE_ALPN_ENABLED=false on the proxy container, which Istio is able to do -- so there is an escape hatch for users.

The Istio logs look like
"transport: authentication handshake failed: credentials: cannot check peer: missing selected ALPN property"

[cert-manager-prow]

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[cert-manager-prow]

Hi @howardjohn. Thanks for your PR.

I'm waiting for a cert-manager member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[howardjohn]

cc @inteon

[ThatsMrTalbot]

/ok-to-test

[ThatsMrTalbot]

Hiya! Thanks for contributing this fix.

Looks like the golangci-lint check is failing.

Specifically its complaining about the indentation now you have added the comment block, running golangci-lint run --fix gave me this diff:

                Certificates: []tls.Certificate{tlsCert},
                // Advertise ALPN, required in modern gRPC versions
                // Typically gRPC sets this for us, but since this tls.Config ultimately gets returned in GetConfigForClient it doesn't.
-               NextProtos:   []string{"h2"},
-               ClientAuth:   tls.VerifyClientCertIfGiven,
-               ClientCAs:    peerCertVerifier.GetGeneralCertPool(),
+               NextProtos: []string{"h2"},
+               ClientAuth: tls.VerifyClientCertIfGiven,
+               ClientCAs:  peerCertVerifier.GetGeneralCertPool(),
                VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
                        err := peerCertVerifier.VerifyPeerCert(rawCerts, verifiedChains)
                        if err != nil {

[inteon]

Thank you for the bug report and fix.

Could you add a tests/ modify our tests to make sure GRPC_ENFORCE_ALPN_ENABLED now works & will keep working in the future?

Also, do you know where in code this is normally auto-set when we are not using GetConfigForClient?