Add ms_tlsx25519 godebug to opt-out from X25519 (#2002)

* add ms_tlsx25519 godebug to opt-out from X25519

* simplify defaultCurvePreferences

* fix test

* fix docs

Author: qmuntal

patches/0012-align-TLS-settings-with-Microsoft-policies.patch

@@ -0,0 +1,98 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: bot-for-go[bot] <199222863+bot-for-go[bot]@users.noreply.github.com>
+Date: Fri, 12 Dec 2025 16:46:44 +0100
+Subject: [PATCH] align TLS settings with Microsoft policies
+
+---
+ doc/godebug.md                 |  4 ++++
+ src/crypto/tls/defaults.go     | 10 +++++++++-
+ src/crypto/tls/tls_test.go     | 24 ++++++++++++++++++++++++
+ src/internal/godebugs/table.go |  1 +
+ 4 files changed, 38 insertions(+), 1 deletion(-)
+
+diff --git a/doc/godebug.md b/doc/godebug.md
+index 28a2dc506ef8ff..8be51c6a136df6 100644
+--- a/doc/godebug.md
++++ b/doc/godebug.md
+@@ -183,6 +183,10 @@ APIs ignore the random `io.Reader` parameter. For Go 1.26, it defaults
+ to `cryptocustomrand=0`, ignoring the random parameters. Using `cryptocustomrand=1`
+ reverts to the pre-Go 1.26 behavior.
+ 
++Microsoft build of Go 1.26 added a new `ms_tlsx25519` setting that controls whether
++`crypto/tls` enables the X25519 and X25519MLKEM768 TLS groups by default. Setting
++`ms_tlsx25519=0` disables these groups. The default value is `ms_tlsx25519=1`.
++
+ ### Go 1.25
+ 
+ Go 1.25 added a new `decoratemappings` setting that controls whether the Go
+diff --git a/src/crypto/tls/defaults.go b/src/crypto/tls/defaults.go
+index 8de8d7e0934b07..0ba8d3ebf0d6e4 100644
+--- a/src/crypto/tls/defaults.go
++++ b/src/crypto/tls/defaults.go
+@@ -15,10 +15,18 @@ import (
+ 
+ var tlsmlkem = godebug.New("tlsmlkem")
+ var tlssecpmlkem = godebug.New("tlssecpmlkem")
++var ms_tlsx25519 = godebug.New("ms_tlsx25519")
+ 
+ // defaultCurvePreferences is the default set of supported key exchanges, as
+ // well as the preference order.
+-func defaultCurvePreferences() []CurveID {
++func defaultCurvePreferences() (groups []CurveID) {
++	defer func() {
++		if ms_tlsx25519.Value() == "0" {
++			groups = slices.DeleteFunc(groups, func(c CurveID) bool {
++				return c == X25519 || c == X25519MLKEM768
++			})
++		}
++	}()
+ 	switch {
+ 	// tlsmlkem=0 restores the pre-Go 1.24 default.
+ 	case tlsmlkem.Value() == "0":
+diff --git a/src/crypto/tls/tls_test.go b/src/crypto/tls/tls_test.go
+index 5f4a2b9de4fd1f..98726c6e9509d5 100644
+--- a/src/crypto/tls/tls_test.go
++++ b/src/crypto/tls/tls_test.go
+@@ -2113,6 +2113,30 @@ func TestHandshakeMLKEM(t *testing.T) {
+ 			expectClient:   []CurveID{X25519MLKEM768, X25519, CurveP256, CurveP384, CurveP521},
+ 			expectSelected: X25519MLKEM768,
+ 		},
++		{
++			name: "GODEBUG ms_tlsx25519=0",
++			preparation: func(t *testing.T) {
++				t.Setenv("GODEBUG", "ms_tlsx25519=0")
++			},
++			expectClient:   []CurveID{SecP256r1MLKEM768, SecP384r1MLKEM1024, CurveP256, CurveP384, CurveP521},
++			expectSelected: SecP256r1MLKEM768,
++		},
++		{
++			name: "GODEBUG ms_tlsx25519=0 and tlsmlkem=0",
++			preparation: func(t *testing.T) {
++				t.Setenv("GODEBUG", "ms_tlsx25519=0,tlsmlkem=0")
++			},
++			expectClient:   []CurveID{CurveP256, CurveP384, CurveP521},
++			expectSelected: CurveP256,
++		},
++		{
++			name: "GODEBUG ms_tlsx25519=0 and tlssecpmlkem=0",
++			preparation: func(t *testing.T) {
++				t.Setenv("GODEBUG", "ms_tlsx25519=0,tlssecpmlkem=0")
++			},
++			expectClient:   []CurveID{CurveP256, CurveP384, CurveP521},
++			expectSelected: CurveP256,
++		},
+ 	}
+ 
+ 	baseConfig := testConfig.Clone()
+diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
+index 8f6d8bbdda656c..089d7172d5985b 100644
+--- a/src/internal/godebugs/table.go
++++ b/src/internal/godebugs/table.go
+@@ -49,6 +49,7 @@ var All = []Info{
+ 	{Name: "httpservecontentkeepheaders", Package: "net/http", Changed: 23, Old: "1"},
+ 	{Name: "installgoroot", Package: "go/build"},
+ 	{Name: "jstmpllitinterp", Package: "html/template", Opaque: true}, // bug #66217: remove Opaque
++	{Name: "ms_tlsx25519", Package: "crypto/tls", Opaque: true},
+ 	//{Name: "multipartfiles", Package: "mime/multipart"},
+ 	{Name: "multipartmaxheaders", Package: "mime/multipart"},
+ 	{Name: "multipartmaxparts", Package: "mime/multipart"},