control/controlclient: use the most recent syspolicy.MachineCertificateSubject value

This PR removes the sync.Once wrapper around retrieving the MachineCertificateSubject policy
setting value, ensuring the most recent version is always used if it changes after the service starts.

Although this policy setting is used by a very limited number of customers, recent support escalations have highlighted issues caused by outdated or incorrect policy values being applied.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>

control/controlclient/sign_supported.go

@@ -13,7 +13,6 @@ import (
 	"crypto/x509"
 	"errors"
 	"fmt"
-	"sync"
 	"time"
 
 	"github.com/tailscale/certstore"
@@ -22,11 +21,6 @@ import (
 	"tailscale.com/util/syspolicy"
 )
 
-var getMachineCertificateSubjectOnce struct {
-	sync.Once
-	v string // Subject of machine certificate to search for
-}
-
 // getMachineCertificateSubject returns the exact name of a Subject that needs
 // to be present in an identity's certificate chain to sign a RegisterRequest,
 // formatted as per pkix.Name.String(). The Subject may be that of the identity
@@ -37,11 +31,8 @@ var getMachineCertificateSubjectOnce struct {
 //
 // Example: "CN=Tailscale Inc Test Root CA,OU=Tailscale Inc Test Certificate Authority,O=Tailscale Inc,ST=ON,C=CA"
 func getMachineCertificateSubject() string {
-	getMachineCertificateSubjectOnce.Do(func() {
-		getMachineCertificateSubjectOnce.v, _ = syspolicy.GetString(syspolicy.MachineCertificateSubject, "")
-	})
-
-	return getMachineCertificateSubjectOnce.v
+	machineCertSubject, _ := syspolicy.GetString(syspolicy.MachineCertificateSubject, "")
+	return machineCertSubject
 }
 
 var (