derp/derpserver,cmd/derper: don't mutate cert provider's shared tls.Certificate (#20478)

ModifyTLSConfigToAddMetaCert (and its inline copy in cmd/derper) appended
the DERP meta cert directly to the *tls.Certificate returned by the
underlying GetCertificate. autocert returns a certificate sharing a cached
chain slice (and, on the TLS-ALPN token path, the same pointer) across
concurrent handshakes, so the in-place append was a data race and could
grow the served chain unboundedly.

Return a shallow copy with the meta cert appended to a fresh backing
array instead, and have cmd/derper reuse ModifyTLSConfigToAddMetaCert
rather than duplicating the wrapper.

Fixes #20352

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

cmd/derper/derper.go

@@ -354,15 +354,7 @@ func main() {
 			log.Fatalf("derper: can not start cert provider: %v", err)
 		}
 		httpsrv.TLSConfig = certManager.TLSConfig()
-		getCert := httpsrv.TLSConfig.GetCertificate
-		httpsrv.TLSConfig.GetCertificate = func(hi *tls.ClientHelloInfo) (*tls.Certificate, error) {
-			cert, err := getCert(hi)
-			if err != nil {
-				return nil, err
-			}
-			cert.Certificate = append(cert.Certificate, s.MetaCert())
-			return cert, nil
-		}
+		s.ModifyTLSConfigToAddMetaCert(httpsrv.TLSConfig)
 		// Disable TLS 1.0 and 1.1, which are obsolete and have security issues.
 		httpsrv.TLSConfig.MinVersion = tls.VersionTLS12
 		httpsrv.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {