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>
bf7d815631