@@ -233,7 +233,15 @@ func (oc *OCSPMonitor) run() {
 	quitCh := s.quitCh
 	s.mu.Unlock()
 
-	defer s.grWG.Done()
+	var doShutdown bool
+	defer func() {
+		// Need to decrement before shuting down, otherwise shutdown
+		// would be stuck waiting on grWG to go down to 0.
+		s.grWG.Done()
+		if doShutdown {
+			s.Shutdown()
+		}
+	}()
 
 	oc.mu.Lock()
 	shutdownOnRevoke := oc.shutdownOnRevoke
// Copyright 2021 The NATS Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package server

import (
	"crypto/sha256"
	"crypto/tls"
	"crypto/x509"
	"encoding/asn1"
	"encoding/base64"
	"encoding/pem"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"time"

	"golang.org/x/crypto/ocsp"
)

const (
	defaultOCSPStoreDir      = "ocsp"
	defaultOCSPCheckInterval = 24 * time.Hour
	minOCSPCheckInterval     = 2 * time.Minute
)

type OCSPMode uint8

const (
	// OCSPModeAuto staples a status, only if "status_request" is set in cert.
	OCSPModeAuto OCSPMode = iota

	// OCSPModeAlways enforces OCSP stapling for certs and shuts down the server in
	// case a server is revoked or cannot get OCSP staples.
	OCSPModeAlways

	// OCSPModeNever disables OCSP stapling even if cert has Must-Staple flag.
	OCSPModeNever

	// OCSPModeMust honors the Must-Staple flag from a certificate but also causing shutdown
	// in case the certificate has been revoked.
	OCSPModeMust
)

// OCSPMonitor monitors the state of a staple per certificate.
type OCSPMonitor struct {
	kind     string
	mu       sync.Mutex
	raw      []byte
	srv      *Server
	certFile string
	resp     *ocsp.Response
	hc       *http.Client
	stopCh   chan struct{}
	Leaf     *x509.Certificate
	Issuer   *x509.Certificate

	shutdownOnRevoke bool
}

func (oc *OCSPMonitor) getNextRun() time.Duration {
	oc.mu.Lock()
	nextUpdate := oc.resp.NextUpdate
	oc.mu.Unlock()

	now := time.Now()
	if nextUpdate.IsZero() {
		// If response is missing NextUpdate, we check the day after.
		// Technically, if NextUpdate is missing, we can try whenever.
		// https://tools.ietf.org/html/rfc6960#section-4.2.2.1
		return defaultOCSPCheckInterval
	}
	dur := nextUpdate.Sub(now) / 2

	// If negative, then wait a couple of minutes before getting another staple.
	if dur < 0 {
		return minOCSPCheckInterval
	}

	return dur
}

func (oc *OCSPMonitor) getStatus() ([]byte, *ocsp.Response, error) {
	raw, resp := oc.getCacheStatus()
	if len(raw) > 0 && resp != nil {
		// Check if the OCSP is still valid.
		if err := validOCSPResponse(resp); err == nil {
			return raw, resp, nil
		}
	}
	var err error
	raw, resp, err = oc.getLocalStatus()
	if err == nil {
		return raw, resp, nil
	}

	return oc.getRemoteStatus()
}

func (oc *OCSPMonitor) getCacheStatus() ([]byte, *ocsp.Response) {
	oc.mu.Lock()
	defer oc.mu.Unlock()
	return oc.raw, oc.resp
}

func (oc *OCSPMonitor) getLocalStatus() ([]byte, *ocsp.Response, error) {
	opts := oc.srv.getOpts()
	storeDir := opts.StoreDir
	if storeDir == _EMPTY_ {
		return nil, nil, fmt.Errorf("store_dir not set")
	}

	// This key must be based upon the current full certificate, not the public key,
	// so MUST be on the full raw certificate and not an SPKI or other reduced form.
	key := fmt.Sprintf("%x", sha256.Sum256(oc.Leaf.Raw))

	oc.mu.Lock()
	raw, err := ioutil.ReadFile(filepath.Join(storeDir, defaultOCSPStoreDir, key))
	oc.mu.Unlock()
	if err != nil {
		return nil, nil, err
	}

	resp, err := ocsp.ParseResponse(raw, oc.Issuer)
	if err != nil {
		return nil, nil, err
	}
	if err := validOCSPResponse(resp); err != nil {
		return nil, nil, err
	}

	// Cache the response.
	oc.mu.Lock()
	oc.raw = raw
	oc.resp = resp
	oc.mu.Unlock()

	return raw, resp, nil
}

func (oc *OCSPMonitor) getRemoteStatus() ([]byte, *ocsp.Response, error) {
	opts := oc.srv.getOpts()
	var overrideURLs []string
	if config := opts.OCSPConfig; config != nil {
		overrideURLs = config.OverrideURLs
	}
	getRequestBytes := func(u string, hc *http.Client) ([]byte, error) {
		resp, err := hc.Get(u)
		if err != nil {
			return nil, err
		}
		defer resp.Body.Close()
		if resp.StatusCode != http.StatusOK {
			return nil, fmt.Errorf("non-ok http status: %d", resp.StatusCode)
		}

		return ioutil.ReadAll(resp.Body)
	}

	// Request documentation:
	// https://tools.ietf.org/html/rfc6960#appendix-A.1

	reqDER, err := ocsp.CreateRequest(oc.Leaf, oc.Issuer, nil)
	if err != nil {
		return nil, nil, err
	}

	reqEnc := base64.StdEncoding.EncodeToString(reqDER)

	responders := oc.Leaf.OCSPServer
	if len(overrideURLs) > 0 {
		responders = overrideURLs
	}
	if len(responders) == 0 {
		return nil, nil, fmt.Errorf("no available ocsp servers")
	}

	oc.mu.Lock()
	hc := oc.hc
	oc.mu.Unlock()
	var raw []byte
	for _, u := range responders {
		u = strings.TrimSuffix(u, "/")
		raw, err = getRequestBytes(fmt.Sprintf("%s/%s", u, reqEnc), hc)
		if err == nil {
			break
		}
	}
	if err != nil {
		return nil, nil, fmt.Errorf("exhausted ocsp servers: %w", err)
	}

	resp, err := ocsp.ParseResponse(raw, oc.Issuer)
	if err != nil {
		return nil, nil, err
	}
	if err := validOCSPResponse(resp); err != nil {
		return nil, nil, err
	}

	if storeDir := opts.StoreDir; storeDir != _EMPTY_ {
		key := fmt.Sprintf("%x", sha256.Sum256(oc.Leaf.Raw))
		if err := oc.writeOCSPStatus(storeDir, key, raw); err != nil {
			return nil, nil, fmt.Errorf("failed to write ocsp status: %w", err)
		}
	}

	oc.mu.Lock()
	oc.raw = raw
	oc.resp = resp
	oc.mu.Unlock()

	return raw, resp, nil
}

func (oc *OCSPMonitor) run() {
	s := oc.srv
	s.mu.Lock()
	quitCh := s.quitCh
	s.mu.Unlock()

	defer s.grWG.Done()

	oc.mu.Lock()
	shutdownOnRevoke := oc.shutdownOnRevoke
	certFile := oc.certFile
	stopCh := oc.stopCh
	kind := oc.kind
	oc.mu.Unlock()

	var nextRun time.Duration
	_, resp, err := oc.getStatus()
	if err == nil && resp.Status == ocsp.Good {
		nextRun = oc.getNextRun()
		t := resp.NextUpdate.Format(time.RFC3339Nano)
		s.Noticef(
			"Found OCSP status for %s certificate at '%s': good, next update %s, checking again in %s",
			kind, certFile, t, nextRun,
		)
	} else if err == nil && shutdownOnRevoke {
		// If resp.Status is ocsp.Revoked, ocsp.Unknown, or any other value.
		s.Errorf("Found OCSP status for %s certificate at '%s': %s", kind, certFile, ocspStatusString(resp.Status))
		s.Shutdown()
		return
	}

	for {
		// On reload, if the certificate changes then need to stop this monitor.
		select {
		case <-time.After(nextRun):
		case <-stopCh:
			// In case of reload and have to restart the OCSP stapling monitoring.
			return
		case <-quitCh:
			// Server quit channel.
			return
		}
		_, resp, err := oc.getRemoteStatus()
		if err != nil {
			nextRun = oc.getNextRun()
			s.Errorf("Bad OCSP status update for certificate '%s': %s, trying again in %v", certFile, err, nextRun)
			continue
		}

		switch n := resp.Status; n {
		case ocsp.Good:
			nextRun = oc.getNextRun()
			t := resp.NextUpdate.Format(time.RFC3339Nano)
			s.Noticef(
				"Received OCSP status for %s certificate '%s': good, next update %s, checking again in %s",
				kind, certFile, t, nextRun,
			)
			continue
		default:
			s.Errorf("Received OCSP status for %s certificate '%s': %s", kind, certFile, ocspStatusString(n))
			if shutdownOnRevoke {
				s.Shutdown()
			}
			return
		}
	}
}

func (oc *OCSPMonitor) stop() {
	oc.mu.Lock()
	stopCh := oc.stopCh
	oc.mu.Unlock()
	stopCh <- struct{}{}
}

// NewOCSPMonitor takes a TLS configuration then wraps it with the callbacks set for OCSP verification
// along with a monitor that will periodically fetch OCSP staples.
func (srv *Server) NewOCSPMonitor(config *tlsConfigKind) (*tls.Config, *OCSPMonitor, error) {
	kind := config.kind
	tc := config.tlsConfig
	tcOpts := config.tlsOpts
	opts := srv.getOpts()
	oc := opts.OCSPConfig

	// We need to track the CA certificate in case the CA is not present
	// in the chain to be able to verify the signature of the OCSP staple.
	var (
		certFile string
		caFile   string
	)
	if kind == kindStringMap[CLIENT] {
		tcOpts = opts.tlsConfigOpts
		if opts.TLSCert != _EMPTY_ {
			certFile = opts.TLSCert
		}
		if opts.TLSCaCert != _EMPTY_ {
			caFile = opts.TLSCaCert
		}
	}
	if tcOpts != nil {
		certFile = tcOpts.CertFile
		caFile = tcOpts.CaFile
	}

	// NOTE: Currently OCSP Stapling is enabled only for the first certificate found.
	var mon *OCSPMonitor
	for _, cert := range tc.Certificates {
		// This is normally non-nil, but can still be nil here when in tests
		// or in some embedded scenarios.
		if cert.Leaf == nil {
			if len(cert.Certificate) <= 0 {
				return nil, nil, fmt.Errorf("no certificate found")
			}
			var err error
			cert.Leaf, err = x509.ParseCertificate(cert.Certificate[0])
			if err != nil {
				return nil, nil, fmt.Errorf("error parsing certificate: %v", err)
			}
		}
		var shutdownOnRevoke bool
		mustStaple := hasOCSPStatusRequest(cert.Leaf)
		if oc != nil {
			switch {
			case oc.Mode == OCSPModeNever:
				if mustStaple {
					srv.Warnf("Certificate at '%s' has MustStaple but OCSP is disabled", certFile)
				}
				return tc, nil, nil
			case oc.Mode == OCSPModeAlways:
				// Start the monitor for this cert even if it does not have
				// the MustStaple flag and shutdown the server in case the
				// staple ever gets revoked.
				mustStaple = true
				shutdownOnRevoke = true
			case oc.Mode == OCSPModeMust && mustStaple:
				shutdownOnRevoke = true
			case oc.Mode == OCSPModeAuto && !mustStaple:
				// "status_request" MustStaple flag not set in certificate. No need to do anything.
				return tc, nil, nil
			}
		}
		if !mustStaple {
			// No explicit OCSP config and cert does not have MustStaple flag either.
			return tc, nil, nil
		}

		if err := srv.setupOCSPStapleStoreDir(); err != nil {
			return nil, nil, err
		}

		// TODO: Add OCSP 'responder_cert' option in case CA cert not available.
		issuer, err := getOCSPIssuer(caFile, cert.Certificate)
		if err != nil {
			return nil, nil, err
		}

		mon = &OCSPMonitor{
			kind:             kind,
			srv:              srv,
			hc:               &http.Client{Timeout: 30 * time.Second},
			shutdownOnRevoke: shutdownOnRevoke,
			certFile:         certFile,
			stopCh:           make(chan struct{}, 1),
			Leaf:             cert.Leaf,
			Issuer:           issuer,
		}

		// Get the certificate status from the memory, then remote OCSP responder.
		if _, resp, err := mon.getStatus(); err != nil {
			return nil, nil, fmt.Errorf("bad OCSP status update for certificate at '%s': %s", certFile, err)
		} else if err == nil && resp != nil && resp.Status != ocsp.Good && shutdownOnRevoke {
			return nil, nil, fmt.Errorf("found existing OCSP status for certificate at '%s': %s", certFile, ocspStatusString(resp.Status))
		}

		// Callbacks below will be in charge of returning the certificate instead,
		// so this has to be nil.
		tc.Certificates = nil

		// GetCertificate returns a certificate that's presented to a client.
		tc.GetCertificate = func(info *tls.ClientHelloInfo) (*tls.Certificate, error) {
			raw, _, err := mon.getStatus()
			if err != nil {
				return nil, err
			}

			return &tls.Certificate{
				OCSPStaple:                   raw,
				Certificate:                  cert.Certificate,
				PrivateKey:                   cert.PrivateKey,
				SupportedSignatureAlgorithms: cert.SupportedSignatureAlgorithms,
				SignedCertificateTimestamps:  cert.SignedCertificateTimestamps,
				Leaf:                         cert.Leaf,
			}, nil
		}

		// Check whether need to verify staples from a client connection depending on the type.
		switch kind {
		case kindStringMap[ROUTER], kindStringMap[GATEWAY], kindStringMap[LEAF]:
			tc.VerifyConnection = func(s tls.ConnectionState) error {
				oresp := s.OCSPResponse
				if oresp == nil {
					return fmt.Errorf("%s client missing OCSP Staple", kind)
				}

				// Client route connections will verify the response of the staple.
				if len(s.VerifiedChains) == 0 {
					return fmt.Errorf("%s client missing TLS verified chains", kind)
				}
				chain := s.VerifiedChains[0]
				resp, err := ocsp.ParseResponseForCert(oresp, chain[0], issuer)
				if err != nil {
					return fmt.Errorf("failed to parse OCSP response from %s client: %w", kind, err)
				}
				if err := resp.CheckSignatureFrom(issuer); err != nil {
					return err
				}
				if resp.Status != ocsp.Good {
					return fmt.Errorf("bad status for OCSP Staple from %s client: %s", kind, ocspStatusString(resp.Status))
				}

				return nil
			}

			// When server makes a client connection, need to also present an OCSP Staple.
			tc.GetClientCertificate = func(info *tls.CertificateRequestInfo) (*tls.Certificate, error) {
				raw, _, err := mon.getStatus()
				if err != nil {
					return nil, err
				}
				cert.OCSPStaple = raw

				return &cert, nil
			}
		default:
			// GetClientCertificate returns a certificate that's presented to a server.
			tc.GetClientCertificate = func(info *tls.CertificateRequestInfo) (*tls.Certificate, error) {
				return &cert, nil
			}
		}

	}
	return tc, mon, nil
}

func (s *Server) setupOCSPStapleStoreDir() error {
	opts := s.getOpts()
	storeDir := opts.StoreDir
	if storeDir == _EMPTY_ {
		return nil
	}
	storeDir = filepath.Join(storeDir, defaultOCSPStoreDir)
	if stat, err := os.Stat(storeDir); os.IsNotExist(err) {
		if err := os.MkdirAll(storeDir, defaultDirPerms); err != nil {
			return fmt.Errorf("could not create OCSP storage directory - %v", err)
		}
	} else if stat == nil || !stat.IsDir() {
		return fmt.Errorf("OCSP storage directory is not a directory")
	}
	return nil
}

type tlsConfigKind struct {
	tlsConfig *tls.Config
	tlsOpts   *TLSConfigOpts
	kind      string
	apply     func(*tls.Config)
}

func (s *Server) configureOCSP() []*tlsConfigKind {
	sopts := s.getOpts()

	configs := make([]*tlsConfigKind, 0)

	if config := sopts.TLSConfig; config != nil {
		opts := sopts.tlsConfigOpts
		o := &tlsConfigKind{
			kind:      kindStringMap[CLIENT],
			tlsConfig: config,
			tlsOpts:   opts,
			apply:     func(tc *tls.Config) { sopts.TLSConfig = tc },
		}
		configs = append(configs, o)
	}
	if config := sopts.Cluster.TLSConfig; config != nil {
		opts := sopts.Cluster.tlsConfigOpts
		o := &tlsConfigKind{
			kind:      kindStringMap[ROUTER],
			tlsConfig: config,
			tlsOpts:   opts,
			apply:     func(tc *tls.Config) { sopts.Cluster.TLSConfig = tc },
		}
		configs = append(configs, o)
	}
	if config := sopts.LeafNode.TLSConfig; config != nil {
		opts := sopts.LeafNode.tlsConfigOpts
		o := &tlsConfigKind{
			kind:      kindStringMap[LEAF],
			tlsConfig: config,
			tlsOpts:   opts,
			apply: func(tc *tls.Config) {

				// RequireAndVerifyClientCert is used to tell a client that it
				// should send the client cert to the server.
				tc.ClientAuth = tls.RequireAndVerifyClientCert
				// GetClientCertificate is used by a client to send the client cert
				// to a server. We're a server, so we must not set this.
				tc.GetClientCertificate = nil
				sopts.LeafNode.TLSConfig = tc
			},
		}
		configs = append(configs, o)
	}
	for i, remote := range sopts.LeafNode.Remotes {
		opts := remote.tlsConfigOpts
		if config := remote.TLSConfig; config != nil {
			o := &tlsConfigKind{
				kind:      kindStringMap[LEAF],
				tlsConfig: config,
				tlsOpts:   opts,
				apply: func(tc *tls.Config) {
					// GetCertificate is used by a server to send the server cert to a
					// client. We're a client, so we must not set this.
					tc.GetCertificate = nil

					sopts.LeafNode.Remotes[i].TLSConfig = tc
				},
			}
			configs = append(configs, o)
		}
	}
	if config := sopts.Gateway.TLSConfig; config != nil {
		opts := sopts.Gateway.tlsConfigOpts
		o := &tlsConfigKind{
			kind:      kindStringMap[GATEWAY],
			tlsConfig: config,
			tlsOpts:   opts,
			apply:     func(tc *tls.Config) { sopts.Gateway.TLSConfig = tc },
		}
		configs = append(configs, o)
	}
	for i, remote := range sopts.Gateway.Gateways {
		opts := remote.tlsConfigOpts
		if config := remote.TLSConfig; config != nil {
			o := &tlsConfigKind{
				kind:      kindStringMap[GATEWAY],
				tlsConfig: config,
				tlsOpts:   opts,
				apply: func(tc *tls.Config) {
					sopts.Gateway.Gateways[i].TLSConfig = tc
				},
			}
			configs = append(configs, o)
		}
	}
	return configs
}

func (s *Server) enableOCSP() error {
	configs := s.configureOCSP()

	for _, config := range configs {
		tc, mon, err := s.NewOCSPMonitor(config)
		if err != nil {
			return err
		}
		// Check if an OCSP stapling monitor is required for this certificate.
		if mon != nil {
			s.ocsps = append(s.ocsps, mon)

			// Override the TLS config with one that follows OCSP.
			config.apply(tc)
		}
	}

	return nil
}

func (s *Server) startOCSPMonitoring() {
	s.mu.Lock()
	ocsps := s.ocsps
	s.mu.Unlock()
	if ocsps == nil {
		return
	}
	for _, mon := range ocsps {
		m := mon
		m.mu.Lock()
		kind := m.kind
		m.mu.Unlock()
		s.Noticef("OCSP Stapling enabled for %s connections", kind)
		s.startGoRoutine(func() { m.run() })
	}
}

func (s *Server) reloadOCSP() error {
	if err := s.setupOCSPStapleStoreDir(); err != nil {
		return err
	}

	s.mu.Lock()
	ocsps := s.ocsps
	s.mu.Unlock()

	// Stop all OCSP Stapling monitors in case there were any running.
	for _, oc := range ocsps {
		oc.stop()
	}

	configs := s.configureOCSP()

	// Restart the monitors under the new configuration.
	ocspm := make([]*OCSPMonitor, 0)
	for _, config := range configs {
		tc, mon, err := s.NewOCSPMonitor(config)
		if err != nil {
			return err
		}
		// Check if an OCSP stapling monitor is required for this certificate.
		if mon != nil {
			ocspm = append(ocspm, mon)

			// Apply latest TLS configuration.
			config.apply(tc)
		}
	}

	// Replace stopped monitors with the new ones.
	s.mu.Lock()
	s.ocsps = ocspm
	s.mu.Unlock()

	// Dispatch all goroutines once again.
	s.startOCSPMonitoring()

	return nil
}

func hasOCSPStatusRequest(cert *x509.Certificate) bool {
	// OID for id-pe-tlsfeature defined in RFC here:
	// https://datatracker.ietf.org/doc/html/rfc7633
	tlsFeatures := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 24}
	const statusRequestExt = 5

	// Example values:
	// * [48 3 2 1 5] - seen when creating own certs locally
	// * [30 3 2 1 5] - seen in the wild
	// Documentation:
	// https://tools.ietf.org/html/rfc6066

	for _, ext := range cert.Extensions {
		if !ext.Id.Equal(tlsFeatures) {
			continue
		}

		var val []int
		rest, err := asn1.Unmarshal(ext.Value, &val)
		if err != nil || len(rest) > 0 {
			return false
		}

		for _, n := range val {
			if n == statusRequestExt {
				return true
			}
		}
		break
	}

	return false
}

// writeOCSPStatus writes an OCSP status to a temporary file then moves it to a
// new path, in an attempt to avoid corrupting existing data.
func (oc *OCSPMonitor) writeOCSPStatus(storeDir, file string, data []byte) error {
	storeDir = filepath.Join(storeDir, defaultOCSPStoreDir)
	tmp, err := ioutil.TempFile(storeDir, "tmp-cert-status")
	if err != nil {
		return err
	}

	if _, err := tmp.Write(data); err != nil {
		tmp.Close()
		os.Remove(tmp.Name())
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}

	oc.mu.Lock()
	err = os.Rename(tmp.Name(), filepath.Join(storeDir, file))
	oc.mu.Unlock()
	if err != nil {
		os.Remove(tmp.Name())
		return err
	}

	return nil
}

func parseCertPEM(name string) (*x509.Certificate, error) {
	data, err := ioutil.ReadFile(name)
	if err != nil {
		return nil, err
	}

	// Ignoring left over byte slice.
	block, _ := pem.Decode(data)
	if block == nil {
		return nil, fmt.Errorf("failed to parse PEM cert %s", name)
	}
	if block.Type != "CERTIFICATE" {
		return nil, fmt.Errorf("unexpected PEM certificate type: %s", block.Type)
	}

	return x509.ParseCertificate(block.Bytes)
}

// getOCSPIssuer returns a CA cert from the given path. If the path is empty,
// then this checks a given cert chain. If both are empty, then it returns an
// error.
func getOCSPIssuer(issuerCert string, chain [][]byte) (*x509.Certificate, error) {
	var issuer *x509.Certificate
	var err error
	switch {
	case len(chain) == 1 && issuerCert == _EMPTY_:
		err = fmt.Errorf("ocsp ca required in chain or configuration")
	case issuerCert != _EMPTY_:
		issuer, err = parseCertPEM(issuerCert)
	case len(chain) > 1 && issuerCert == _EMPTY_:
		issuer, err = x509.ParseCertificate(chain[1])
	default:
		err = fmt.Errorf("invalid ocsp ca configuration")
	}
	if err != nil {
		return nil, err
	} else if !issuer.IsCA {
		return nil, fmt.Errorf("%s invalid ca basic constraints: is not ca", issuerCert)
	}

	return issuer, nil
}

func ocspStatusString(n int) string {
	switch n {
	case ocsp.Good:
		return "good"
	case ocsp.Revoked:
		return "revoked"
	default:
		return "unknown"
	}
}

func validOCSPResponse(r *ocsp.Response) error {
	// Time validation not handled by ParseResponse.
	// https://tools.ietf.org/html/rfc6960#section-4.2.2.1
	if !r.NextUpdate.IsZero() && r.NextUpdate.Before(time.Now()) {
		t := r.NextUpdate.Format(time.RFC3339Nano)
		return fmt.Errorf("invalid ocsp NextUpdate, is past time: %s", t)
	}
	if r.ThisUpdate.After(time.Now()) {
		t := r.ThisUpdate.Format(time.RFC3339Nano)
		return fmt.Errorf("invalid ocsp ThisUpdate, is future time: %s", t)
	}

	return nil
}

@wallyqs @variadico Could you please review this part: what was happening is that when run() decides to call s.Shutdown(), this go routine would be blocked because Shutdown() would be waiting on grWG.Wait() that would never be done because this go routine itself had not yet called s.grWG.Done(). Can you think of if this would have been user-visible? And if so, what would be the symptoms? If not user-visible, this is just a fix that will help make sure that there are not left-over go routines when a test completes.