Safely get certificate's leaf (#150)

- It's not guaranteed that `tls.X509KeyPair` will set `c.Leaf`.
- This patch fixes this by using a wrapper that parses the leaf
certificate(in bytes) if `c.Leaf` wasn't set.
- Resolves #149

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/150
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Gusted <gusted@noreply.codeberg.org>
Co-committed-by: Gusted <gusted@noreply.codeberg.org>

server/certificates/certificates.go

@@ -303,7 +303,11 @@ func obtainCert(acmeClient *lego.Client, domains []string, renew *certificate.Re
 		log.Error().Err(err).Msgf("Couldn't obtain again a certificate or %v", domains)
 		if renew != nil && renew.CertURL != "" {
 			tlsCertificate, err := tls.X509KeyPair(renew.Certificate, renew.PrivateKey)
-			if err == nil && tlsCertificate.Leaf.NotAfter.After(time.Now()) {
+			if err != nil {
+				return mockCert(domains[0], err.Error(), mainDomainSuffix, keyDatabase), err
+			}
+			leaf, err := leaf(&tlsCertificate)
+			if err == nil && leaf.NotAfter.After(time.Now()) {
 				// avoid sending a mock cert instead of a still valid cert, instead abuse CSR field to store time to try again at
 				renew.CSR = []byte(strconv.FormatInt(time.Now().Add(6*time.Hour).Unix(), 10))
 				if err := keyDatabase.Put(name, renew); err != nil {
@@ -529,3 +533,12 @@ func MaintainCertDB(ctx context.Context, interval time.Duration, mainDomainSuffi
 		}
 	}
 }
+
+// leaf returns the parsed leaf certificate, either from c.leaf or by parsing
+// the corresponding c.Certificate[0].
+func leaf(c *tls.Certificate) (*x509.Certificate, error) {
+	if c.Leaf != nil {
+		return c.Leaf, nil
+	}
+	return x509.ParseCertificate(c.Certificate[0])
+}