Cache parsed certificate instead of re-parsing for each field

Use @cached_property decorator to cache the results of parse_certificate().

This significantly cuts down on the number of times certs need to be
parsed for a list view.

lemur/tests/test_certificates.py

@@ -9,9 +9,11 @@ from cryptography import x509
 from cryptography.hazmat.backends import default_backend
 from marshmallow import ValidationError
 from freezegun import freeze_time
+from mock import patch
 
 from lemur.certificates.service import create_csr
 from lemur.certificates.views import *  # noqa
+from lemur.common import utils
 from lemur.domains.models import Domain
 
 
@@ -66,6 +68,21 @@ def test_get_certificate_primitives(certificate):
         assert len(primitives) == 24
 
 
+def test_certificate_output_schema(session, certificate, issuer_plugin):
+    from lemur.certificates.schemas import CertificateOutputSchema
+
+    # Clear the cached attribute first
+    if 'parsed_cert' in certificate.__dict__:
+        del certificate.__dict__['parsed_cert']
+
+    # Make sure serialization parses the cert only once (uses cached 'parsed_cert' attribute)
+    with patch('lemur.common.utils.parse_certificate', side_effect=utils.parse_certificate) as wrapper:
+        data, errors = CertificateOutputSchema().dump(certificate)
+        assert data['issuer'] == 'Example'
+
+    assert wrapper.call_count == 1
+
+
 def test_certificate_edit_schema(session):
     from lemur.certificates.schemas import CertificateEditInputSchema