Fixed #3747 -- cache extensions on x.509 objects (#3769)

* Fixed #3747 -- cache extensions on x.509 objects

* be kind to cpython, save a dict lookup

* flake8

* changelog

2 files changed, 18 insertions, 4 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/x509.py b/src/cryptography/hazmat/backends/openssl/x509.py
index a04d6d53..5bf0438e 100644
--- a/src/cryptography/hazmat/backends/openssl/x509.py
+++ b/src/cryptography/hazmat/backends/openssl/x509.py
@@ -126,7 +126,7 @@ class _Certificate(object):
         oid = _obj2txt(self._backend, alg[0].algorithm)
         return x509.ObjectIdentifier(oid)
 
-    @property
+    @utils.cached_property
     def extensions(self):
         if self._backend._lib.CRYPTOGRAPHY_OPENSSL_110_OR_GREATER:
             return _CERTIFICATE_EXTENSION_PARSER.parse(
@@ -200,7 +200,7 @@ class _RevokedCertificate(object):
             )
         )
 
-    @property
+    @utils.cached_property
     def extensions(self):
         return _REVOKED_CERTIFICATE_EXTENSION_PARSER.parse(
             self._backend, self._x509_revoked
@@ -334,7 +334,7 @@ class _CertificateRevocationList(object):
         else:
             return self._backend._lib.sk_X509_REVOKED_num(revoked)
 
-    @property
+    @utils.cached_property
     def extensions(self):
         return _CRL_EXTENSION_PARSER.parse(self._backend, self._x509_crl)
 
@@ -391,7 +391,7 @@ class _CertificateSigningRequest(object):
         oid = _obj2txt(self._backend, alg[0].algorithm)
         return x509.ObjectIdentifier(oid)
 
-    @property
+    @utils.cached_property
     def extensions(self):
         x509_exts = self._backend._lib.X509_REQ_get_extensions(self._x509_req)
         return _CSR_EXTENSION_PARSER.parse(self._backend, x509_exts)
diff --git a/src/cryptography/utils.py b/src/cryptography/utils.py
index d28dc71d..efb12e21 100644
--- a/src/cryptography/utils.py
+++ b/src/cryptography/utils.py
@@ -145,3 +145,17 @@ def deprecated(value, module_name, message, warning_class):
     if not isinstance(module, _ModuleWithDeprecations):
         sys.modules[module_name] = _ModuleWithDeprecations(module)
     return _DeprecatedValue(value, message, warning_class)
+
+
+def cached_property(func):
+    cached_name = "_cached_{0}".format(func)
+    sentinel = object()
+
+    def inner(instance):
+        cache = getattr(instance, cached_name, sentinel)
+        if cache is not sentinel:
+            return cache
+        result = func(instance)
+        setattr(instance, cached_name, result)
+        return result
+    return property(inner)