Lazily load the library on first use instead of on import

3 files changed, 20 insertions, 4 deletions

diff --git a/cryptography/hazmat/bindings/utils.py b/cryptography/hazmat/bindings/utils.py
index f741aada..0fceac28 100644
--- a/cryptography/hazmat/bindings/utils.py
+++ b/cryptography/hazmat/bindings/utils.py
@@ -14,6 +14,7 @@
 from __future__ import absolute_import, division, print_function
 
 import binascii
+import threading
 
 import sys
 
@@ -21,6 +22,21 @@ from cffi import FFI
 from cffi.verifier import Verifier
 
 
+class LazyLibrary(object):
+    def __init__(self, ffi):
+        self._ffi = ffi
+        self._lib = None
+        self._lock = threading.Lock()
+
+    def __getattr__(self, name):
+        if self._lib is None:
+            with self._lock:
+                if self._lib is None:
+                    self._lib = self._ffi.verifier.load_library()
+
+        return getattr(self._lib, name)
+
+
 def load_library_for_binding(ffi, module_prefix, modules):
     lib = ffi.verifier.load_library()
 
diff --git a/cryptography/hazmat/primitives/constant_time.py b/cryptography/hazmat/primitives/constant_time.py
index 8ba045aa..a14eda85 100644
--- a/cryptography/hazmat/primitives/constant_time.py
+++ b/cryptography/hazmat/primitives/constant_time.py
@@ -16,7 +16,7 @@ from __future__ import absolute_import, division, print_function
 import hmac
 import os
 
-from cryptography.hazmat.bindings.utils import build_ffi
+from cryptography.hazmat.bindings.utils import LazyLibrary, build_ffi
 
 
 with open(os.path.join(os.path.dirname(__file__), "src/constant_time.h")) as f:
@@ -27,7 +27,7 @@ with open(os.path.join(os.path.dirname(__file__), "src/constant_time.c")) as f:
 
 
 _ffi = build_ffi(cdef_source=TYPES, verify_source=FUNCTIONS)
-_lib = _ffi.verifier.load_library()
+_lib = LazyLibrary(_ffi)
 
 
 if hasattr(hmac, "compare_digest"):
diff --git a/cryptography/hazmat/primitives/padding.py b/cryptography/hazmat/primitives/padding.py
index 6bb51ab9..7aeeff7c 100644
--- a/cryptography/hazmat/primitives/padding.py
+++ b/cryptography/hazmat/primitives/padding.py
@@ -17,7 +17,7 @@ import six
 
 from cryptography import utils
 from cryptography.exceptions import AlreadyFinalized
-from cryptography.hazmat.bindings.utils import build_ffi
+from cryptography.hazmat.bindings.utils import LazyLibrary, build_ffi
 from cryptography.hazmat.primitives import interfaces
 
 
@@ -65,7 +65,7 @@ uint8_t Cryptography_check_pkcs7_padding(const uint8_t *data,
 
 
 _ffi = build_ffi(cdef_source=TYPES, verify_source=FUNCTIONS)
-_lib = _ffi.verifier.load_library()
+_lib = LazyLibrary(_ffi)
 
 
 class PKCS7(object):