[1.25] Add hardcoded paths to macOS frameworks for Big Sur compatibil…

…ity. (#1905)

ctypes.util.find_library is checking for the presence of the framework
files, which will fail due to macOS Big Sur's "dynamic linker cache".
Since these files really can't be anywhere else, we just hardcode the
path and load them.

See: https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11-beta-release-notes
(relevent excerpt provided below)

This update takes the approach of leaving the existing behavior as is
for macOS 10.8 (lowest supported version) through 10.15.

Allegedly Apple is going to number Big Sur as macOS 11, although right
now the beta returns a version of "10.16", so this code handles both as
being greater than or equal to "10.16".

Co-authored-by: Seth Michael Larson <sethmichaellarson@gmail.com>

src/urllib3/contrib/_securetransport/bindings.py

@@ -45,17 +45,11 @@
     c_bool,
 )
 from ctypes import CDLL, POINTER, CFUNCTYPE
+from urllib3.packages.six import raise_from
 
 
-security_path = find_library("Security")
-if not security_path:
-    raise ImportError("The library Security could not be found")
-
-
-core_foundation_path = find_library("CoreFoundation")
-if not core_foundation_path:
-    raise ImportError("The library CoreFoundation could not be found")
-
+if platform.system() != "Darwin":
+    raise ImportError("Only macOS is supported")
 
 version = platform.mac_ver()[0]
 version_info = tuple(map(int, version.split(".")))
@@ -65,8 +59,31 @@
         % (version_info[0], version_info[1])
     )
 
-Security = CDLL(security_path, use_errno=True)
-CoreFoundation = CDLL(core_foundation_path, use_errno=True)
+
+def load_cdll(name, macos10_16_path):
+    """Loads a CDLL by name, falling back to known path on 10.16+"""
+    try:
+        # Big Sur is technically 11 but we use 10.16 due to the Big Sur
+        # beta being labeled as 10.16.
+        if version_info >= (10, 16):
+            path = macos10_16_path
+        else:
+            path = find_library(name)
+        if not path:
+            raise OSError  # Caught and reraised as 'ImportError'
+        return CDLL(path, use_errno=True)
+    except OSError:
+        raise_from(ImportError("The library %s failed to load" % name), None)
+
+
+Security = load_cdll(
+    "Security", "/System/Library/Frameworks/Security.framework/Security"
+)
+CoreFoundation = load_cdll(
+    "CoreFoundation",
+    "/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation",
+)
+
 
 Boolean = c_bool
 CFIndex = c_long