ctypes: check _dyld_shared_cache_contains_path in find_library

System libraries in Mac OS 11 may be present only in the shared cache,
with the actual mach-o file not present in the corresponding location
on the filesystem.   ctypes.util.find_library should check the
shared cache in order to behave consistently across Mac OS 10.15
and earlier and Mac OS 11.0 and later.

Lib/ctypes/macholib/dyld.py

@@ -7,6 +7,12 @@
 from ctypes.macholib.dylib import dylib_info
 from itertools import *
 
+try:
+    from _ctypes import _dyld_shared_cache_contains_path
+except ImportError:
+    def _dyld_shared_cache_contains_path(*args):
+        raise NotImplementedError
+
 __all__ = [
     'dyld_find', 'framework_find',
     'framework_info', 'dylib_info',
@@ -124,6 +130,12 @@ def dyld_find(name, executable_path=None, env=None):
             ), env):
         if os.path.isfile(path):
             return path
+        try:
+            if _dyld_shared_cache_contains_path(path):
+                return path
+        except NotImplementedError:
+            pass
+
     raise ValueError("dylib %s could not be found" % (name,))
 
 def framework_find(fn, executable_path=None, env=None):

Misc/NEWS.d/next/Core and Builtins/2020-06-30-18-24-28.bpo-41100._QkcD_.rst

@@ -0,0 +1 @@
+ctypes: Mac OS 11: check for system libraries in the shared cache

Modules/_ctypes/callproc.c

@@ -64,6 +64,10 @@
 #include "ctypes_dlfcn.h"
 #endif
 
+#ifdef __APPLE__
+extern bool _dyld_shared_cache_contains_path(const char* path) __attribute__((weak_import));
+#endif
+
 #ifdef MS_WIN32
 #include <malloc.h>
 #endif
@@ -1398,6 +1402,37 @@ copy_com_pointer(PyObject *self, PyObject *args)
 }
 #else
 
+#ifdef __APPLE__
+static PyObject *py_dyld_shared_cache_contains_path(PyObject *self, PyObject *args)
+{
+    PyObject *name, *name2;
+    char *name_str;
+
+    if (_dyld_shared_cache_contains_path == NULL) {
+        PyErr_SetString(PyExc_NotImplementedError, "_dyld_shared_cache_contains_path symbol is missing");
+        return NULL;
+    }
+
+    if (!PyArg_ParseTuple(args, "O", &name))
+        return NULL;
+
+    if (name == Py_None)
+        Py_RETURN_FALSE;
+
+    if (PyUnicode_FSConverter(name, &name2) == 0)
+        return NULL;
+    if (PyBytes_Check(name2))
+        name_str = PyBytes_AS_STRING(name2);
+    else
+        name_str = PyByteArray_AS_STRING(name2);
+
+    if(_dyld_shared_cache_contains_path(name_str))
+        Py_RETURN_TRUE;
+    else
+        Py_RETURN_FALSE;
+}
+#endif
+
 static PyObject *py_dl_open(PyObject *self, PyObject *args)
 {
     PyObject *name, *name2;
@@ -1908,6 +1943,9 @@ PyMethodDef _ctypes_module_methods[] = {
      "dlopen(name, flag={RTLD_GLOBAL|RTLD_LOCAL}) open a shared library"},
     {"dlclose", py_dl_close, METH_VARARGS, "dlclose a library"},
     {"dlsym", py_dl_sym, METH_VARARGS, "find symbol in shared library"},
+#endif
+#ifdef __APPLE__
+    {"_dyld_shared_cache_contains_path", py_dyld_shared_cache_contains_path, METH_VARARGS, "check if path is in the shared cache"},
 #endif
     {"alignment", align_func, METH_O, alignment_doc},
     {"sizeof", sizeof_func, METH_O, sizeof_doc},