bpo-44689: ctypes.util.find_library() now finds macOS 11+ system libraries when built on older macOS systems (#27251)

Previously, when built on older macOS systems, `find_library` was not able to find macOS system libraries when running on Big Sur due to changes in how system libraries are stored.

Misc/NEWS.d/next/macOS/2021-07-20-22-27-01.bpo-44689.mmT_xH.rst

@@ -0,0 +1,5 @@
+ :meth:`ctypes.util.find_library` now works correctly on macOS 11 Big Sur
+ even if Python is built on an older version of macOS.  Previously, when
+ built on older macOS systems, ``find_library`` was not able to find
+ macOS system libraries when running on Big Sur due to changes in
+ how system libraries are stored.

Modules/_ctypes/callproc.c

@@ -1442,14 +1442,37 @@ copy_com_pointer(PyObject *self, PyObject *args)
     return r;
 }
 #else
-
+#ifdef __APPLE__
 #ifdef HAVE_DYLD_SHARED_CACHE_CONTAINS_PATH
+#define HAVE_DYLD_SHARED_CACHE_CONTAINS_PATH_RUNTIME \
+    __builtin_available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)
+#else
+// Support the deprecated case of compiling on an older macOS version
+static void *libsystem_b_handle;
+static bool (*_dyld_shared_cache_contains_path)(const char *path);
+
+__attribute__((constructor)) void load_dyld_shared_cache_contains_path(void) {
+    libsystem_b_handle = dlopen("/usr/lib/libSystem.B.dylib", RTLD_LAZY);
+    if (libsystem_b_handle != NULL) {
+        _dyld_shared_cache_contains_path = dlsym(libsystem_b_handle, "_dyld_shared_cache_contains_path");
+    }
+}
+
+__attribute__((destructor)) void unload_dyld_shared_cache_contains_path(void) {
+    if (libsystem_b_handle != NULL) {
+        dlclose(libsystem_b_handle);
+    }
+}
+#define HAVE_DYLD_SHARED_CACHE_CONTAINS_PATH_RUNTIME \
+    _dyld_shared_cache_contains_path != NULL
+#endif
+
 static PyObject *py_dyld_shared_cache_contains_path(PyObject *self, PyObject *args)
 {
      PyObject *name, *name2;
      char *name_str;
 
-     if (__builtin_available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)) {
+     if (HAVE_DYLD_SHARED_CACHE_CONTAINS_PATH_RUNTIME) {
          int r;
 
          if (!PyArg_ParseTuple(args, "O", &name))
@@ -1992,7 +2015,7 @@ PyMethodDef _ctypes_module_methods[] = {
     {"dlclose", py_dl_close, METH_VARARGS, "dlclose a library"},
     {"dlsym", py_dl_sym, METH_VARARGS, "find symbol in shared library"},
 #endif
-#ifdef HAVE_DYLD_SHARED_CACHE_CONTAINS_PATH
+#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},