File feature-detection.patch of Package python-zstandard

diff --git a/c-ext/backend_c.c b/c-ext/backend_c.c
index 9aa7a5c..f67191a 100644
--- a/c-ext/backend_c.c
+++ b/c-ext/backend_c.c
@@ -210,6 +210,20 @@ void zstd_module_init(PyObject *m) {
     Py_DECREF(feature);
 #endif
 
+#ifdef SYSTEM_ZSTD
+    feature = PyUnicode_FromString("system_zstd");
+    if (NULL == feature) {
+        PyErr_SetString(PyExc_ImportError, "could not create feature string");
+        return;
+    }
+
+    if (PySet_Add(features, feature) == -1) {
+        return;
+    }
+
+    Py_DECREF(feature);
+#endif
+
     if (PyObject_SetAttrString(m, "backend_features", features) == -1) {
         return;
     }
diff --git a/setup_zstd.py b/setup_zstd.py
index 5aefdd0..3d8fe73 100644
--- a/setup_zstd.py
+++ b/setup_zstd.py
@@ -78,6 +78,7 @@ def get_c_extension(
 
     if system_zstd:
         extra_args.append("-DZSTD_MULTITHREAD")
+        extra_args.append("-DSYSTEM_ZSTD")
     else:
         extra_args.append("-DZSTD_SINGLE_FILE")
         extra_args.append("-DZSTDLIB_VISIBLE=")
diff --git a/tests/test_module_attributes.py b/tests/test_module_attributes.py
index a540bfe..a081b1a 100644
--- a/tests/test_module_attributes.py
+++ b/tests/test_module_attributes.py
@@ -26,7 +26,15 @@ class TestModuleAttributes(unittest.TestCase):
             },
         }[zstd.backend]
 
-        self.assertEqual(zstd.backend_features, expected)
+        # The following features are available only with
+        # statically linked version of the module.
+        available_features = set(zstd.backend_features)
+        if 'system_zstd' in available_features:
+            available_features.remove('system_zstd')
+            expected.discard('multi_compress_to_buffer')
+            expected.discard('multi_decompress_to_buffer')
+
+        self.assertEqual(available_features, expected)
 
     def test_constants(self):
         self.assertEqual(zstd.MAX_COMPRESSION_LEVEL, 22)