bpo-27535: Fix memory leak with warnings ignore

Lib/test/test_warnings/__init__.py

@@ -125,6 +125,7 @@ def test_ignore(self):
             self.module.filterwarnings("ignore", category=UserWarning)
             self.module.warn("FilterTests.test_ignore", UserWarning)
             self.assertEqual(len(w), 0)
+            self.assertEqual(list(__warningregistry__), ['version'])
 
     def test_ignore_after_default(self):
         with original_warnings.catch_warnings(record=True,

Lib/warnings.py

@@ -364,7 +364,6 @@ def warn_explicit(message, category, filename, lineno,
         action = defaultaction
     # Early exit actions
     if action == "ignore":
-        registry[key] = 1
         return
 
     # Prime the linecache for formatting, in case the

Misc/NEWS.d/next/Library/2017-11-21-16-05-35.bpo-27535.JLhcNz.rst

@@ -0,0 +1,4 @@
+The warnings module doesn't leak memory anymore in the hidden warnings
+registry for the "ignore" action of warnings filters. warn_explicit()
+function doesn't add the warning key to the registry anymore for the
+"ignore" action.

Python/_warnings.c

@@ -525,16 +525,21 @@ warn_explicit(PyObject *category, PyObject *message,
         goto cleanup;
     }
 
+    if (_PyUnicode_EqualToASCIIString(action, "ignore")) {
+        goto return_none;
+    }
+
     /* Store in the registry that we've been here, *except* when the action
        is "always". */
     rc = 0;
     if (!_PyUnicode_EqualToASCIIString(action, "always")) {
         if (registry != NULL && registry != Py_None &&
-                PyDict_SetItem(registry, key, Py_True) < 0)
+            PyDict_SetItem(registry, key, Py_True) < 0)
+        {
             goto cleanup;
-        else if (_PyUnicode_EqualToASCIIString(action, "ignore"))
-            goto return_none;
-        else if (_PyUnicode_EqualToASCIIString(action, "once")) {
+        }
+
+        if (_PyUnicode_EqualToASCIIString(action, "once")) {
             if (registry == NULL || registry == Py_None) {
                 registry = get_once_registry();
                 if (registry == NULL)