bpo-35742: Fix test_envar_unimportable in test_builtin. (GH-11561)

Handle the case of an empty module name in PYTHONBREAKPOINT.

Fixes a regression introduced in bpo-34756.

Lib/test/test_builtin.py

@@ -1608,6 +1608,7 @@ class TestBreakpoint(unittest.TestCase):
     def test_envar_unimportable(self):
         for envar in (
                 '.', '..', '.foo', 'foo.', '.int', 'int.',
+                '.foo.bar', '..foo.bar', '/./',
                 'nosuchbuiltin',
                 'nosuchmodule.nosuchcallable',
                 ):

Python/sysmodule.c

@@ -141,11 +141,14 @@ sys_breakpointhook(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyOb
         modulepath = PyUnicode_FromString("builtins");
         attrname = envar;
     }
-    else {
+    else if (last_dot != envar) {
         /* Split on the last dot; */
         modulepath = PyUnicode_FromStringAndSize(envar, last_dot - envar);
         attrname = last_dot + 1;
     }
+    else {
+        goto warn;
+    }
     if (modulepath == NULL) {
         PyMem_RawFree(envar);
         return NULL;
@@ -155,27 +158,29 @@ sys_breakpointhook(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyOb
     Py_DECREF(modulepath);
 
     if (module == NULL) {
-        goto error;
+        if (PyErr_ExceptionMatches(PyExc_ImportError)) {
+            goto warn;
+        }
+        PyMem_RawFree(envar);
+        return NULL;
     }
 
     PyObject *hook = PyObject_GetAttrString(module, attrname);
     Py_DECREF(module);
 
     if (hook == NULL) {
-        goto error;
+        if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
+            goto warn;
+        }
+        PyMem_RawFree(envar);
+        return NULL;
     }
     PyMem_RawFree(envar);
     PyObject *retval = _PyObject_FastCallKeywords(hook, args, nargs, keywords);
     Py_DECREF(hook);
     return retval;
 
-  error:
-    if (!PyErr_ExceptionMatches(PyExc_ImportError)
-        && !PyErr_ExceptionMatches(PyExc_AttributeError))
-    {
-        PyMem_RawFree(envar);
-        return NULL;
-    }
+  warn:
     /* If any of the imports went wrong, then warn and ignore. */
     PyErr_Clear();
     int status = PyErr_WarnFormat(