gh-94526: getpath_dirname() no longer encodes the path (#97645)

Fix the Python path configuration used to initialized sys.path at
Python startup. Paths are no longer encoded to UTF-8/strict to avoid
encoding errors if it contains surrogate characters (bytes paths are
decoded with the surrogateescape error handler).

getpath_basename() and getpath_dirname() functions no longer encode
the path to UTF-8/strict, but work directly on Unicode strings. These
functions now use PyUnicode_FindChar() and PyUnicode_Substring() on
the Unicode path, rather than strrchr() on the encoded bytes string.

Misc/NEWS.d/next/Core and Builtins/2022-09-29-15-19-29.gh-issue-94526.wq5m6T.rst

@@ -0,0 +1,4 @@
+Fix the Python path configuration used to initialized :data:`sys.path` at
+Python startup. Paths are no longer encoded to UTF-8/strict to avoid encoding
+errors if it contains surrogate characters (bytes paths are decoded with the
+surrogateescape error handler). Patch by Victor Stinner.

Modules/getpath.c

@@ -82,27 +82,32 @@ getpath_abspath(PyObject *Py_UNUSED(self), PyObject *args)
 static PyObject *
 getpath_basename(PyObject *Py_UNUSED(self), PyObject *args)
 {
-    const char *path;
-    if (!PyArg_ParseTuple(args, "s", &path)) {
+    PyObject *path;
+    if (!PyArg_ParseTuple(args, "U", &path)) {
         return NULL;
     }
-    const char *name = strrchr(path, SEP);
-    return PyUnicode_FromString(name ? name + 1 : path);
+    Py_ssize_t end = PyUnicode_GET_LENGTH(path);
+    Py_ssize_t pos = PyUnicode_FindChar(path, SEP, 0, end, -1);
+    if (pos < 0) {
+        return Py_NewRef(path);
+    }
+    return PyUnicode_Substring(path, pos + 1, end);
 }
 
 
 static PyObject *
 getpath_dirname(PyObject *Py_UNUSED(self), PyObject *args)
 {
-    const char *path;
-    if (!PyArg_ParseTuple(args, "s", &path)) {
+    PyObject *path;
+    if (!PyArg_ParseTuple(args, "U", &path)) {
         return NULL;
     }
-    const char *name = strrchr(path, SEP);
-    if (!name) {
+    Py_ssize_t end = PyUnicode_GET_LENGTH(path);
+    Py_ssize_t pos = PyUnicode_FindChar(path, SEP, 0, end, -1);
+    if (pos < 0) {
         return PyUnicode_FromStringAndSize(NULL, 0);
     }
-    return PyUnicode_FromStringAndSize(path, (name - path));
+    return PyUnicode_Substring(path, 0, pos);
 }