fix: improve bytes to str decoding error handling (#4294)

* (bugfix): Improve bytes to str decoding error handling

* regroup test

* Further broaden tests

* Add another decode error test

* Fix bug in tests

* Reviewer suggestions

include/pybind11/pytypes.h

@@ -1432,6 +1432,9 @@ class str : public object {
     str(const char *c, const SzType &n)
         : object(PyUnicode_FromStringAndSize(c, ssize_t_cast(n)), stolen_t{}) {
         if (!m_ptr) {
+            if (PyErr_Occurred()) {
+                throw error_already_set();
+            }
             pybind11_fail("Could not allocate string object!");
         }
     }
@@ -1441,6 +1444,9 @@ class str : public object {
     // NOLINTNEXTLINE(google-explicit-constructor)
     str(const char *c = "") : object(PyUnicode_FromString(c), stolen_t{}) {
         if (!m_ptr) {
+            if (PyErr_Occurred()) {
+                throw error_already_set();
+            }
             pybind11_fail("Could not allocate string object!");
         }
     }
@@ -1598,6 +1604,9 @@ inline str::str(const bytes &b) {
     }
     auto obj = reinterpret_steal<object>(PyUnicode_FromStringAndSize(buffer, length));
     if (!obj) {
+        if (PyErr_Occurred()) {
+            throw error_already_set();
+        }
         pybind11_fail("Could not allocate string object!");
     }
     m_ptr = obj.release().ptr();

tests/test_pytypes.cpp

@@ -206,7 +206,12 @@ TEST_SUBMODULE(pytypes, m) {
     m.def("str_from_char_ssize_t", []() { return py::str{"red", (py::ssize_t) 3}; });
     m.def("str_from_char_size_t", []() { return py::str{"blue", (py::size_t) 4}; });
     m.def("str_from_string", []() { return py::str(std::string("baz")); });
+    m.def("str_from_std_string_input", [](const std::string &stri) { return py::str(stri); });
+    m.def("str_from_cstr_input", [](const char *c_str) { return py::str(c_str); });
     m.def("str_from_bytes", []() { return py::str(py::bytes("boo", 3)); });
+    m.def("str_from_bytes_input",
+          [](const py::bytes &encoded_str) { return py::str(encoded_str); });
+
     m.def("str_from_object", [](const py::object &obj) { return py::str(obj); });
     m.def("repr_from_object", [](const py::object &obj) { return py::repr(obj); });
     m.def("str_from_handle", [](py::handle h) { return py::str(h); });

tests/test_pytypes.py

@@ -244,6 +244,20 @@ def __repr__(self):
         m.str_from_string_from_str(ucs_surrogates_str)
 
 
+@pytest.mark.parametrize(
+    "func",
+    [
+        m.str_from_bytes_input,
+        m.str_from_cstr_input,
+        m.str_from_std_string_input,
+    ],
+)
+def test_surrogate_pairs_unicode_error(func):
+    input_str = "\ud83d\ude4f".encode("utf-8", "surrogatepass")
+    with pytest.raises(UnicodeDecodeError):
+        func(input_str)
+
+
 def test_bytes(doc):
     assert m.bytes_from_char_ssize_t().decode() == "green"
     assert m.bytes_from_char_size_t().decode() == "purple"