fix: segfault when moving scoped_ostream_redirect

The default move constructor left the stream (`std::cout`) pointing at
the moved-from `pythonbuf`, whose internal buffer and streambuf pointers
were nulled by the move. Any subsequent write through the stream
dereferenced null, causing a segfault.

Replace `= default` with an explicit move constructor that re-points
the stream to the new buffer and disarms the moved-from destructor.

Author: kounelisagis

include/pybind11/iostream.h

@@ -178,10 +178,18 @@ class scoped_ostream_redirect {
         old = costream.rdbuf(&buffer);
     }
 
-    ~scoped_ostream_redirect() { costream.rdbuf(old); }
+    ~scoped_ostream_redirect() {
+        if (old) {
+            costream.rdbuf(old);
+        }
+    }
 
     scoped_ostream_redirect(const scoped_ostream_redirect &) = delete;
-    scoped_ostream_redirect(scoped_ostream_redirect &&other) = default;
+    scoped_ostream_redirect(scoped_ostream_redirect &&other)
+        : old(other.old), costream(other.costream), buffer(std::move(other.buffer)) {
+        other.old = nullptr;     // Disarm moved-from destructor
+        costream.rdbuf(&buffer); // Re-point stream to our buffer
+    }
     scoped_ostream_redirect &operator=(const scoped_ostream_redirect &) = delete;
     scoped_ostream_redirect &operator=(scoped_ostream_redirect &&) = delete;
 };

tests/test_iostream.cpp

@@ -123,4 +123,11 @@ TEST_SUBMODULE(iostream, m) {
         .def("stop", &TestThread::stop)
         .def("join", &TestThread::join)
         .def("sleep", &TestThread::sleep);
+
+    m.def("move_redirect_output", [](const std::string &msg) {
+        py::scoped_ostream_redirect redir1(std::cout, py::module_::import("sys").attr("stdout"));
+        std::cout << "before" << std::flush;
+        py::scoped_ostream_redirect redir2(std::move(redir1));
+        std::cout << msg << std::flush;
+    });
 }

tests/test_iostream.py

@@ -284,6 +284,13 @@ def test_redirect_both(capfd):
     assert stream2.getvalue() == msg2
 
 
+def test_move_redirect(capsys):
+    m.move_redirect_output("after")
+    stdout, stderr = capsys.readouterr()
+    assert stdout == "beforeafter"
+    assert not stderr
+
+
 @pytest.mark.skipif(sys.platform.startswith("emscripten"), reason="Requires threads")
 def test_threading():
     with m.ostream_redirect(stdout=True, stderr=False):