Re: [Libguestfs] [libnbd PATCH 3/2] python: Slice pread_structured buffer from original

This patch fixes the corner-case regression introduced by the previous patch where the pread_structured callback buffer lifetime ends as soon as the callback (that is, where accessing a stashed callback parameter can result in ValueError instead of modifying a harmless copy). With careful effort, we can create a memoryview of the Python object that we read into, then slice that memoryview as part of the callback; now the slice will be valid as long as the original object is also valid.

| @@ -76,8 +77,24 @@ chunk_wrapper (void *user_data, const vo | PyObject *py_subbuf = NULL; | PyObject *py_error = NULL; | | - /* Cast away const */ | - py_subbuf = PyMemoryView_FromMemory ((char *) subbuf, count, PyBUF_READ); | + if (data->buf) { | + PyObject *start, *end, *slice; | + assert (PyMemoryView_Check (data->buf)); | + ptrdiff_t offs = subbuf - PyMemoryView_GET_BUFFER (data->buf)->buf; | + start = PyLong_FromLong (offs); | + if (!start) { PyErr_PrintEx (0); goto out; } | + end = PyLong_FromLong (offs + count); | + if (!end) { Py_DECREF (start); PyErr_PrintEx (0); goto out; } | + slice = PySlice_New (start, end, NULL); | + Py_DECREF (start); | + Py_DECREF (end); | + if (!slice) { PyErr_PrintEx (0); goto out; } | + py_subbuf = PyObject_GetItem (data->buf, slice);

+++ b/generator/Python.ml @@ -187,8 +187,24 @@ let pr " PyList_SET_ITEM (py_%s, i_%s, py_e_%s);\n" n n n; pr " }\n" | CBBytesIn (n, len) -> - pr " /* Cast away const */\n"; - pr " py_%s = PyMemoryView_FromMemory ((char *) %s, %s, PyBUF_READ);\n" n n len; + pr " if (data->buf) {\n"; + pr " PyObject *start, *end, *slice;\n"; + pr " assert (PyMemoryView_Check (data->buf));\n"; + pr " ptrdiff_t offs = %s - PyMemoryView_GET_BUFFER (data->buf)->buf;\n" n; + pr " start = PyLong_FromLong (offs);\n"; + pr " if (!start) { PyErr_PrintEx (0); goto out; }\n"; + pr " end = PyLong_FromLong (offs + %s);\n" len; + pr " if (!end) { Py_DECREF (start); PyErr_PrintEx (0); goto out; }\n"; + pr " slice = PySlice_New (start, end, NULL);\n"; + pr " Py_DECREF (start);\n"; + pr " Py_DECREF (end);\n"; + pr " if (!slice) { PyErr_PrintEx (0); goto out; }\n"; + pr " py_%s = PyObject_GetItem (data->buf, slice);\n" n;