C API: Detect buffer overflow in PyBytesWriter

[vstinner]

PyBytesWriter gives a directy access to its content with PyBytesWriter_GetData(). The caller is responsible to handle correctly the buffer size and write the correct amount of bytes. But programming is hard, and mistakes are easy to do. I propose adding a mechanism to detect buffer overflow to help developers detecting bugs in in their code.

Example of bug:

    PyBytesWriter *writer = PyBytesWriter_Create(0);
    if (writer == NULL) {
        goto error;
    }
    if (PyBytesWriter_Grow(writer, 3) < 0) {
        goto error;
    }
    char *data = PyBytesWriter_GetData(writer);
    strcpy(data, "abc");

    return PyBytesWriter_Finish(writer);

This code allocates 3 bytes but writes 4 bytes because strcat() also writes a trailing NUL byte.

Linked PRs

• gh-156939: Detect buffer overflow in PyBytesWriter in debug mode #156943