rust/hg-cpython/src/utils.rs

stream-clone: smoothly detect and handle a case were a revlog is split

This detect and handle the most common case for a race condition around stream
and revlog splitting. The one were the revlog is split between the initial
collection of data and the time were we start considering stream that data.

In such case, we repatch an inlined version of that revlog together when this
happens. This is necessary as stream-v2 promised a specific number of bytes and
a specific number of files to the client. In stream-v3, we will have the
opportunity to just send a split revlog instead.

Getting a better version of the protocol for stream-v3 is still useful, but it
is no longer a blocket to fix that race condition.

Note that another, rarer race condition exist, were the revlog is split while
we creating the revlog and extracing content from it. This can be dealt with
later.

use cpython::exc::ValueError;
use cpython::{PyBytes, PyDict, PyErr, PyObject, PyResult, PyTuple, Python};
use hg::revlog::Node;

#[allow(unused)]
pub fn print_python_trace(py: Python) -> PyResult<PyObject> {
    eprintln!("===============================");
    eprintln!("Printing Python stack from Rust");
    eprintln!("===============================");
    let traceback = py.import("traceback")?;
    let sys = py.import("sys")?;
    let kwargs = PyDict::new(py);
    kwargs.set_item(py, "file", sys.get(py, "stderr")?)?;
    traceback.call(py, "print_stack", PyTuple::new(py, &[]), Some(&kwargs))
}

// Necessary evil for the time being, could maybe be moved to
// a TryFrom in Node itself
const NODE_BYTES_LENGTH: usize = 20;
type NodeData = [u8; NODE_BYTES_LENGTH];

/// Copy incoming Python bytes given as `PyObject` into `Node`,
/// doing the necessary checks
pub fn node_from_py_object<'a>(
    py: Python,
    bytes: &'a PyObject,
) -> PyResult<Node> {
    let as_py_bytes: &'a PyBytes = bytes.extract(py)?;
    node_from_py_bytes(py, as_py_bytes)
}

/// Clone incoming Python bytes given as `PyBytes` as a `Node`,
/// doing the necessary checks.
pub fn node_from_py_bytes(py: Python, bytes: &PyBytes) -> PyResult<Node> {
    <NodeData>::try_from(bytes.data(py))
        .map_err(|_| {
            PyErr::new::<ValueError, _>(
                py,
                format!("{}-byte hash required", NODE_BYTES_LENGTH),
            )
        })
        .map(Into::into)
}