view rust/hg-cpython/src/copy_tracing.rs @ 50979:4c5f6e95df84

rust: make `Revision` a newtype

This change is the one we've been building towards during this series.
The aim is to make `Revision` mean more than a simple integer, holding
the information that it is valid for a given revlog index.
While this still allows for programmer error, since creating a revision
directly and querying a different index with a "checked" revision are
still possible, the friction created by the newtype will hopefully make
us think twice about which type to use.

Enough of the Rust ecosystem relies on the newtype pattern to be
efficiently optimized away (even compiler in codegen tests¹), so I'm not
worried about this being a fundamental problem.

[1] https://github.com/rust-lang/rust/blob/7a70647f195f6b0a0f1ebd72b1542ba91a32f43a/tests/codegen/vec-in-place.rs#L47

use cpython::ObjectProtocol;
use cpython::PyBytes;
use cpython::PyDict;
use cpython::PyDrop;
use cpython::PyList;
use cpython::PyModule;
use cpython::PyObject;
use cpython::PyResult;
use cpython::PyTuple;
use cpython::Python;

use hg::copy_tracing::ChangedFiles;
use hg::copy_tracing::CombineChangesetCopies;
use hg::Revision;

use crate::pybytes_deref::PyBytesDeref;
use crate::PyRevision;

/// Combines copies information contained into revision `revs` to build a copy
/// map.
///
/// See mercurial/copies.py for details
pub fn combine_changeset_copies_wrapper(
    py: Python,
    revs: PyList,
    children_count: PyDict,
    target_rev: PyRevision,
    rev_info: PyObject,
    multi_thread: bool,
) -> PyResult<PyDict> {
    let target_rev = Revision(target_rev.0);
    let children_count = children_count
        .items(py)
        .iter()
        .map(|(k, v)| {
            Ok((Revision(k.extract::<PyRevision>(py)?.0), v.extract(py)?))
        })
        .collect::<PyResult<_>>()?;

    /// (Revision number, parent 1, parent 2, copy data for this revision)
    type RevInfo<Bytes> = (Revision, Revision, Revision, Option<Bytes>);

    let revs_info =
        revs.iter(py).map(|rev_py| -> PyResult<RevInfo<PyBytes>> {
            let rev = Revision(rev_py.extract::<PyRevision>(py)?.0);
            let tuple: PyTuple =
                rev_info.call(py, (rev_py,), None)?.cast_into(py)?;
            let p1 =
                Revision(tuple.get_item(py, 0).extract::<PyRevision>(py)?.0);
            let p2 =
                Revision(tuple.get_item(py, 1).extract::<PyRevision>(py)?.0);
            let opt_bytes = tuple.get_item(py, 2).extract(py)?;
            Ok((rev, p1, p2, opt_bytes))
        });

    let path_copies;
    if !multi_thread {
        let mut combine_changeset_copies =
            CombineChangesetCopies::new(children_count);

        for rev_info in revs_info {
            let (rev, p1, p2, opt_bytes) = rev_info?;
            let files = match &opt_bytes {
                Some(bytes) => ChangedFiles::new(bytes.data(py)),
                // Python None was extracted to Option::None,
                // meaning there was no copy data.
                None => ChangedFiles::new_empty(),
            };

            combine_changeset_copies.add_revision(rev, p1, p2, files)
        }
        path_copies = combine_changeset_copies.finish(target_rev)
    } else {
        // Use a bounded channel to provide back-pressure:
        // if the child thread is slower to process revisions than this thread
        // is to gather data for them, an unbounded channel would keep
        // growing and eat memory.
        //
        // TODO: tweak the bound?
        let (rev_info_sender, rev_info_receiver) =
            crossbeam_channel::bounded::<RevInfo<PyBytesDeref>>(1000);

        // This channel (going the other way around) however is unbounded.
        // If they were both bounded, there might potentially be deadlocks
        // where both channels are full and both threads are waiting on each
        // other.
        let (pybytes_sender, pybytes_receiver) =
            crossbeam_channel::unbounded();

        // Start a thread that does CPU-heavy processing in parallel with the
        // loop below.
        //
        // If the parent thread panics, `rev_info_sender` will be dropped and
        // “disconnected”. `rev_info_receiver` will be notified of this and
        // exit its own loop.
        let thread = std::thread::spawn(move || {
            let mut combine_changeset_copies =
                CombineChangesetCopies::new(children_count);
            for (rev, p1, p2, opt_bytes) in rev_info_receiver {
                let files = match &opt_bytes {
                    Some(raw) => ChangedFiles::new(raw.as_ref()),
                    // Python None was extracted to Option::None,
                    // meaning there was no copy data.
                    None => ChangedFiles::new_empty(),
                };
                combine_changeset_copies.add_revision(rev, p1, p2, files);

                // Send `PyBytes` back to the parent thread so the parent
                // thread can drop it. Otherwise the GIL would be implicitly
                // acquired here through `impl Drop for PyBytes`.
                if let Some(bytes) = opt_bytes {
                    if pybytes_sender.send(bytes.unwrap()).is_err() {
                        // The channel is disconnected, meaning the parent
                        // thread panicked or returned
                        // early through
                        // `?` to propagate a Python exception.
                        break;
                    }
                }
            }

            combine_changeset_copies.finish(target_rev)
        });

        for rev_info in revs_info {
            let (rev, p1, p2, opt_bytes) = rev_info?;
            let opt_bytes = opt_bytes.map(|b| PyBytesDeref::new(py, b));

            // We’d prefer to avoid the child thread calling into Python code,
            // but this avoids a potential deadlock on the GIL if it does:
            py.allow_threads(|| {
                rev_info_sender.send((rev, p1, p2, opt_bytes)).expect(
                    "combine_changeset_copies: channel is disconnected",
                );
            });

            // Drop anything in the channel, without blocking
            for pybytes in pybytes_receiver.try_iter() {
                pybytes.release_ref(py)
            }
        }
        // We’d prefer to avoid the child thread calling into Python code,
        // but this avoids a potential deadlock on the GIL if it does:
        path_copies = py.allow_threads(|| {
            // Disconnect the channel to signal the child thread to stop:
            // the `for … in rev_info_receiver` loop will end.
            drop(rev_info_sender);

            // Wait for the child thread to stop, and propagate any panic.
            thread.join().unwrap_or_else(|panic_payload| {
                std::panic::resume_unwind(panic_payload)
            })
        });

        // Drop anything left in the channel
        for pybytes in pybytes_receiver.iter() {
            pybytes.release_ref(py)
        }
    };

    let out = PyDict::new(py);
    for (dest, source) in path_copies.into_iter() {
        out.set_item(
            py,
            PyBytes::new(py, &dest.into_vec()),
            PyBytes::new(py, &source.into_vec()),
        )?;
    }
    Ok(out)
}

/// Create the module, with `__package__` given from parent
pub fn init_module(py: Python, package: &str) -> PyResult<PyModule> {
    let dotted_name = &format!("{}.copy_tracing", package);
    let m = PyModule::new(py, dotted_name)?;

    m.add(py, "__package__", package)?;
    m.add(py, "__doc__", "Copy tracing - Rust implementation")?;

    m.add(
        py,
        "combine_changeset_copies",
        py_fn!(
            py,
            combine_changeset_copies_wrapper(
                revs: PyList,
                children: PyDict,
                target_rev: PyRevision,
                rev_info: PyObject,
                multi_thread: bool
            )
        ),
    )?;

    let sys = PyModule::import(py, "sys")?;
    let sys_modules: PyDict = sys.get(py, "modules")?.extract(py)?;
    sys_modules.set_item(py, dotted_name, &m)?;

    Ok(m)
}