copies-rust: introduce PyBytesWithData to reduce GIL requirement
See explanations in new doc-comments.
Differential Revision: https://phab.mercurial-scm.org/D9685
use cpython::ObjectProtocol;
use cpython::PyBytes;
use cpython::PyDict;
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 self::pybytes_with_data::PyBytesWithData;
// Module to encapsulate private fields
mod pybytes_with_data {
use cpython::{PyBytes, Python};
/// Safe abstraction over a `PyBytes` together with the `&[u8]` slice
/// that borrows it.
///
/// Calling `PyBytes::data` requires a GIL marker but we want to access the
/// data in a thread that (ideally) does not need to acquire the GIL.
/// This type allows separating the call an the use.
pub(super) struct PyBytesWithData {
#[allow(unused)]
keep_alive: PyBytes,
/// Borrows the buffer inside `self.keep_alive`,
/// but the borrow-checker cannot express self-referential structs.
data: *const [u8],
}
fn require_send<T: Send>() {}
#[allow(unused)]
fn static_assert_pybytes_is_send() {
require_send::<PyBytes>;
}
// Safety: PyBytes is Send. Raw pointers are not by default,
// but here sending one to another thread is fine since we ensure it stays
// valid.
unsafe impl Send for PyBytesWithData {}
impl PyBytesWithData {
pub fn new(py: Python, bytes: PyBytes) -> Self {
Self {
data: bytes.data(py),
keep_alive: bytes,
}
}
pub fn data(&self) -> &[u8] {
// Safety: the raw pointer is valid as long as the PyBytes is still
// alive, and the returned slice borrows `self`.
unsafe { &*self.data }
}
}
}
/// 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: Revision,
rev_info: PyObject,
multi_thread: bool,
) -> PyResult<PyDict> {
let children_count = children_count
.items(py)
.iter()
.map(|(k, v)| Ok((k.extract(py)?, 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 = rev_py.extract(py)?;
let tuple: PyTuple =
rev_info.call(py, (rev_py,), None)?.cast_into(py)?;
let p1 = tuple.get_item(py, 0).extract(py)?;
let p2 = tuple.get_item(py, 1).extract(py)?;
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)
}
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<PyBytesWithData>>(1000);
// 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.data()),
// 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)
// The GIL is (still) implicitly acquired here through
// `impl Drop for PyBytes`.
}
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| PyBytesWithData::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",
);
});
}
// 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(|| {
// 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)
})
})
};
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: Revision,
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)
}