move bounded_traversal from mononoke directory to common/rust/shed

Summary: I'd like to make use of the `bounded_traversal` crate outside of mononoke's codebase to limit memory usage.

Reviewed By: mitrandir77, liubov-dmitrieva

Differential Revision: D70098058

fbshipit-source-id: 3f0cf6d007ef7bc3d37b4bbf4ecc28b62005708d

Cargo.toml

@@ -27,6 +27,7 @@ members = [
   "shed/ascii_ext",
   "shed/async_once_cell",
   "shed/borrowed",
+  "shed/bounded_traversal",
   "shed/buffered_weighted",
   "shed/cached_config",
   "shed/cachelib_stub",

shed/bounded_traversal/Cargo.toml

@@ -0,0 +1,26 @@
+# @generated by autocargo from //common/rust/shed/bounded_traversal:bounded_traversal
+
+[package]
+name = "bounded_traversal"
+version = "0.1.0"
+authors = ["Facebook <opensource+rust-shed@fb.com>"]
+edition = "2021"
+readme = "../../README.md"
+repository = "https://github.com/facebookexperimental/rust-shed"
+license = "MIT OR Apache-2.0"
+
+[dependencies]
+either = "1.5"
+futures = { version = "0.3.30", features = ["async-await", "compat"] }
+smallvec = { version = "1.13.2", features = ["serde", "specialization", "union"] }
+thiserror = "2"
+
+[dev-dependencies]
+anyhow = "1.0.95"
+cloned = { version = "0.1.0", path = "../cloned" }
+lock_ext = { version = "0.1.0", path = "../lock_ext" }
+maplit = "1.0"
+pretty_assertions = { version = "1.2", features = ["alloc"], default-features = false }
+quickcheck = "1.0"
+quickcheck_async = "0.1.1"
+tokio = { version = "1.41.0", features = ["full", "test-util", "tracing"] }

shed/bounded_traversal/src/common.rs

@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under both the MIT license found in the
+ * LICENSE-MIT file in the root directory of this source tree and the Apache
+ * License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+ * of this source tree.
+ */
+
+use std::future::Future;
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
+
+use either::Either;
+use futures::ready;
+
+/// Return value from `unfold` callbacks for ordered traversals.  Each element
+/// in the unfolded vector can be either an item to output from the traversal,
+/// or a recursive step with an associated weight.
+///
+/// The associated weight should be an estimate of the number of eventual
+/// output items this recursive step will expand into.
+pub enum OrderedTraversal<Out, In> {
+    Output(Out),
+    Recurse(usize, In),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) struct NodeLocation<Index> {
+    pub node_index: Index,  // node index inside execution tree
+    pub child_index: usize, // index inside parents children list
+}
+
+impl<Index> NodeLocation<Index> {
+    pub fn new(node_index: Index, child_index: usize) -> Self {
+        NodeLocation {
+            node_index,
+            child_index,
+        }
+    }
+}
+
+/// Equivalent of `futures::future::Either` but with heterogeneous output
+/// types using `either::Either`.
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub(crate) enum Either2<A, B> {
+    Left(A),
+    Right(B),
+}
+
+impl<A, B> Future for Either2<A, B>
+where
+    A: Future,
+    B: Future,
+{
+    type Output = Either<A::Output, B::Output>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+        // see `impl<Left, Right> Future for Either<Left, Right>`
+        unsafe {
+            let result = match self.get_unchecked_mut() {
+                Either2::Left(future) => Either::Left(ready!(Pin::new_unchecked(future).poll(cx))),
+                Either2::Right(future) => {
+                    Either::Right(ready!(Pin::new_unchecked(future).poll(cx)))
+                }
+            };
+            Poll::Ready(result)
+        }
+    }
+}