feat: Add critical-section based implementation

The current no-std implementation is somewhat ineffecient, potentially
broken and relies on some slow data structures. The main reason why we
use this implementation is because we don't have a way to "lock" the
linked list, since there's no way to lock things in no-std. However,
many embedded platforms have a concept of a "critical section" that can
be used to exclusively lock something.

This PR adds an option to the "std" implementation that replaces the
existing Mutex with a usage of the critical-section crate. This is
enabled with the "critical-section" feature. This allows us to have the
advantages of the std-based implementation without needing to rely on
std for platforms that don't have it.

Signed-off-by: John Nunley <[email protected]>

.github/workflows/ci.yml

@@ -58,6 +58,7 @@ jobs:
         run: cargo hack build --all --no-dev-deps
       - run: cargo hack build --all --target thumbv7m-none-eabi --no-default-features --no-dev-deps
       - run: cargo hack build --target thumbv7m-none-eabi --no-default-features --no-dev-deps --features portable-atomic
+      - run: cargo hack build --target thumbv7m-none-eabi --no-default-features --no-dev-deps --features critical-section
       #- name: Install wasm-pack
       #  uses: taiki-e/install-action@wasm-pack
       #- run: wasm-pack test --node
@@ -93,6 +94,7 @@ jobs:
         uses: taiki-e/install-action@cargo-hack
       - run: cargo hack build --all --rust-version
       - run: cargo hack build --all --no-default-features --rust-version
+      - run: cargo hack build --all --no-default-features --rust-version --features critical-section
 
   clippy:
     runs-on: ubuntu-latest

Cargo.toml

@@ -29,6 +29,7 @@ unexpected_cfgs = { level = "warn", check-cfg = ['cfg(loom)'] }
 
 [dependencies]
 concurrent-queue = { version = "2.4.0", default-features = false }
+critical-section = { version = "1.2.0", default-features = false, optional = true }
 pin-project-lite = "0.2.12"
 portable-atomic-util = { version = "0.2.0", default-features = false, optional = true, features = ["alloc"] }
 
@@ -45,6 +46,7 @@ default-features = false
 optional = true
 
 [dev-dependencies]
+critical-section = { version = "1.2.0", features = ["std"] }
 futures-lite = "2.0.0"
 try-lock = "0.2.5"
 waker-fn = "1"

examples/mutex.rs

@@ -2,7 +2,7 @@
 //!
 //! This mutex exposes both blocking and async methods for acquiring a lock.
 
-#[cfg(not(target_family = "wasm"))]
+#[cfg(all(feature = "std", not(target_family = "wasm")))]
 mod example {
     #![allow(dead_code)]
 
@@ -171,7 +171,7 @@ mod example {
     }
 }
 
-#[cfg(target_family = "wasm")]
+#[cfg(any(target_family = "wasm", not(feature = "std")))]
 mod example {
     pub(super) fn entry() {
         println!("This example is not supported on wasm yet.");

src/lib.rs

@@ -85,8 +85,11 @@ extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std as alloc;
 
-#[cfg_attr(feature = "std", path = "std.rs")]
-#[cfg_attr(not(feature = "std"), path = "no_std.rs")]
+#[cfg_attr(any(feature = "std", feature = "critical-section"), path = "std.rs")]
+#[cfg_attr(
+    not(any(feature = "std", feature = "critical-section")),
+    path = "no_std.rs"
+)]
 mod sys;
 
 mod notify;