Port wasmtime-fiber to no_std and allow async feature in no_std…

… Wasmtime. (#9689)

This PR allows a `no_std` Wasmtime build to be configured with the
`async` feature. (Previously, a minimal `no_std` configuration could
only run with sync entry points, without suspending of stacks.)

The main hurdle to this support was the `wasmtime-fiber` crate.
Fortunately, the "unix" variant of fibers was almost entirely portable
to a `no_std` environment, owing to the fact that it implements
stack-switching manually in assembly itself. I moved the per-ISA
implementations to a shared submodule and built the nostd platform
backend for `wasmtime-fiber` with a stripped-down version of the unix
backend.

The nostd backend does not support mmap'd stacks, does not support
custom stack allocators, and does not propagate panics.

prtest:full

.github/workflows/main.yml

@@ -356,11 +356,12 @@ jobs:
               -p wasmtime --no-default-features --features profiling
               -p wasmtime --no-default-features --features cache
               -p wasmtime --no-default-features --features async
+              -p wasmtime --no-default-features --features std
               -p wasmtime --no-default-features --features pooling-allocator
               -p wasmtime --no-default-features --features cranelift
               -p wasmtime --no-default-features --features component-model
               -p wasmtime --no-default-features --features runtime,component-model
-              -p wasmtime --no-default-features --features cranelift,wat,async,cache
+              -p wasmtime --no-default-features --features cranelift,wat,async,std,cache
               -p wasmtime --no-default-features --features winch
               -p wasmtime --no-default-features --features wmemcheck
               -p wasmtime --no-default-features --features wmemcheck,cranelift,runtime
@@ -384,6 +385,12 @@ jobs:
               -p wasmtime --features incremental-cache
               -p wasmtime --all-features
 
+          - name: wasmtime-fiber
+            checks: |
+              -p wasmtime-fiber --no-default-features
+              -p wasmtime-fiber --no-default-features --features std
+              -p wasmtime-fiber --all-features
+
           - name: wasmtime-cli
             checks: |
               -p wasmtime-cli --no-default-features
@@ -432,6 +439,18 @@ jobs:
       env:
         GH_TOKEN: ${{ github.token }}
 
+  fiber_tests:
+    name: wasmtime-fiber tests
+    runs-on: ubuntu-latest
+    env:
+      CARGO_NDK_VERSION: 2.12.2
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: true
+    - uses: ./.github/actions/install-rust
+    - run: cargo test -p wasmtime-fiber --no-default-features
+
   # Checks for no_std support, ensure that crates can build on a no_std target
   no_std_checks:
     name: no_std checks
@@ -1192,6 +1211,7 @@ jobs:
       - cargo_vet
       - doc
       - micro_checks
+      - fiber_tests
       - no_std_checks
       - clippy
       - monolith_checks

crates/asm-macros/src/lib.rs

@@ -13,7 +13,7 @@ cfg_if::cfg_if! {
         #[macro_export]
         macro_rules! asm_func {
             ($name:expr, $body:expr $(, $($args:tt)*)?) => {
-                std::arch::global_asm!(
+                core::arch::global_asm!(
                     concat!(
                         ".p2align 4\n",
                         ".private_extern _", $name, "\n",
@@ -29,7 +29,7 @@ cfg_if::cfg_if! {
         #[macro_export]
         macro_rules! asm_func {
             ($name:expr, $body:expr $(, $($args:tt)*)?) => {
-                std::arch::global_asm!(
+                core::arch::global_asm!(
                     concat!(
                         ".def ", $name, "\n",
                         ".scl 2\n",
@@ -65,7 +65,7 @@ cfg_if::cfg_if! {
         #[macro_export]
         macro_rules! asm_func {
             ($name:expr, $body:expr $(, $($args:tt)*)?) => {
-                std::arch::global_asm!(
+                core::arch::global_asm!(
                     concat!(
                         ".p2align 4\n",
                         ".hidden ", $name, "\n",

crates/fiber/Cargo.toml

@@ -15,10 +15,10 @@ workspace = true
 anyhow = { workspace = true }
 cfg-if = { workspace = true }
 wasmtime-versioned-export-macros = { workspace = true }
+wasmtime-asm-macros = { workspace = true }
 
 [target.'cfg(unix)'.dependencies]
 rustix = { workspace = true, features = ["mm", "param"] }
-wasmtime-asm-macros = { workspace = true }
 
 [target.'cfg(windows)'.dependencies.windows-sys]
 workspace = true
@@ -33,3 +33,9 @@ wasmtime-versioned-export-macros = { workspace = true }
 
 [dev-dependencies]
 backtrace = "0.3.68"
+
+[features]
+
+# Assume presence of the standard library. Allows propagating
+# panic-unwinds across fiber invocations.
+std = []

crates/fiber/build.rs

@@ -23,8 +23,8 @@ fn main() {
         build.file("src/windows.c");
         build.define("VERSIONED_SUFFIX", Some(versioned_suffix!()));
     } else if arch == "s390x" {
-        println!("cargo:rerun-if-changed=src/unix/s390x.S");
-        build.file("src/unix/s390x.S");
+        println!("cargo:rerun-if-changed=src/stackswitch/s390x.S");
+        build.file("src/stackswitch/s390x.S");
         build.define("VERSIONED_SUFFIX", Some(versioned_suffix!()));
     } else {
         // assume that this is included via inline assembly in the crate itself,

crates/fiber/src/lib.rs

@@ -1,15 +1,22 @@
 #![expect(clippy::allow_attributes, reason = "crate not migrated yet")]
+#![no_std]
 
+#[cfg(any(feature = "std", unix, windows))]
+#[macro_use]
+extern crate std;
+extern crate alloc;
+
+use alloc::boxed::Box;
 use anyhow::Error;
-use std::any::Any;
-use std::cell::Cell;
-use std::io;
-use std::marker::PhantomData;
-use std::ops::Range;
-use std::panic::{self, AssertUnwindSafe};
+use core::cell::Cell;
+use core::marker::PhantomData;
+use core::ops::Range;
 
 cfg_if::cfg_if! {
-    if #[cfg(windows)] {
+    if #[cfg(not(feature = "std"))] {
+        mod nostd;
+        use nostd as imp;
+    } else if #[cfg(windows)] {
         mod windows;
         use windows as imp;
     } else if #[cfg(unix)] {
@@ -20,6 +27,11 @@ cfg_if::cfg_if! {
     }
 }
 
+// Our own stack switcher routines are used on Unix and no_std
+// platforms, but not on Windows (it has its own fiber API).
+#[cfg(any(unix, not(feature = "std")))]
+pub(crate) mod stackswitch;
+
 /// Represents an execution stack to use for a fiber.
 pub struct FiberStack(imp::FiberStack);
 
@@ -31,14 +43,16 @@ fn _assert_send_sync() {
     _assert_sync::<FiberStack>();
 }
 
+pub type Result<T, E = imp::Error> = core::result::Result<T, E>;
+
 impl FiberStack {
     /// Creates a new fiber stack of the given size.
-    pub fn new(size: usize) -> io::Result<Self> {
+    pub fn new(size: usize) -> Result<Self> {
         Ok(Self(imp::FiberStack::new(size)?))
     }
 
     /// Creates a new fiber stack of the given size.
-    pub fn from_custom(custom: Box<dyn RuntimeFiberStack>) -> io::Result<Self> {
+    pub fn from_custom(custom: Box<dyn RuntimeFiberStack>) -> Result<Self> {
         Ok(Self(imp::FiberStack::from_custom(custom)?))
     }
 
@@ -55,11 +69,7 @@ impl FiberStack {
     ///
     /// The caller must properly allocate the stack space with a guard page and
     /// make the pages accessible for correct behavior.
-    pub unsafe fn from_raw_parts(
-        bottom: *mut u8,
-        guard_size: usize,
-        len: usize,
-    ) -> io::Result<Self> {
+    pub unsafe fn from_raw_parts(bottom: *mut u8, guard_size: usize, len: usize) -> Result<Self> {
         Ok(Self(imp::FiberStack::from_raw_parts(
             bottom, guard_size, len,
         )?))
@@ -128,7 +138,8 @@ enum RunResult<Resume, Yield, Return> {
     Resuming(Resume),
     Yield(Yield),
     Returned(Return),
-    Panicked(Box<dyn Any + Send>),
+    #[cfg(feature = "std")]
+    Panicked(Box<dyn core::any::Any + Send>),
 }
 
 impl<'a, Resume, Yield, Return> Fiber<'a, Resume, Yield, Return> {
@@ -140,7 +151,7 @@ impl<'a, Resume, Yield, Return> Fiber<'a, Resume, Yield, Return> {
     pub fn new(
         stack: FiberStack,
         func: impl FnOnce(Resume, &mut Suspend<Resume, Yield, Return>) -> Return + 'a,
-    ) -> io::Result<Self> {
+    ) -> Result<Self> {
         let inner = imp::Fiber::new(&stack.0, func)?;
 
         Ok(Self {
@@ -177,7 +188,11 @@ impl<'a, Resume, Yield, Return> Fiber<'a, Resume, Yield, Return> {
                 Err(y)
             }
             RunResult::Returned(r) => Ok(r),
-            RunResult::Panicked(payload) => std::panic::resume_unwind(payload),
+            #[cfg(feature = "std")]
+            RunResult::Panicked(_payload) => {
+                use std::panic;
+                panic::resume_unwind(_payload);
+            }
         }
     }
 
@@ -222,11 +237,27 @@ impl<Resume, Yield, Return> Suspend<Resume, Yield, Return> {
             inner,
             _phantom: PhantomData,
         };
-        let result = panic::catch_unwind(AssertUnwindSafe(|| (func)(initial, &mut suspend)));
-        suspend.inner.switch::<Resume, Yield, Return>(match result {
-            Ok(result) => RunResult::Returned(result),
-            Err(panic) => RunResult::Panicked(panic),
-        });
+
+        #[cfg(feature = "std")]
+        {
+            use std::panic::{self, AssertUnwindSafe};
+            let result = panic::catch_unwind(AssertUnwindSafe(|| (func)(initial, &mut suspend)));
+            suspend.inner.switch::<Resume, Yield, Return>(match result {
+                Ok(result) => RunResult::Returned(result),
+                Err(panic) => RunResult::Panicked(panic),
+            });
+        }
+        // Note that it is sound to omit the `catch_unwind` here: it
+        // will not result in unwinding going off the top of the fiber
+        // stack, because the code on the fiber stack is invoked via
+        // an extern "C" boundary which will panic on unwinds.
+        #[cfg(not(feature = "std"))]
+        {
+            let result = (func)(initial, &mut suspend);
+            suspend
+                .inner
+                .switch::<Resume, Yield, Return>(RunResult::Returned(result));
+        }
     }
 }
 
@@ -236,11 +267,11 @@ impl<A, B, C> Drop for Fiber<'_, A, B, C> {
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test))]
 mod tests {
     use super::{Fiber, FiberStack};
+    use alloc::string::ToString;
     use std::cell::Cell;
-    use std::panic::{self, AssertUnwindSafe};
     use std::rc::Rc;
 
     #[test]
@@ -332,7 +363,10 @@ mod tests {
     }
 
     #[test]
+    #[cfg(feature = "std")]
     fn panics_propagated() {
+        use std::panic::{self, AssertUnwindSafe};
+
         let a = Rc::new(Cell::new(false));
         let b = SetOnDrop(a.clone());
         let fiber =