scaffold a scheduler

crates/wasi-c2/TEST_FAILURES

@@ -25,5 +25,3 @@ wasi_tests::path_filestat
 
 wasi_tests::poll_oneoff
     - no sched yet
-wasi_tests::sched_yield
-    - no sched yet

crates/wasi-c2/src/ctx.rs

@@ -1,6 +1,7 @@
 use crate::clocks::{WasiMonotonicClock, WasiSystemClock};
 use crate::dir::{DirCaps, DirEntry, WasiDir};
 use crate::file::{FileCaps, FileEntry, WasiFile};
+use crate::sched::{SyncSched, WasiSched};
 use crate::string_array::{StringArray, StringArrayError};
 use crate::table::Table;
 use crate::Error;
@@ -14,6 +15,7 @@ pub struct WasiCtx {
     pub(crate) env: StringArray,
     pub(crate) random: RefCell<Box<dyn RngCore>>,
     pub(crate) clocks: WasiCtxClocks,
+    pub(crate) sched: Box<dyn WasiSched>,
     table: Rc<RefCell<Table>>,
 }
 
@@ -28,6 +30,7 @@ impl WasiCtx {
             env: StringArray::new(),
             random: RefCell::new(Box::new(unsafe { cap_rand::rngs::OsRng::default() })),
             clocks: WasiCtxClocks::default(),
+            sched: Box::new(SyncSched::default()),
             table: Rc::new(RefCell::new(Table::new())),
         }
     }

crates/wasi-c2/src/lib.rs

@@ -6,6 +6,7 @@ mod dir;
 mod error;
 mod file;
 pub mod random;
+pub mod sched;
 pub mod snapshots;
 pub mod stdio;
 mod string_array;

crates/wasi-c2/src/sched.rs

@@ -0,0 +1,20 @@
+use crate::Error;
+
+pub trait WasiSched {
+    // XXX poll oneoff needs args and results.
+    fn poll_oneoff(&self) -> Result<(), Error>;
+    fn sched_yield(&self) -> Result<(), Error>;
+}
+
+#[derive(Default)]
+pub struct SyncSched {}
+
+impl WasiSched for SyncSched {
+    fn poll_oneoff(&self) -> Result<(), Error> {
+        todo!()
+    }
+    fn sched_yield(&self) -> Result<(), Error> {
+        std::thread::yield_now();
+        Ok(())
+    }
+}

crates/wasi-c2/src/snapshots/preview_1.rs

@@ -886,7 +886,8 @@ impl<'a> wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
         events: &GuestPtr<types::Event>,
         nsubscriptions: types::Size,
     ) -> Result<types::Size, Error> {
-        unimplemented!()
+        self.sched.poll_oneoff()?;
+        Ok(0)
     }
 
     fn proc_exit(&self, status: types::Exitcode) -> wiggle::Trap {
@@ -903,7 +904,7 @@ impl<'a> wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
     }
 
     fn sched_yield(&self) -> Result<(), Error> {
-        unimplemented!()
+        self.sched.sched_yield()
     }
 
     fn random_get(&self, buf: &GuestPtr<u8>, buf_len: types::Size) -> Result<(), Error> {