Wasi: preview2 poll-oneoff returns list<bool> (bytecodealliance#6498)

* poll.wit: we can returl a list<bool> now

* adapter: fixes for poll-oneoff returning list<bool>

* wasi preview2: fixes for poll-oneoff returning list<bool>

* adapter: manually import poll-oneoff to avoid pulling in std to allocate return vec

* comment describing the skip functions

crates/wasi-preview1-component-adapter/src/lib.rs

@@ -34,9 +34,13 @@ pub mod bindings {
         world: "wasi:preview/command",
         std_feature,
         raw_strings,
-        // The generated definition of command will pull in std, so we are defining it
-        // manually below instead
-        skip: ["run", "get-directories", "get-environment"],
+        // Automatically generated bindings for these functions will allocate
+        // Vecs, which in turn pulls in the panic machinery from std, which
+        // creates vtables that end up in the wasm elem section, which we
+        // can't support in these special core-wasm adapters.
+        // Instead, we manually define the bindings for these functions in
+        // terms of raw pointers.
+        skip: ["run", "get-environment", "poll-oneoff"],
     });
 
     #[cfg(feature = "reactor")]
@@ -45,7 +49,13 @@ pub mod bindings {
         world: "wasi:preview/reactor",
         std_feature,
         raw_strings,
-        skip: ["get-directories", "get-environment"],
+        // Automatically generated bindings for these functions will allocate
+        // Vecs, which in turn pulls in the panic machinery from std, which
+        // creates vtables that end up in the wasm elem section, which we
+        // can't support in these special core-wasm adapters.
+        // Instead, we manually define the bindings for these functions in
+        // terms of raw pointers.
+        skip: ["get-environment", "poll-oneoff"],
     });
 }
 
@@ -1686,24 +1696,39 @@ pub unsafe extern "C" fn poll_oneoff(
             });
         }
 
-        let vec = state.import_alloc.with_buffer(
+        #[link(wasm_import_module = "wasi:poll/poll")]
+        extern "C" {
+            #[link_name = "poll-oneoff"]
+            fn poll_oneoff_import(pollables: *const Pollable, len: usize, rval: *mut BoolList);
+        }
+        let mut ready_list = BoolList {
+            base: std::ptr::null(),
+            len: 0,
+        };
+
+        state.import_alloc.with_buffer(
             results,
             nsubscriptions
                 .checked_mul(size_of::<bool>())
                 .trapping_unwrap(),
-            || poll::poll_oneoff(slice::from_raw_parts(pollables.pointer, pollables.length)),
+            || {
+                poll_oneoff_import(
+                    pollables.pointer,
+                    pollables.length,
+                    &mut ready_list as *mut _,
+                )
+            },
         );
 
-        assert_eq!(vec.len(), nsubscriptions);
-        assert_eq!(vec.as_ptr(), results);
-        forget(vec);
+        assert_eq!(ready_list.len, nsubscriptions);
+        assert_eq!(ready_list.base, results as *const bool);
 
         drop(pollables);
 
         let ready = subscriptions
             .iter()
             .enumerate()
-            .filter_map(|(i, s)| (*results.add(i) != 0).then_some(s));
+            .filter_map(|(i, s)| (*ready_list.base.add(i)).then_some(s));
 
         let mut count = 0;
 
@@ -2176,6 +2201,13 @@ pub struct StrTupleList {
     len: usize,
 }
 
+#[derive(Copy, Clone)]
+#[repr(C)]
+pub struct BoolList {
+    base: *const bool,
+    len: usize,
+}
+
 const fn bump_arena_size() -> usize {
     // The total size of the struct should be a page, so start there
     let mut start = PAGE_SIZE;

crates/wasi/src/preview2/preview2/poll.rs

@@ -34,7 +34,7 @@ impl<T: WasiView> poll::Host for T {
         Ok(())
     }
 
-    async fn poll_oneoff(&mut self, futures: Vec<Pollable>) -> anyhow::Result<Vec<u8>> {
+    async fn poll_oneoff(&mut self, futures: Vec<Pollable>) -> anyhow::Result<Vec<bool>> {
         use crate::preview2::sched::{sync::SyncSched, Poll, Userdata, WasiSched};
 
         // Convert `futures` into `Poll` subscriptions.
@@ -74,10 +74,9 @@ impl<T: WasiView> poll::Host for T {
         // Do the poll.
         SyncSched.poll_oneoff(&mut poll).await?;
 
-        // Convert the results into a list of `u8` to return.
-        let mut results = vec![0_u8; len];
+        let mut results = vec![false; len];
         for (_result, data) in poll.results() {
-            results[u64::from(data) as usize] = u8::from(true);
+            results[u64::from(data) as usize] = true;
         }
         Ok(results)
     }

crates/wasi/wit/deps/poll/poll.wit

@@ -32,10 +32,8 @@ interface poll {
     /// component model async proposal, which will include a scalable waiting
     /// facility.
     ///
-    /// Note that the return type would ideally be `list<bool>`, but that would
-    /// be more difficult to polyfill given the current state of `wit-bindgen`.
-    /// See <https://github.com/bytecodealliance/preview2-prototyping/pull/11#issuecomment-1329873061>
-    /// for details.  For now, we use zero to mean "not ready" and non-zero to
-    /// mean "ready".
-    poll-oneoff: func(in: list<pollable>) -> list<u8>
+    /// The result list<bool> is the same length as the argument
+    /// list<pollable>, and indicates the readiness of each corresponding
+    /// element in that / list, with true indicating ready.
+    poll-oneoff: func(in: list<pollable>) -> list<bool>
 }