Forbid empty poll list.

Fixes: WebAssembly/wasi-io#67

crates/test-programs/src/bin/preview2_pollable_traps.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // Polling an empty list should trap:
+    test_programs::wasi::io::poll::poll(&[]);
+}

crates/wasi-http/wit/deps/io/poll.wit

@@ -27,8 +27,9 @@ interface poll {
     /// The result `list<u32>` contains one or more indices of handles in the
     /// argument list that is ready for I/O.
     ///
-    /// If the list contains more elements than can be indexed with a `u32`
-    /// value, this function traps.
+    /// This function traps if either:
+    /// - the list is empty, or:
+    /// - the list contains more elements than can be indexed with a `u32` value.
     ///
     /// A timeout can be implemented by adding a pollable from the
     /// wasi-clocks API to the list.

crates/wasi/src/preview2/poll.rs

@@ -1,5 +1,5 @@
 use crate::preview2::{bindings::io::poll, WasiView};
-use anyhow::Result;
+use anyhow::{anyhow, Result};
 use futures::Future;
 use smallvec::{smallvec, SmallVec};
 use std::{
@@ -155,8 +155,7 @@ impl PollableResource {
 impl<T: WasiView> poll::Host for T {
     async fn poll(&mut self, pollables: Vec<Resource<PollableResource>>) -> Result<Vec<u32>> {
         if pollables.is_empty() {
-            futures::future::pending::<()>().await;
-            unreachable!();
+            return Err(anyhow!("empty poll list"));
         }
 
         type PollableRep = u32;

crates/wasi/tests/all/async_.rs

@@ -355,8 +355,16 @@ async fn preview2_stream_pollable_traps() {
     )
 }
 #[test_log::test(tokio::test(flavor = "multi_thread"))]
-async fn preview2_pollable() {
-    run(PREVIEW2_POLLABLE_COMPONENT, false).await.unwrap()
+async fn preview2_pollable_correct() {
+    run(PREVIEW2_POLLABLE_CORRECT_COMPONENT, false)
+        .await
+        .unwrap()
+}
+#[test_log::test(tokio::test(flavor = "multi_thread"))]
+async fn preview2_pollable_traps() {
+    run(PREVIEW2_POLLABLE_TRAPS_COMPONENT, false)
+        .await
+        .unwrap_err();
 }
 #[test_log::test(tokio::test(flavor = "multi_thread"))]
 async fn preview2_adapter_badfd() {

crates/wasi/tests/all/sync.rs

@@ -294,8 +294,12 @@ fn preview2_stream_pollable_traps() {
     )
 }
 #[test_log::test]
-fn preview2_pollable() {
-    run(PREVIEW2_POLLABLE_COMPONENT, false).unwrap()
+fn preview2_pollable_correct() {
+    run(PREVIEW2_POLLABLE_CORRECT_COMPONENT, false).unwrap()
+}
+#[test_log::test]
+fn preview2_pollable_traps() {
+    run(PREVIEW2_POLLABLE_TRAPS_COMPONENT, false).unwrap_err();
 }
 #[test_log::test]
 fn preview2_adapter_badfd() {

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

@@ -27,8 +27,9 @@ interface poll {
     /// The result `list<u32>` contains one or more indices of handles in the
     /// argument list that is ready for I/O.
     ///
-    /// If the list contains more elements than can be indexed with a `u32`
-    /// value, this function traps.
+    /// This function traps if either:
+    /// - the list is empty, or:
+    /// - the list contains more elements than can be indexed with a `u32` value.
     ///
     /// A timeout can be implemented by adding a pollable from the
     /// wasi-clocks API to the list.