wasip2: Fix zero-wait pollables

crates/test-programs/src/bin/p2_clocks_zero_wait.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let pollable = wasip2::clocks::monotonic_clock::subscribe_duration(0);
+    for _ in 0..20 {
+        if pollable.ready() {
+            return;
+        }
+        wasip2::clocks::monotonic_clock::subscribe_duration(0).block();
+    }
+
+    panic!("pollable should eventually be ready");
+}

crates/test-programs/src/bin/p2_tcp_busy_poll.rs

@@ -11,8 +11,6 @@ use test_programs::wasi::sockets::tcp::TcpSocket;
 // prevent e.g. socket readiness from being delivered.  Here we verify that such
 // starvation does not happen.
 fn test_tcp_busy_poll(family: IpAddressFamily, address: IpSocketAddress) {
-    let zero_wait = monotonic_clock::subscribe_duration(0);
-
     let net = Network::default();
 
     let listener = TcpSocket::new(family).unwrap();
@@ -33,6 +31,7 @@ fn test_tcp_busy_poll(family: IpAddressFamily, address: IpSocketAddress) {
         let rx_ready = rx.subscribe();
         let mut counter = 0;
         loop {
+            let zero_wait = monotonic_clock::subscribe_duration(0);
             if counter > 1_000_000 {
                 panic!("socket still not ready!");
             }

crates/wasi/src/p2/host/clocks.rs

@@ -73,7 +73,7 @@ fn subscribe_to_duration(
     duration: tokio::time::Duration,
 ) -> wasmtime::Result<Resource<DynPollable>> {
     let sleep = if duration.is_zero() {
-        table.push(Deadline::Past)?
+        table.push(Deadline::Past { yielded: false })?
     } else if let Some(deadline) = tokio::time::Instant::now().checked_add(duration) {
         // NB: this resource created here is not actually exposed to wasm, it's
         // only an internal implementation detail used to match the signature
@@ -115,7 +115,7 @@ impl monotonic_clock::Host for WasiClocksCtxView<'_> {
 }
 
 enum Deadline {
-    Past,
+    Past { yielded: bool },
     Instant(tokio::time::Instant),
     Never,
 }
@@ -124,7 +124,8 @@ enum Deadline {
 impl Pollable for Deadline {
     async fn ready(&mut self) {
         match self {
-            Deadline::Past => {
+            Deadline::Past { yielded: true } => {}
+            Deadline::Past { yielded } => {
                 // It is important we yield to Tokio here; otherwise we risk
                 // starving `mio` such that it is unable to signal readiness for
                 // other pollables (e.g. TCP sockets) when the guest is polling
@@ -142,6 +143,7 @@ impl Pollable for Deadline {
                 // are hypothetically other ways to generate a pollable that's
                 // always immediately ready, which this hack doesn't cover, but
                 // we consider this sufficient for now.
+                *yielded = true;
                 tokio::task::yield_now().await
             }
             Deadline::Instant(instant) => tokio::time::sleep_until(*instant).await,

crates/wasi/tests/all/p2/async_.rs

@@ -435,3 +435,8 @@ async fn file_truncation_readonly(component_path: &str) {
     let contents = std::fs::read(&file).expect("read truncation test file");
     assert_eq!(EXPECTED_CONTENTS, contents);
 }
+
+#[test_log::test(tokio::test(flavor = "multi_thread"))]
+async fn p2_clocks_zero_wait() {
+    run(P2_CLOCKS_ZERO_WAIT_COMPONENT, |_| {}).await.unwrap()
+}

crates/wasi/tests/all/p2/sync.rs

@@ -403,3 +403,8 @@ fn file_truncation_readonly(component_path: &str) {
     let contents = std::fs::read(&file).expect("read truncation test file");
     assert_eq!(EXPECTED_CONTENTS, contents);
 }
+
+#[test_log::test]
+fn p2_clocks_zero_wait() {
+    run(P2_CLOCKS_ZERO_WAIT_COMPONENT, |_| {}).unwrap()
+}