WIP implementation of poll_oneoff on Windows

crates/wasi-common/src/sys/windows/hostcalls_impl/misc.rs

@@ -76,12 +76,160 @@ pub(crate) fn clock_time_get(clock_id: wasi::__wasi_clockid_t) -> Result<wasi::_
     duration.as_nanos().try_into().map_err(Into::into)
 }
 
+fn stdin_nonempty() -> bool {
+    use std::io::Read;
+    std::io::stdin().bytes().peekable().peek().is_some()
+}
+
 pub(crate) fn poll_oneoff(
     timeout: Option<ClockEventData>,
     fd_events: Vec<FdEventData>,
     events: &mut Vec<wasi::__wasi_event_t>,
-) -> Result<Vec<wasi::__wasi_event_t>> {
-    unimplemented!("poll_oneoff")
+) -> Result<()> {
+    use crate::fdentry::Descriptor;
+    if fd_events.is_empty() && timeout.is_none() {
+        return Ok(());
+    }
+
+    // Currently WASI file support is only (a) regular files (b) directories (c) symlinks on Windows,
+    // which are always ready to write on Unix.
+    //
+    // We need to consider stdin/stdout/stderr separately.
+    // We treat stdout/stderr as always ready to write. I'm not sure if it's correct
+    // on Windows but I have not find any way of checking if a write to stdout would block.
+    // Therefore, we only poll the stdin.
+    let mut stdin_events = vec![];
+    let mut immediate_events = vec![];
+
+    for event in fd_events {
+        match event.descriptor {
+            Descriptor::Stdin if event.r#type == wasi::__WASI_EVENTTYPE_FD_READ => {
+                stdin_events.push(event)
+            }
+            _ => immediate_events.push(event),
+        }
+    }
+
+    // we have at least one immediate event, so we don't need to care about stdin
+    if immediate_events.len() > 0 {
+        for event in immediate_events {
+            let size = match event.descriptor {
+                Descriptor::OsHandle(os_handle)
+                    if event.r#type == wasi::__WASI_EVENTTYPE_FD_READ =>
+                {
+                    os_handle
+                        .metadata()
+                        .expect("FIXME return a proper error")
+                        .len()
+                }
+                Descriptor::Stdin => panic!("Descriptor::Stdin should have been filtered out"),
+                // On Unix, ioctl(FIONREAD) will return 0 for stdout/stderr. Emulate the same behavior on Windows.
+                //
+                // Besides, the spec is unclear what nbytes should actually be for __WASI_EVENTTYPE_FD_WRITE and
+                // the implementation on Unix just returns 0 here, so it's probably fine to do the same on Windows for now.
+                // cf. https://github.com/WebAssembly/WASI/issues/148
+                _ => 0,
+            };
+
+            events.push(wasi::__wasi_event_t {
+                userdata: event.userdata,
+                r#type: event.r#type,
+                error: wasi::__WASI_ERRNO_SUCCESS,
+                u: wasi::__wasi_event_u_t {
+                    fd_readwrite: wasi::__wasi_event_fd_readwrite_t {
+                        nbytes: size,
+                        flags: 0,
+                    },
+                },
+            })
+        }
+    } else {
+        assert_ne!(stdin_events.len(), 0, "stdin_events should not be empty");
+
+        // We are busy-polling the stdin with delay, unfortunately.
+        //
+        // We'd like to do the following:
+        // (1) wait in a non-blocking way for data to be available in stdin, with timeout
+        // (2) find out, how many bytes are there available to be read.
+        // For one, using `WaitForSingleObject` on the stdin handle could possibly be one way of
+        // achieving (1).
+        // I don't know of any way to achieve (2).
+        //
+        // While both of these are not as trivial on Windows as they are on Linux, there's a much
+        // more fundamental issue preventing us from achieving such behavior with the current
+        // implementation of wasi-common.
+        //
+        // Precisely, in `fd_read` we are using `io::stdin` via the `BufRead` trait, which does
+        // buffering on the libstd side. This means that even if there's still some unread data
+        // in stdin, the Windows system calls may return false negatives, indicating that stdin is empty.
+        // Therefore, avoiding the busy-poll here would require us to ditch libstd for the interaction
+        // with stdin altogether.
+        //
+        // However, polling stdin is a relatively infrequent use case, so this hopefully won't be
+        // a major issue.
+        let poll_interval = Duration::from_millis(10);
+        let poll_start = Instant::now();
+        let timeout_duration = timeout
+            .map(|t| t.delay.try_into().map(Duration::from_nanos))
+            .transpose()?;
+        let timeout = loop {
+            if let Some(timeout_duration) = timeout_duration {
+                if poll_start.elapsed() >= timeout_duration {
+                    break timeout;
+                }
+            }
+            if stdin_nonempty() {
+                break None;
+            }
+            std::thread::sleep(poll_interval);
+        };
+
+        // TODO try refactoring pushing to events
+        match timeout {
+            // timeout occurred
+            Some(timeout) => {
+                for event in stdin_events {
+                    events.push(wasi::__wasi_event_t {
+                        userdata: timeout.userdata,
+                        r#type: wasi::__WASI_EVENTTYPE_CLOCK,
+                        error: wasi::__WASI_ERRNO_SUCCESS,
+                        u: wasi::__wasi_event_u_t {
+                            fd_readwrite: wasi::__wasi_event_fd_readwrite_t {
+                                nbytes: 0,
+                                flags: 0,
+                            },
+                        },
+                    });
+                }
+            }
+            // stdin is ready for reading
+            None => {
+                for event in stdin_events {
+                    assert_eq!(
+                        event.r#type,
+                        wasi::__WASI_EVENTTYPE_FD_READ,
+                        "stdin was expected to be polled for reading"
+                    );
+                    events.push(wasi::__wasi_event_t {
+                        userdata: event.userdata,
+                        r#type: event.r#type,
+                        error: wasi::__WASI_ERRNO_SUCCESS,
+                        // Another limitation is that `std::io::BufRead` doesn't allow us
+                        // to find out the number bytes available in the buffer,
+                        // so we return the only universally correct lower bound.
+                        u: wasi::__wasi_event_u_t {
+                            fd_readwrite: wasi::__wasi_event_fd_readwrite_t {
+                                nbytes: 1,
+                                flags: 0,
+                            },
+                        },
+                    });
+                }
+            }
+        }
+    }
+
+    Ok(())
 }
 
 fn get_monotonic_time() -> Duration {