fix(wasi): enable all WasiFiles to be pollable (#3913)

Currently, the use of the downcast method means that you have to use one
of the hard-coded types. But Enarx needs to define its own `WasiFile`
implementations. This works fine, except the resulting files cannot be
used in poll because they aren't part of the hard-coded list.

Replace this with an accessor method for the pollable type in
`WasiFile`. Because we provide a default implementation of the method
and manually implement it on all the hard-coded types, this is backwards
compatible.

Signed-off-by: Nathaniel McCallum <nathaniel@profian.com>

crates/wasi-common/cap-std-sync/src/stdio.rs

@@ -31,6 +31,15 @@ impl WasiFile for Stdin {
     fn as_any(&self) -> &dyn Any {
         self
     }
+    #[cfg(unix)]
+    fn pollable(&self) -> Option<rustix::fd::BorrowedFd> {
+        Some(self.0.as_fd())
+    }
+
+    #[cfg(windows)]
+    fn pollable(&self) -> Option<io_extras::os::windows::RawHandleOrSocket> {
+        Some(self.0.as_raw_handle_or_socket())
+    }
     async fn get_filetype(&mut self) -> Result<FileType, Error> {
         if self.isatty() {
             Ok(FileType::CharacterDevice)
@@ -98,6 +107,15 @@ macro_rules! wasi_file_write_impl {
             fn as_any(&self) -> &dyn Any {
                 self
             }
+            #[cfg(unix)]
+            fn pollable(&self) -> Option<rustix::fd::BorrowedFd> {
+                Some(self.0.as_fd())
+            }
+
+            #[cfg(windows)]
+            fn pollable(&self) -> Option<io_extras::os::windows::RawHandleOrSocket> {
+                Some(self.0.as_raw_handle_or_socket())
+            }
             async fn get_filetype(&mut self) -> Result<FileType, Error> {
                 if self.isatty() {
                     Ok(FileType::CharacterDevice)