jump through enough hoops for the poll lifetime to work out

you program rust for a few years and you think you're done tearing your
hair out over lifetimes, well, you'll find yourself wrong

crates/wasi-common/src/snapshots/preview_0.rs

@@ -1,12 +1,14 @@
-use crate::file::{FileCaps, FileEntryExt, TableFileExt};
+use crate::file::{FileCaps, FileEntryExt, FileEntryMutExt, TableFileExt, WasiFile};
 use crate::sched::{
     subscription::{RwEventFlags, SubscriptionResult},
-    Poll,
+    Poll, Userdata,
 };
 use crate::snapshots::preview_1::types as snapshot1_types;
 use crate::snapshots::preview_1::wasi_snapshot_preview1::WasiSnapshotPreview1 as Snapshot1;
 use crate::{Error, ErrorExt, WasiCtx};
 use cap_std::time::Duration;
+use std::cell::RefMut;
+use std::collections::HashSet;
 use std::convert::{TryFrom, TryInto};
 use std::io::{IoSlice, IoSliceMut};
 use std::ops::Deref;
@@ -778,7 +780,11 @@ impl wasi_unstable::WasiUnstable for WasiCtx {
         }
 
         let table = self.table();
-        let mut poll = Poll::new(&table);
+        let mut sub_fds: HashSet<types::Fd> = HashSet::new();
+        // We need these refmuts to outlive Poll, which will hold the &mut dyn WasiFile inside
+        let mut read_refs: Vec<(RefMut<'_, dyn WasiFile>, Userdata)> = Vec::new();
+        let mut write_refs: Vec<(RefMut<'_, dyn WasiFile>, Userdata)> = Vec::new();
+        let mut poll = Poll::new();
 
         let subs = subs.as_array(nsubscriptions);
         for sub_elem in subs.iter() {
@@ -816,11 +822,29 @@ impl wasi_unstable::WasiUnstable for WasiCtx {
                 },
                 types::SubscriptionU::FdRead(readsub) => {
                     let fd = readsub.file_descriptor;
-                    poll.subscribe_read(u32::from(fd), sub.userdata.into())?;
+                    if sub_fds.contains(&fd) {
+                        return Err(Error::invalid_argument()
+                            .context("Fd can be subscribed to at most once per poll"));
+                    } else {
+                        sub_fds.insert(fd);
+                    }
+                    let file_ref = table
+                        .get_file_mut(u32::from(fd))?
+                        .get_cap(FileCaps::POLL_READWRITE)?;
+                    read_refs.push((file_ref, sub.userdata.into()));
                 }
                 types::SubscriptionU::FdWrite(writesub) => {
                     let fd = writesub.file_descriptor;
-                    poll.subscribe_write(u32::from(fd), sub.userdata.into())?;
+                    if sub_fds.contains(&fd) {
+                        return Err(Error::invalid_argument()
+                            .context("Fd can be subscribed to at most once per poll"));
+                    } else {
+                        sub_fds.insert(fd);
+                    }
+                    let file_ref = table
+                        .get_file_mut(u32::from(fd))?
+                        .get_cap(FileCaps::POLL_READWRITE)?;
+                    write_refs.push((file_ref, sub.userdata.into()));
                 }
             }
         }