sketch of poll_oneoff polyfill (#11)

This adds a `poll_oneoff` implementation to src/lib.rs.  It's completely
untested.  I was planning on adding a host implementation and using that to test
end-to-end, but that raised some tough questions about how much of the existing
`wasi-common` scheduler(s) should be reused.  I've decided to focus on other,
more widely-used parts of WASI first, but I wanted to share the work I've
already done here.

Note that I've moved the clock- and socket-specific functions out of `wasi-poll`
and into `wasi-clocks` and `wasi-tcp`, respectively.  The latter is a new
interface which will eventually contain functions and types resembling
@npmccallum's https://github.com/npmccallum/wasi-snapshot-preview2#wasi-tcp
proposal.

Per discussion with @sunfishcode:

- `wasi-tcp` includes an `error` enum type, intended to represent only socket-related errors.
- It also includes a `socket` pseudo-handle type, distinct from `wasi-filesystem`'s `descriptor` type.

These fine-grained types help move us away from the "everything is a file
descriptor" and "all errors are errnos" approaches of Preview 1.

If we decide `poll-oneoff` should be usable with files as well as sockets, we
can add `subscribe-read` and `subscribe-write` functions to `wasi-filesystem`
which accept file `descriptor`s.  Likewise for any other pseudo-handle type from
which we'd like to create futures.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

host/src/clocks.rs

@@ -66,6 +66,24 @@ impl wasi_default_clocks::WasiDefaultClocks for WasiCtx {
 }
 
 impl wasi_clocks::WasiClocks for WasiCtx {
+    fn subscribe_wall_clock(
+        &mut self,
+        when: wasi_clocks::Datetime,
+        absolute: bool,
+    ) -> anyhow::Result<wasi_clocks::WasiFuture> {
+        drop((when, absolute));
+        todo!()
+    }
+
+    fn subscribe_monotonic_clock(
+        &mut self,
+        when: wasi_clocks::Instant,
+        absolute: bool,
+    ) -> anyhow::Result<wasi_clocks::WasiFuture> {
+        drop((when, absolute));
+        todo!()
+    }
+
     fn monotonic_clock_now(
         &mut self,
         fd: wasi_clocks::MonotonicClock,

host/src/lib.rs

@@ -4,6 +4,7 @@ mod logging;
 mod poll;
 mod random;
 mod table;
+mod tcp;
 pub use table::Table;
 
 wit_bindgen_host_wasmtime_rust::generate!({
@@ -21,6 +22,7 @@ pub fn add_to_linker<T>(
     wasi_logging::add_to_linker(l, f)?;
     wasi_poll::add_to_linker(l, f)?;
     wasi_random::add_to_linker(l, f)?;
+    wasi_tcp::add_to_linker(l, f)?;
     Ok(())
 }
 

host/src/poll.rs

@@ -1,57 +1,17 @@
 #![allow(unused_variables)]
 
-use crate::{wasi_poll, WasiCtx};
+use crate::{
+    wasi_poll::{WasiFuture, WasiPoll},
+    WasiCtx,
+};
+use anyhow::Result;
 
-impl wasi_poll::WasiPoll for WasiCtx {
-    fn drop_future(&mut self, future: wasi_poll::WasiFuture) -> anyhow::Result<()> {
+impl WasiPoll for WasiCtx {
+    fn drop_future(&mut self, future: WasiFuture) -> Result<()> {
         todo!()
     }
 
-    fn bytes_readable(
-        &mut self,
-        fd: wasi_poll::Descriptor,
-    ) -> anyhow::Result<wasi_poll::BytesResult> {
-        todo!()
-    }
-
-    fn bytes_writable(
-        &mut self,
-        fd: wasi_poll::Descriptor,
-    ) -> anyhow::Result<wasi_poll::BytesResult> {
-        todo!()
-    }
-
-    fn subscribe_read(
-        &mut self,
-        fd: wasi_poll::Descriptor,
-    ) -> anyhow::Result<wasi_poll::WasiFuture> {
-        todo!()
-    }
-
-    fn subscribe_write(
-        &mut self,
-        fd: wasi_poll::Descriptor,
-    ) -> anyhow::Result<wasi_poll::WasiFuture> {
-        todo!()
-    }
-
-    fn subscribe_wall_clock(
-        &mut self,
-        when: wasi_poll::Datetime,
-        absolute: bool,
-    ) -> anyhow::Result<wasi_poll::WasiFuture> {
-        todo!()
-    }
-
-    fn subscribe_monotonic_clock(
-        &mut self,
-        when: wasi_poll::Instant,
-        absolute: bool,
-    ) -> anyhow::Result<wasi_poll::WasiFuture> {
-        todo!()
-    }
-
-    fn poll_oneoff(&mut self, futures: Vec<wasi_poll::WasiFuture>) -> anyhow::Result<Vec<bool>> {
+    fn poll_oneoff(&mut self, futures: Vec<WasiFuture>) -> Result<Vec<u8>> {
         todo!()
     }
 }

host/src/tcp.rs

@@ -0,0 +1,28 @@
+use crate::{
+    wasi_tcp::{self, BytesResult, Socket, WasiFuture, WasiTcp},
+    WasiCtx,
+};
+use anyhow::Result;
+use wit_bindgen_host_wasmtime_rust::Error;
+
+impl WasiTcp for WasiCtx {
+    fn bytes_readable(&mut self, socket: Socket) -> Result<BytesResult, Error<wasi_tcp::Error>> {
+        drop(socket);
+        todo!()
+    }
+
+    fn bytes_writable(&mut self, socket: Socket) -> Result<BytesResult, Error<wasi_tcp::Error>> {
+        drop(socket);
+        todo!()
+    }
+
+    fn subscribe_read(&mut self, socket: Socket) -> Result<WasiFuture> {
+        drop(socket);
+        todo!()
+    }
+
+    fn subscribe_write(&mut self, socket: Socket) -> Result<WasiFuture> {
+        drop(socket);
+        todo!()
+    }
+}