Add wasm32-unknown-emscripten target support

.github/workflows/ci.yml

@@ -101,6 +101,44 @@ jobs:
       run: rustup target add wasm32-wasip2
     - name: Tests
       run: cargo test --target wasm32-wasip2 --all-features
+  TestEmscripten:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    # TEMPORARY: this job builds against the guybedford/emscripten `cf` fork,
+    # which carries the epoll, AF_UNIX pathname sockets, and multicast
+    # getsockopt patches this branch depends on. Once those land upstream the
+    # fork checkout can be dropped and this can move to a released emsdk.
+    steps:
+    - uses: actions/checkout@v4
+    - uses: dtolnay/rust-toolchain@nightly
+      with:
+        components: rust-src
+    # JSPI is the return-to-host bridge; it needs a Node with JSPI on by
+    # default (V8 13.x+). Recorded here so the cargo runner uses it rather than
+    # the Node bundled with emsdk.
+    - uses: actions/setup-node@v4
+      with:
+        node-version: 26
+    - name: Record JSPI-capable node
+      run: echo "JSPI_NODE=$(which node)" >> "$GITHUB_ENV"
+    - name: Install emsdk (LLVM + binaryen toolchain)
+      run: |
+        git clone --depth 1 https://github.com/emscripten-core/emsdk.git
+        ./emsdk/emsdk install latest
+        ./emsdk/emsdk activate latest
+    - name: Clone guybedford/emscripten (cf) fork over the emsdk tree
+      run: |
+        git clone --depth 1 --branch cf https://github.com/guybedford/emscripten.git em-fork
+        python3 em-fork/bootstrap.py
+        rm -rf ./emsdk/upstream/emscripten
+        ln -s "$(pwd)/em-fork" ./emsdk/upstream/emscripten
+    - name: Tests
+      run: |
+        source ./emsdk/emsdk_env.sh
+        export EM_CONFIG="$EMSDK/.emscripten"
+        CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_RUNNER="$JSPI_NODE" \
+        RUSTFLAGS="-A linker_messages -C target-feature=+atomics,+bulk-memory,+mutable-globals -C link-arg=-pthread -C link-arg=-sPROXY_TO_PTHREAD -C link-arg=-sNODERAWSOCKETS -C link-arg=-sNODERAWFS -C link-arg=-sALLOW_MEMORY_GROWTH=1 -C link-arg=-sEXIT_RUNTIME=1 -C link-arg=-sJSPI" \
+        cargo +nightly test --target wasm32-unknown-emscripten -Zbuild-std --all-features --tests --examples --lib
   Nightly:
     runs-on: ubuntu-latest
     timeout-minutes: 10
@@ -190,6 +228,7 @@ jobs:
           #- powerpc64-ibm-aix
           - riscv32imc-esp-espidf
           - sparcv9-sun-solaris
+          - wasm32-unknown-emscripten
           - wasm32-wasip1
           - x86_64-apple-darwin
           - x86_64-apple-ios

Cargo.toml

@@ -47,7 +47,7 @@ net = []
 log = { version = "0.4.8", optional = true }
 
 [target.'cfg(any(unix, target_os = "hermit", target_os = "wasi"))'.dependencies]
-libc = "0.2.183"
+libc = { git = "https://github.com/guybedford/libc", branch = "emscripten" }
 
 [target.'cfg(windows)'.dependencies.windows-sys]
 version = "0.61"

src/lib.rs

@@ -40,8 +40,11 @@
 //!
 //! The available features are described in the [`features`] module.
 
-#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
-compile_error!("This wasm target is unsupported by mio. If using Tokio, disable the net feature.");
+#[cfg(all(
+    target_family = "wasm",
+    not(any(target_os = "wasi", target_os = "emscripten"))
+))]
+compile_error!("This wasm target is unsupported by mio (only the WASI and Emscripten wasm targets are supported). If using Tokio, disable the net feature.");
 
 // macros used internally
 #[macro_use]

src/net/mod.rs

@@ -36,4 +36,7 @@ pub use self::udp::UdpSocket;
 #[cfg(unix)]
 mod uds;
 #[cfg(unix)]
-pub use self::uds::{UnixDatagram, UnixListener, UnixStream};
+pub use self::uds::{UnixListener, UnixStream};
+// Emscripten's node-backed AF_UNIX is stream-only (no datagram primitive).
+#[cfg(all(unix, not(target_os = "emscripten")))]
+pub use self::uds::UnixDatagram;

src/net/uds/mod.rs

@@ -1,4 +1,6 @@
+#[cfg(not(target_os = "emscripten"))]
 mod datagram;
+#[cfg(not(target_os = "emscripten"))]
 pub use self::datagram::UnixDatagram;
 
 mod listener;

src/net/uds/stream.rs

@@ -52,6 +52,8 @@ impl UnixStream {
     /// Creates an unnamed pair of connected sockets.
     ///
     /// Returns two `UnixStream`s which are connected to each other.
+    // Emscripten has no `socketpair(2)` (no `uv_socketpair` exposed by node).
+    #[cfg(not(target_os = "emscripten"))]
     pub fn pair() -> io::Result<(UnixStream, UnixStream)> {
         sys::uds::stream::pair().map(|(stream1, stream2)| {
             (UnixStream::from_std(stream1), UnixStream::from_std(stream2))

src/sys/shell/uds.rs

@@ -1,3 +1,5 @@
+// Emscripten's node-backed AF_UNIX is stream-only (no datagram primitive).
+#[cfg(not(target_os = "emscripten"))]
 pub(crate) mod datagram {
     use std::io;
     use std::os::unix::net::{self, SocketAddr};
@@ -38,6 +40,8 @@ pub(crate) mod stream {
         os_required!()
     }
 
+    // Emscripten has no `socketpair(2)` (no `uv_socketpair` exposed by node).
+    #[cfg(not(target_os = "emscripten"))]
     pub(crate) fn pair() -> io::Result<(net::UnixStream, net::UnixStream)> {
         os_required!()
     }

src/sys/unix/mod.rs

@@ -19,6 +19,7 @@ cfg_os_poll! {
         not(mio_unsupported_force_poll_poll),
         any(
             target_os = "android",
+            target_os = "emscripten",
             target_os = "illumos",
             target_os = "linux",
             target_os = "redox",
@@ -105,6 +106,7 @@ cfg_os_poll! {
         ),
         target_os = "aix",
         target_os = "dragonfly",
+        target_os = "emscripten",
         target_os = "haiku",
         target_os = "hurd",
         target_os = "netbsd",
@@ -158,6 +160,7 @@ cfg_os_poll! {
             // NOTE: also add to the list for the `pipe` module below.
             target_os = "aix",
             target_os = "dragonfly",
+            target_os = "emscripten",
             target_os = "haiku",
             target_os = "hurd",
             target_os = "netbsd",

src/sys/unix/net.rs

@@ -56,8 +56,11 @@ pub(crate) fn new_socket(domain: libc::c_int, socket_type: libc::c_int) -> io::R
     }
 
     // Darwin (and others) doesn't have SOCK_NONBLOCK or SOCK_CLOEXEC.
+    // Emscripten's `socket(2)` silently strips both flags, so set `O_NONBLOCK`
+    // via `fcntl(2)` instead (`FD_CLOEXEC` is a no-op there).
     #[cfg(any(
         target_os = "aix",
+        target_os = "emscripten",
         target_os = "ios",
         target_os = "macos",
         target_os = "tvos",

src/sys/unix/pipe.rs

@@ -11,6 +11,7 @@ pub(crate) fn new_raw() -> io::Result<[RawFd; 2]> {
     #[cfg(any(
         target_os = "android",
         target_os = "dragonfly",
+        target_os = "emscripten",
         target_os = "freebsd",
         target_os = "fuchsia",
         target_os = "hurd",

src/sys/unix/selector/epoll.rs

@@ -238,6 +238,9 @@ pub mod event {
 }
 
 // No special requirement from the implementation around waking.
+// On emscripten the public `Waker` is not compiled (the runtime wakes through
+// its own event loop), so this re-export is unused there.
+#[cfg_attr(target_os = "emscripten", allow(unused_imports))]
 pub(crate) use crate::sys::unix::waker::Waker;
 
 cfg_io_source! {

src/sys/unix/tcp.rs

@@ -65,6 +65,7 @@ pub(crate) fn accept(listener: &net::TcpListener) -> io::Result<(net::TcpStream,
         // See https://github.com/tokio-rs/mio/issues/1445 for details
         all(not(target_arch="x86"), target_os = "android"),
         target_os = "dragonfly",
+        target_os = "emscripten",
         target_os = "freebsd",
         target_os = "fuchsia",
         target_os = "hurd",

src/sys/unix/uds/mod.rs

@@ -1,12 +1,16 @@
+#[cfg(not(target_os = "emscripten"))]
+use std::io;
 #[cfg(target_os = "android")]
 use std::os::android::net::SocketAddrExt;
 #[cfg(target_os = "linux")]
 use std::os::linux::net::SocketAddrExt;
 use std::os::unix::ffi::OsStrExt;
+#[cfg(not(target_os = "emscripten"))]
 use std::os::unix::io::FromRawFd;
 use std::os::unix::net::SocketAddr;
-use std::{io, mem, ptr};
+use std::{mem, ptr};
 
+#[cfg(not(target_os = "emscripten"))]
 pub(crate) mod datagram;
 pub(crate) mod listener;
 pub(crate) mod stream;
@@ -81,6 +85,7 @@ fn unix_addr(address: &SocketAddr) -> (libc::sockaddr_un, libc::socklen_t) {
     (sockaddr, addrlen as _)
 }
 
+#[cfg(not(target_os = "emscripten"))]
 fn pair<T>(flags: libc::c_int) -> io::Result<(T, T)>
 where
     T: FromRawFd,

src/sys/unix/uds/stream.rs

@@ -20,6 +20,7 @@ pub(crate) fn connect_addr(address: &SocketAddr) -> io::Result<net::UnixStream>
     Ok(socket)
 }
 
+#[cfg(not(target_os = "emscripten"))]
 pub(crate) fn pair() -> io::Result<(net::UnixStream, net::UnixStream)> {
     super::pair(libc::SOCK_STREAM)
 }

tests/regressions.rs

@@ -109,7 +109,7 @@ fn issue_1205() {
 }
 
 #[test]
-#[cfg(unix)]
+#[cfg(all(unix, not(target_os = "emscripten")))]
 #[cfg_attr(miri, ignore = "Miri doesn't support Unix domain sockets")]
 fn issue_1403() {
     use mio::net::UnixDatagram;

tests/tcp_stream.rs

@@ -464,6 +464,10 @@ fn shutdown_both() {
 }
 
 #[cfg(unix)]
+#[cfg_attr(
+    target_os = "emscripten",
+    ignore = "REMOVE BEFORE LANDING - pending Node.js 26.6.0 in runner"
+)]
 #[test]
 fn raw_fd() {
     init();
@@ -636,6 +640,7 @@ fn tcp_shutdown_client_read_close_event() {
 #[cfg_attr(
     any(
         target_os = "android",
+        target_os = "emscripten",
         target_os = "hurd",
         target_os = "illumos",
         target_os = "solaris",

tests/udp_socket.rs

@@ -407,6 +407,10 @@ fn smoke_test_connected_udp_socket(mut socket1: UdpSocket, mut socket2: UdpSocke
     assert!(socket2.take_error().unwrap().is_none());
 }
 
+#[cfg_attr(
+    target_os = "emscripten",
+    ignore = "libuv does not re-associate a connected UDP socket on reconnect"
+)]
 #[test]
 fn reconnect_udp_socket_sending() {
     let (mut poll, mut events) = init_with_poll();
@@ -470,6 +474,10 @@ fn reconnect_udp_socket_sending() {
     assert!(socket3.take_error().unwrap().is_none());
 }
 
+#[cfg_attr(
+    target_os = "emscripten",
+    ignore = "libuv does not re-associate a connected UDP socket on reconnect"
+)]
 #[test]
 fn reconnect_udp_socket_receiving() {
     let (mut poll, mut events) = init_with_poll();
@@ -652,6 +660,7 @@ fn connected_udp_socket_unconnected_methods() {
         target_os = "android",
         target_os = "hurd",
         target_os = "linux",
+        target_os = "emscripten",
         target_os = "windows",
         target_os = "cygwin",
         target_os = "wasi",
@@ -664,6 +673,7 @@ fn connected_udp_socket_unconnected_methods() {
         target_os = "android",
         target_os = "hurd",
         target_os = "linux",
+        target_os = "emscripten",
         target_os = "windows",
         target_os = "cygwin",
         target_os = "wasi",

tests/unix_datagram.rs

@@ -1,4 +1,12 @@
-#![cfg(all(unix, feature = "os-poll", feature = "net", not(miri)))] // Miri doesn't support Unix domain sockets.
+// Emscripten's node-backed AF_UNIX is stream-only (no datagram sockets).
+// Miri doesn't support Unix domain sockets.
+#![cfg(all(
+    unix,
+    not(target_os = "emscripten"),
+    feature = "os-poll",
+    feature = "net",
+    not(miri)
+))]
 
 use mio::net::UnixDatagram;
 use mio::{Interest, Token};

tests/unix_pipe.rs

@@ -1,12 +1,15 @@
 #![cfg(all(unix, feature = "os-poll", feature = "os-ext", feature = "net"))]
 
 use std::io::{Read, Write};
+#[cfg(not(target_os = "emscripten"))]
 use std::process::{Command, Stdio};
 use std::sync::{Arc, Barrier};
 use std::thread;
 use std::time::Duration;
 
-use mio::unix::pipe::{self, Receiver, Sender};
+use mio::unix::pipe;
+#[cfg(not(target_os = "emscripten"))]
+use mio::unix::pipe::{Receiver, Sender};
 use mio::{Events, Interest, Poll, Token};
 
 mod util;
@@ -132,6 +135,8 @@ fn event_when_receiver_is_dropped() {
 }
 
 #[test]
+// Emscripten/wasm has no fork/exec, so no subprocesses.
+#[cfg(not(target_os = "emscripten"))]
 #[cfg_attr(
     any(target_os = "hurd", target_os = "nto", target_os = "cygwin"),
     ignore = "Writer fd close events do not trigger POLLHUP on nto and GNU/Hurd targets"
@@ -184,6 +189,7 @@ fn from_child_process_io() {
 }
 
 #[test]
+#[cfg(not(target_os = "emscripten"))]
 #[cfg_attr(miri, ignore = "Miri doesn't support process spawning")]
 fn nonblocking_child_process_io() {
     // `cat` simply echo everything that we write via standard in.

tests/unix_stream.rs

@@ -24,6 +24,7 @@ const DATA1_LEN: usize = 16;
 const DATA2_LEN: usize = 14;
 const DEFAULT_BUF_SIZE: usize = 64;
 const TOKEN_1: Token = Token(0);
+#[cfg(not(target_os = "emscripten"))]
 const TOKEN_2: Token = Token(1);
 
 #[test]
@@ -145,6 +146,8 @@ fn unix_stream_from_std() {
     )
 }
 
+// Emscripten has no `socketpair(2)`.
+#[cfg(not(target_os = "emscripten"))]
 #[test]
 fn unix_stream_pair() {
     let (mut poll, mut events) = init_with_poll();

tests/util/mod.rs

@@ -224,7 +224,7 @@ pub fn assert_socket_non_blocking<S>(_: &S) {
 }
 
 /// Assert that `CLOEXEC` is set on `socket`.
-#[cfg(any(unix, target_os = "wasi"))]
+#[cfg(any(all(unix, not(target_os = "emscripten")), target_os = "wasi"))]
 pub fn assert_socket_close_on_exec<S>(socket: &S)
 where
     S: AsRawFd,
@@ -242,6 +242,11 @@ where
     }
 }
 
+// Emscripten is a single process with no `exec(2)`, so `FD_CLOEXEC` is a no-op
+// (`F_GETFD` always returns 0); the concept doesn't apply.
+#[cfg(target_os = "emscripten")]
+pub fn assert_socket_close_on_exec<S>(_: &S) {}
+
 #[cfg(windows)]
 pub fn assert_socket_close_on_exec<S>(_: &S) {
     // Windows doesn't have this concept.