Implement poll-based backend

Introduces a new backend for UNIX using the level triggered poll()
syscall instead of epoll or kqueue.  This support is crucial for
embedded systems like the esp32 family but also for alternative
operating systems like Haiku.

This diff does not introduce any new platform support targets itself but
provides the core technical implementation necessary to support these
other targets.  Future PRs will introduce specific platform support
however due to reasons outlined in tokio-rs#1602 (many thanks for this initial
effort BTW!) it is not possible to automate tests for those platforms.
We will instead rely on the fact that Linux can serve as a proxy to
prove that the mio code is working nominally.

Note that only Linux has a sufficiently complex implementation to pass
all tests.  This is due to SIGRDHUP missing on other platforms and is
required for about a dozen or so tests that check is_read_closed().

.github/workflows/ci.yml

@@ -31,6 +31,18 @@ jobs:
       run: cargo test --all-features
     - name: Tests release build
       run: cargo test --release --all-features
+  TestPoll:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    env:
+      RUSTFLAGS="--cfg mio_unsupported_force_poll_poll"
+    steps:
+    - uses: actions/checkout@v3
+    - uses: dtolnay/rust-toolchain@stable
+    - name: Tests
+      run: cargo test --all-features
+    - name: Tests release build
+      run: cargo test --release --all-features
   MinimalVersions:
     runs-on: ${{ matrix.os }}
     timeout-minutes: 10
@@ -133,6 +145,7 @@ jobs:
     runs-on: ubuntu-latest
     needs:
       - Test
+      - TestPoll
       - MinimalVersions
       - MSRV
       - Nightly

src/io_source.rs

@@ -142,9 +142,7 @@ where
     ) -> io::Result<()> {
         #[cfg(debug_assertions)]
         self.selector_id.associate(registry)?;
-        registry
-            .selector()
-            .register(self.inner.as_raw_fd(), token, interests)
+        self.state.register(registry, token, interests, self.inner.as_raw_fd())
     }
 
     fn reregister(
@@ -155,15 +153,13 @@ where
     ) -> io::Result<()> {
         #[cfg(debug_assertions)]
         self.selector_id.check_association(registry)?;
-        registry
-            .selector()
-            .reregister(self.inner.as_raw_fd(), token, interests)
+        self.state.reregister(registry, token, interests, self.inner.as_raw_fd())
     }
 
     fn deregister(&mut self, registry: &Registry) -> io::Result<()> {
         #[cfg(debug_assertions)]
         self.selector_id.remove_association(registry)?;
-        registry.selector().deregister(self.inner.as_raw_fd())
+        self.state.deregister(registry, self.inner.as_raw_fd())
     }
 }
 

src/poll.rs

@@ -1,5 +1,5 @@
 use crate::{event, sys, Events, Interest, Token};
-#[cfg(unix)]
+#[cfg(all(unix, not(mio_unsupported_force_poll_poll)))]
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::time::Duration;
 use std::{fmt, io};
@@ -411,7 +411,7 @@ impl Poll {
     }
 }
 
-#[cfg(unix)]
+#[cfg(all(unix, not(mio_unsupported_force_poll_poll)))]
 impl AsRawFd for Poll {
     fn as_raw_fd(&self) -> RawFd {
         self.registry.as_raw_fd()
@@ -696,15 +696,18 @@ impl fmt::Debug for Registry {
     }
 }
 
-#[cfg(unix)]
+#[cfg(all(unix, not(mio_unsupported_force_poll_poll)))]
 impl AsRawFd for Registry {
     fn as_raw_fd(&self) -> RawFd {
         self.selector.as_raw_fd()
     }
 }
 
 cfg_os_poll! {
-    #[cfg(unix)]
+    #[cfg(all(
+        unix,
+        not(mio_unsupported_force_poll_poll)
+    ))]
     #[test]
     pub fn as_raw_fd() {
         let poll = Poll::new().unwrap();

src/sys/unix/mod.rs

@@ -33,25 +33,65 @@ cfg_os_poll! {
     }
 
     cfg_io_source! {
-        use std::io;
-
         // Both `kqueue` and `epoll` don't need to hold any user space state.
-        pub(crate) struct IoSourceState;
+        #[cfg(not(mio_unsupported_force_poll_poll))]
+        mod stateless_io_source {
+            use std::io;
+            use std::os::unix::io::RawFd;
+            use crate::Registry;
+            use crate::Token;
+            use crate::Interest;
 
-        impl IoSourceState {
-            pub fn new() -> IoSourceState {
-                IoSourceState
-            }
+            pub(crate) struct IoSourceState;
+
+            impl IoSourceState {
+                pub fn new() -> IoSourceState {
+                    IoSourceState
+                }
+
+                pub fn do_io<T, F, R>(&self, f: F, io: &T) -> io::Result<R>
+                where
+                    F: FnOnce(&T) -> io::Result<R>,
+                {
+                    // We don't hold state, so we can just call the function and
+                    // return.
+                    f(io)
+                }
 
-            pub fn do_io<T, F, R>(&self, f: F, io: &T) -> io::Result<R>
-            where
-                F: FnOnce(&T) -> io::Result<R>,
-            {
-                // We don't hold state, so we can just call the function and
-                // return.
-                f(io)
+                pub fn register(
+                    &mut self,
+                    registry: &Registry,
+                    token: Token,
+                    interests: Interest,
+                    fd: RawFd,
+                ) -> io::Result<()> {
+                    // Pass through, we don't have any state
+                    registry.selector().register(fd, token, interests)
+                }
+
+                pub fn reregister(
+                    &mut self,
+                    registry: &Registry,
+                    token: Token,
+                    interests: Interest,
+                    fd: RawFd,
+                ) -> io::Result<()> {
+                    // Pass through, we don't have any state
+                    registry.selector().reregister(fd, token, interests)
+                }
+
+                pub fn deregister(&mut self, registry: &Registry, fd: RawFd) -> io::Result<()> {
+                    // Pass through, we don't have any state
+                    registry.selector().deregister(fd)
+                }
             }
         }
+
+        #[cfg(not(mio_unsupported_force_poll_poll))]
+        pub(crate) use self::stateless_io_source::IoSourceState;
+
+        #[cfg(mio_unsupported_force_poll_poll)]
+        pub(crate) use self::selector::IoSourceState;
     }
 
     cfg_os_ext! {

src/sys/unix/selector/mod.rs

@@ -1,40 +1,58 @@
-#[cfg(any(
-    target_os = "android",
-    target_os = "illumos",
-    target_os = "linux",
-    target_os = "redox",
+#[cfg(all(
+    not(mio_unsupported_force_poll_poll),
+    any(
+        target_os = "android",
+        target_os = "illumos",
+        target_os = "linux",
+        target_os = "redox",
+    )
 ))]
 mod epoll;
 
-#[cfg(any(
-    target_os = "android",
-    target_os = "illumos",
-    target_os = "linux",
-    target_os = "redox",
+#[cfg(all(
+    not(mio_unsupported_force_poll_poll),
+    any(
+        target_os = "android",
+        target_os = "illumos",
+        target_os = "linux",
+        target_os = "redox",
+    )
 ))]
 pub(crate) use self::epoll::{event, Event, Events, Selector};
 
-#[cfg(any(
-    target_os = "dragonfly",
-    target_os = "freebsd",
-    target_os = "ios",
-    target_os = "macos",
-    target_os = "netbsd",
-    target_os = "openbsd",
-    target_os = "tvos",
-    target_os = "watchos",
+#[cfg(mio_unsupported_force_poll_poll)]
+mod poll;
+
+#[cfg(mio_unsupported_force_poll_poll)]
+pub(crate) use self::poll::{event, Event, Events, Selector, IoSourceState};
+
+#[cfg(all(
+    not(mio_unsupported_force_poll_poll),
+    any(
+        target_os = "dragonfly",
+        target_os = "freebsd",
+        target_os = "ios",
+        target_os = "macos",
+        target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "tvos",
+        target_os = "watchos",
+    )
 ))]
 mod kqueue;
 
-#[cfg(any(
-    target_os = "dragonfly",
-    target_os = "freebsd",
-    target_os = "ios",
-    target_os = "macos",
-    target_os = "netbsd",
-    target_os = "openbsd",
-    target_os = "tvos",
-    target_os = "watchos",
+#[cfg(all(
+    not(mio_unsupported_force_poll_poll),
+    any(
+        target_os = "dragonfly",
+        target_os = "freebsd",
+        target_os = "ios",
+        target_os = "macos",
+        target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "tvos",
+        target_os = "watchos",
+    ),
 ))]
 pub(crate) use self::kqueue::{event, Event, Events, Selector};