Add a port_getn_query function. (#1216)

* Add a `port_getn_query` function.

The solarish [`port_getn` function] special-cases a `max` argument value
of 0 to be a query of the number of events available. #1215 added a
special-case to protect the code from doing a resize in that case. And
in case users actually do want to do a query, this PR adds a new
`port_getn_query` function that passes a zero.

[`port_getn` function]: https://illumos.org/man/3C/port_getn

* Fix errors.

* Update src/event/port.rs

Co-authored-by: 王宇逸 <[email protected]>

---------

Co-authored-by: 王宇逸 <[email protected]>

Author: sunfishcode

Diff for: src/backend/libc/event/syscalls.rs

@@ -314,9 +314,13 @@ pub(crate) fn port_getn(
     events: &mut Vec<Event>,
     mut nget: u32,
 ) -> io::Result<()> {
+    // `port_getn` special-cases a max value of 0 to be a query that returns
+    // the number of events. We don't want to do the `set_len` in that case, so
+    // so bail out early if needed.
     if events.capacity() == 0 {
         return Ok(());
     }
+
     let timeout = timeout.map_or(null_mut(), as_mut_ptr);
     unsafe {
         ret(c::port_getn(
@@ -336,6 +340,24 @@ pub(crate) fn port_getn(
     Ok(())
 }
 
+#[cfg(solarish)]
+pub(crate) fn port_getn_query(port: BorrowedFd<'_>) -> io::Result<u32> {
+    let mut nget: u32 = 0;
+
+    // Pass a `max` of 0 to query the number of available events.
+    unsafe {
+        ret(c::port_getn(
+            borrowed_fd(port),
+            null_mut(),
+            0,
+            &mut nget,
+            null_mut(),
+        ))?;
+    }
+
+    Ok(nget)
+}
+
 #[cfg(solarish)]
 pub(crate) fn port_send(
     port: BorrowedFd<'_>,

Diff for: src/event/port.rs

@@ -110,6 +110,13 @@ pub fn port_get(port: impl AsFd, timeout: Option<Duration>) -> io::Result<Event>
 /// `port_getn(port, events, min_events, timeout)`—Gets multiple events from a
 /// port.
 ///
+/// This requests up to a max of `events.capacity()` events, and then resizes
+/// `events` to the number of events retrieved. If `events.capacity()` is 0,
+/// this does nothing and returns immediately.
+///
+/// To query the number of events without retrieving any, use
+/// [`port_getn_query`].
+///
 /// # References
 ///  - [OpenSolaris]
 ///  - [illumos]
@@ -138,6 +145,21 @@ pub fn port_getn(
     )
 }
 
+/// `port_getn(port, NULL, 0, NULL)`—Queries the number of events
+/// available from a port.
+///
+/// To retrieve the events, use [`port_getn`].
+///
+/// # References
+///  - [OpenSolaris]
+///  - [illumos]
+///
+/// [OpenSolaris]: https://www.unix.com/man-page/opensolaris/3C/port_getn/
+/// [illumos]: https://illumos.org/man/3C/port_getn
+pub fn port_getn_query(port: impl AsFd) -> io::Result<u32> {
+    syscalls::port_getn_query(port.as_fd())
+}
+
 /// `port_send(port, events, userdata)`—Sends an event to a port.
 ///
 /// # References