Round up when converting timeout to ms for epoll_wait()

src/sys/unix/selector/epoll.rs

@@ -87,7 +87,19 @@ impl Selector {
         const MAX_SAFE_TIMEOUT: u128 = libc::c_int::max_value() as u128;
 
         let timeout = timeout
-            .map(|to| cmp::min(to.as_millis(), MAX_SAFE_TIMEOUT) as libc::c_int)
+            .map(|to| {
+                let to_ms = to.as_millis();
+                // as_millis() truncates, so round up to 1 ms as the documentation says can happen.
+                // This avoids turning submillisecond timeouts into immediate returns unless the
+                // caller explicitly requests that by specifying a zero timeout.
+                let to_ms = to_ms
+                    + if to_ms == 0 && to.subsec_nanos() != 0 {
+                        1
+                    } else {
+                        0
+                    };
+                cmp::min(MAX_SAFE_TIMEOUT, to_ms) as libc::c_int
+            })
             .unwrap_or(-1);
 
         events.clear();