Reduce CPU load from try_read_inner() by performing a sleeping wait

Currently TryStream::try_read_inner() are part of a busy-loop that waits
for characters. We can make that wait use significantly less CPU by
sleeping until characters are available instead.

This should have no impact on performance providing characters or EOF
are received before reaching the timeout. The 100ms timeout could impact
the responsiveness of Sessions with a short timeouts. Happily these
sessions should be uncommon and their needs could be met by disabling
the polling feature.

Author: daniel-thompson

src/session/sync_session.rs

@@ -508,6 +508,12 @@ where
 
     // non-buffered && non-blocking read
     fn try_read_inner(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        // We don't know what the session timeout is but it is unlikely to be
+        // less than 100ms (and if it is then polling is optional anyway).
+        if !self.stream.get_mut().ready(Duration::from_millis(100))? {
+            return Err(io::Error::from(io::ErrorKind::WouldBlock));
+        }
+
         self.stream.get_mut().set_blocking(false)?;
 
         let result = self.stream.get_mut().read(buf);