Nonblocking/timeout-compatible reads.

Adds a new method to a `WebSocket`: `try_read_message`.
Calling `read_message` would always enter a loop until receipt of a
message, invalidating any modifications to the underlying stream's
timeout or blocking mode. The new method allows these to be respected.

This is structured so that the semantics/behaviour of `read_message`
remain unchanged, exposing the inner part of its loop publicly.

src/protocol/mod.rs

@@ -141,17 +141,38 @@ impl<Stream: Read + Write> WebSocket<Stream> {
     /// However, it never blocks on write.
     pub fn read_message(&mut self) -> Result<Message> {
         loop {
+            if let Some(msg) = self.try_read_message()? {
+                return Ok(msg);
+            }
+        }
+    }
+
+    /// Read a message from a stream (without blocking), if possible.
+    ///
+    /// As in [`read_message`], this function sends pong and close
+    /// responses automatically in a non-blocking fashion. Unlike
+    /// [`read_message`], this allows early exit if the underlying
+    /// stream is set to nonblocking mode or has a read timeout.
+    ///
+    /// [`read_message`]: struct.WebSocket.html#read_message
+    pub fn try_read_message(&mut self) -> Result<Option<Message>> {
         // Since we may get ping or close, we need to reply to the messages even during read.
         // Thus we call write_pending() but ignore its blocking.
         self.write_pending().no_block()?;
-            // If we get here, either write blocks or we have nothing to write.
-            // Thus if read blocks, just let it return WouldBlock.
-            let res = self.read_message_frame();
-            if let Some(message) = self.translate_close(res)? {
-                trace!("Received message {}", message);
-                return Ok(message)
-            }
+
+        // Try to get a frame, convert to None if we get a WouldBlock,
+        // strip off any unnecessary wrapping. Additionally, wrap up any
+        // closing logic.
+        let frame = self.read_message_frame();
+        let out = self.translate_close(frame)
+            .no_block()?
+            .and_then(|x| x);
+
+        if log_enabled!(log::Level::Trace) && out.is_some() {
+            trace!("Received message {}", out.as_ref().unwrap());
         }
+
+        Ok(out)
     }
 
     /// Send a message to stream, if possible.