Body: don't call poll_ready on tx when 0 bytes remaining. (#479)

Some web servers (if IIS is a web server) may close their request
stream early when they consider the input is complete. That leads to
poll_ready returning an error of kind "Closed" which is legitimate as
the receiver disappeared. So this change ignores the case when the body
has been fully transmitted.

src/body.rs

@@ -237,27 +237,32 @@ impl Sender {
                 return Ok(().into());
             }
 
-            try_ready!(tx
-                .as_mut()
-                .expect("tx only taken on error")
-                .poll_ready()
-                .map_err(::error::from));
-
+            // The input stream is read only if the buffer is empty so
+            // that there is only one read in the buffer at any time.
+            //
+            // We need to know whether there is any data to send before
+            // we check the transmission channel (with poll_ready below)
+            // because somestimes the receiver disappears as soon as is
+            // considers the data is completely transmitted, which may
+            // be true.
+            //
+            // The use case is a web server that closes its
+            // input stream as soon as the data received is valid JSON.
+            // This behaviour is questionable, but it exists and the
+            // fact is that there is actually no remaining data to read.
+            if buf.len() == 0 {
                 if buf.remaining_mut() == 0 {
                     buf.reserve(8192);
                 }
 
                 match body.read(unsafe { buf.bytes_mut() }) {
                     Ok(0) => {
-                    return Ok(().into())
-                },
+                        // The buffer was empty and nothing's left to
+                        // read. Return.
+                        return Ok(().into());
+                    }
                     Ok(n) => {
                         unsafe { buf.advance_mut(n); }
-                    written += n as u64;
-                    let tx = tx.as_mut().expect("tx only taken on error");
-                    if let Err(_) = tx.send_data(buf.take().freeze().into()) {
-                        return Err(::error::timedout(None));
-                    }
                     }
                     Err(e) => {
                         let ret = io::Error::new(e.kind(), e.to_string());
@@ -268,6 +273,21 @@ impl Sender {
                         return Err(::error::from(ret));
                     }
                 }
+            }
+
+            // The only way to get here is when the buffer is not empty.
+            // We can check the transmission channel
+            try_ready!(tx
+                .as_mut()
+                .expect("tx only taken on error")
+                .poll_ready()
+                .map_err(::error::from));
+
+            written += buf.len() as u64;
+            let tx = tx.as_mut().expect("tx only taken on error");
+            if let Err(_) = tx.send_data(buf.take().freeze().into()) {
+                return Err(::error::timedout(None));
+            }
         })
     }
 }