Statically track the status of a Response by using a Phantom Type

Introduces two Phantom Types, Fresh and Streaming, which indicate the status
of a Response.

Response::start translates an Response<Fresh> into a
Response<Streaming> by writing the StatusCode and Headers.

Response<Fresh> allows modification of Headers and StatusCode, but does
not allow writing to the body. Response<Streaming> has the opposite privileges.

examples/server.rs

@@ -36,8 +36,13 @@ impl Handler for Echo {
                     },
                     (&Post, "/echo") => (), // fall through, fighting mutable borrows
                     _ => {
+<<<<<<< Updated upstream
                         res.status = hyper::status::NotFound;
                         try_continue!(res.end());
+=======
+                        *res.status_mut() = hyper::status::NotFound;
+                        try_continue!(res.start().and_then(|res| res.end()));
+>>>>>>> Stashed changes
                         continue;
                     }
                 },

src/server/mod.rs

@@ -4,7 +4,7 @@ use std::io::{Acceptor, Listener, IoResult, EndOfFile, IncomingConnections};
 use std::io::net::ip::{IpAddr, Port, SocketAddr};
 
 pub use self::request::Request;
-pub use self::response::Response;
+pub use self::response::{Response, Fresh, Streaming};
 
 pub mod request;
 pub mod response;
@@ -55,8 +55,8 @@ pub struct Incoming<'a> {
     from: IncomingConnections<'a, TcpAcceptor>
 }
 
-impl<'a> Iterator<(Request, Response)> for Incoming<'a> {
-    fn next(&mut self) -> Option<(Request, Response)> {
+impl<'a> Iterator<(Request, Response<Fresh>)> for Incoming<'a> {
+    fn next(&mut self) -> Option<(Request, Response<Fresh>)> {
         for conn in self.from {
             match conn {
                 Ok(stream) => {

src/server/response.rs

@@ -12,39 +12,52 @@ use status;
 use version;
 use rfc7230::{CR, LF, LINE_ENDING};
 
+/// Phantom type indicating Headers and StatusCode have not been written.
+pub struct Fresh;
+
+/// Phantom type indicating Headers and StatusCode have been written.
+pub struct Streaming;
+
+/// The status of a Response, indicating if the headers and status have been written.
+pub trait WriteStatus {}
+
+impl WriteStatus for Streaming {}
+impl WriteStatus for Fresh {}
 
 /// The outgoing half for a Tcp connection, created by a `Server` and given to a `Handler`.
-pub struct Response {
-    /// The status code for the request.
-    pub status: status::StatusCode,
-    /// The outgoing headers on this response.
-    pub headers: header::Headers,
+pub struct Response<W: WriteStatus> {
     /// The HTTP version of this response.
     pub version: version::HttpVersion,
-
-    headers_written: bool, // TODO: can this check be moved to compile time?
+    // Stream the Response is writing to, not accessible through UnwrittenResponse
     body: BufferedWriter<TcpStream>, // TODO: use a HttpWriter from rfc7230
+    // The status code for the request.
+    status: status::StatusCode,
+    // The outgoing headers on this response.
+    headers: header::Headers
 }
 
-impl Response {
+impl<W: WriteStatus> Response<W> {
+    /// The status of this response.
+    #[inline]
+    pub fn status(&self) -> status::StatusCode { self.status }
 
+    /// The headers of this response.
+    pub fn headers(&self) -> &header::Headers { &self.headers }
+}
+
+impl Response<Fresh> {
     /// Creates a new Response that can be used to write to a network stream.
-    pub fn new(tcp: TcpStream) -> Response {
+    pub fn new(tcp: TcpStream) -> Response<Fresh> {
         Response {
             status: status::Ok,
             version: version::Http11,
             headers: header::Headers::new(),
-            headers_written: false,
             body: BufferedWriter::new(tcp)
         }
     }
 
-    fn write_head(&mut self) -> IoResult<()> {
-        if self.headers_written {
-            debug!("headers previously written, nooping");
-            return Ok(());
-        }
-        self.headers_written = true;
+    /// Consume this Response<Fresh>, writing the Headers and Status and creating a Response<Streaming>
+    pub fn start(mut self) -> IoResult<Response<Streaming>> {
         debug!("writing head: {} {}", self.version, self.status);
         try!(write!(self.body, "{} {}{}{}", self.version, self.status, CR as char, LF as char));
 
@@ -58,9 +71,26 @@ impl Response {
             try!(self.body.write(LINE_ENDING));
         }
 
-        self.body.write(LINE_ENDING)
+        try!(self.body.write(LINE_ENDING));
+
+        // "copy" to change the phantom type
+        Ok(Response {
+            version: self.version,
+            body: self.body,
+            status: self.status,
+            headers: self.headers
+        })
     }
 
+    /// Get a mutable reference to the status.
+    #[inline]
+    pub fn status_mut(&mut self) -> &mut status::StatusCode { &mut self.status }
+
+    /// Get a mutable reference to the Headers.
+    pub fn headers_mut(&mut self) -> &mut header::Headers { &mut self.headers }
+}
+
+impl Response<Streaming> {
     /// Flushes all writing of a response to the client.
     pub fn end(mut self) -> IoResult<()> {
         debug!("ending");
@@ -68,20 +98,14 @@ impl Response {
     }
 }
 
-
-impl Writer for Response {
+impl Writer for Response<Streaming> {
     fn write(&mut self, msg: &[u8]) -> IoResult<()> {
-        if !self.headers_written {
-            try!(self.write_head());
-        }
         debug!("write {:u} bytes", msg.len());
         self.body.write(msg)
     }
 
     fn flush(&mut self) -> IoResult<()> {
-        if !self.headers_written {
-            try!(self.write_head());
-        }
         self.body.flush()
     }
 }
+