Batch command sequence writes

This allows command sequences like Prepare/Describe/Sync to be sent in
fewer TCP packets.

src/lib.rs

@@ -5,7 +5,8 @@ use extra::md5::Md5;
 use extra::url::{UserInfo, Url};
 use std::cell::Cell;
 use std::hashmap::HashMap;
-use std::rt::io::io_error;
+use std::rt::io::{io_error, Decorator};
+use std::rt::io::mem::MemWriter;
 use std::rt::io::net::ip::SocketAddr;
 use std::rt::io::net::tcp::TcpStream;
 
@@ -183,6 +184,16 @@ impl PostgresConnection {
         Ok(conn)
     }
 
+    fn write_messages(&self, messages: &[&FrontendMessage]) {
+        let mut buf = MemWriter::new();
+        for &message in messages.iter() {
+            buf.write_message(message);
+        }
+        do self.stream.with_mut_ref |s| {
+            s.write(buf.inner_ref().as_slice());
+        }
+    }
+
     fn write_message(&self, message: &FrontendMessage) {
         do self.stream.with_mut_ref |s| {
             s.write_message(message);
@@ -249,13 +260,17 @@ impl PostgresConnection {
         self.next_stmt_id.put_back(id + 1);
 
         let types = [];
-        self.write_message(&Parse {
+        self.write_messages([
+                &Parse {
                     name: stmt_name,
                     query: query,
                     param_types: types
-        });
+                },
-        self.write_message(&Describe { variant: 'S' as u8, name: stmt_name });
+                &Describe {
-        self.write_message(&Sync);
+                    variant: 'S' as u8,
+                    name: stmt_name
+                },
+                &Sync]);
 
         match_read_message!(self, {
             ParseComplete => (),
@@ -371,11 +386,12 @@ pub struct PostgresStatement<'self> {
 impl<'self> Drop for PostgresStatement<'self> {
     fn drop(&self) {
         do io_error::cond.trap(|_| {}).inside {
-            self.conn.write_message(&Close {
+            self.conn.write_messages([
+                    &Close {
                         variant: 'S' as u8,
                         name: self.name.as_slice()
-            });
+                    },
-            self.conn.write_message(&Sync);
+                    &Sync]);
             loop {
                 match_read_message!(self.conn, {
                     ReadyForQuery {_} => break,
@@ -399,18 +415,19 @@ impl<'self> PostgresStatement<'self> {
 
         let result_formats = [];
 
-        self.conn.write_message(&Bind {
+        self.conn.write_messages([
+                &Bind {
                     portal: portal_name,
                     statement: self.name.as_slice(),
                     formats: formats,
                     values: values,
                     result_formats: result_formats
-        });
+                },
-        self.conn.write_message(&Execute {
+                &Execute {
                     portal: portal_name.as_slice(),
                     max_rows: 0
-        });
+                },
-        self.conn.write_message(&Sync);
+                &Sync]);
 
         match_read_message!(self.conn, {
             BindComplete => None,
@@ -521,11 +538,12 @@ pub struct PostgresResult<'self> {
 impl<'self> Drop for PostgresResult<'self> {
     fn drop(&self) {
         do io_error::cond.trap(|_| {}).inside {
-            self.stmt.conn.write_message(&Close {
+            self.stmt.conn.write_messages([
+                    &Close {
                         variant: 'P' as u8,
                         name: self.name.as_slice()
-            });
+                    },
-            self.stmt.conn.write_message(&Sync);
+                    &Sync]);
             loop {
                 match_read_message!(self.stmt.conn, {
                     ReadyForQuery {_} => break,