perf(server): cache renderings of the Date header

This is actually one of the biggest impacts to benchmark performances at
this point. Caching the rendering of the Date header improves "hello
world" benchmarks by around 10%.

src/http/h1/parse.rs

@@ -1,13 +1,12 @@
 use std::borrow::Cow;
 use std::fmt::{self, Write};
-use std::time::SystemTime;
 
 use httparse;
 use bytes::{BytesMut, Bytes};
 
 use header::{self, Headers, ContentLength, TransferEncoding};
 use http::{ByteStr, MessageHead, RawStatus, Http1Transaction, ParseResult, ServerTransaction, ClientTransaction, RequestLine};
-use http::h1::{Encoder, Decoder};
+use http::h1::{Encoder, Decoder, date};
 use method::Method;
 use status::StatusCode;
 use version::HttpVersion::{Http10, Http11};
@@ -90,10 +89,6 @@ impl Http1Transaction for ServerTransaction {
         use ::header;
         trace!("writing head: {:?}", head);
 
-        if !head.headers.has::<header::Date>() {
-            head.headers.set(header::Date(SystemTime::now().into()));
-        }
-
         let len = head.headers.get::<header::ContentLength>().map(|n| **n);
 
         let body = if let Some(len) = len {
@@ -121,10 +116,19 @@ impl Http1Transaction for ServerTransaction {
         debug!("writing headers = {:?}", head.headers);
         if head.version == ::HttpVersion::Http11 && head.subject == ::StatusCode::Ok {
             extend(dst, b"HTTP/1.1 200 OK\r\n");
-            let _ = write!(FastWrite(dst), "{}\r\n", head.headers);
+            let _ = write!(FastWrite(dst), "{}", head.headers);
         } else {
-            let _ = write!(FastWrite(dst), "{} {}\r\n{}\r\n", head.version, head.subject, head.headers);
+            let _ = write!(FastWrite(dst), "{} {}\r\n{}", head.version, head.subject, head.headers);
         }
+        // using http::h1::date is quite a lot faster than generating a unique Date header each time
+        // like req/s goes up about 10%
+        if !head.headers.has::<header::Date>() {
+            dst.reserve(date::DATE_VALUE_LENGTH + 8);
+            extend(dst, b"Date: ");
+            date::extend(dst);
+            extend(dst, b"\r\n");
+        }
+        extend(dst, b"\r\n");
         body
     }
 
@@ -316,16 +320,9 @@ impl<'a> fmt::Write for FastWrite<'a> {
     }
 }
 
+#[inline]
 fn extend(dst: &mut Vec<u8>, data: &[u8]) {
-    use std::ptr;
-    dst.reserve(data.len());
-    let prev = dst.len();
-    unsafe {
-        ptr::copy_nonoverlapping(data.as_ptr(),
-                                 dst.as_mut_ptr().offset(prev as isize),
-                                 data.len());
-        dst.set_len(prev + data.len());
-    }
+    dst.extend_from_slice(data);
 }
 
 #[cfg(test)]