perf(http1): improve parsing of sequentially partial messages

If request headers are received in incremental partial chunks, hyper
would restart parsing each time. This is because the HTTP/1 parser is
stateless, since the most common case is a full message and stateless
parses faster.

However, if continuing to receive more partial chunks of the request,
each subsequent full parse is slower and slower. Since partial parses is
less common, we can store a little bit of state to improve performance
in general.

Now, if a partial request is received, hyper will check for the end of
the message quickly, and if not found, simply save the length to allow
the next partial chunk to start its search from there. Only once the end
is found will a fill parse happen.

Reported-by: Datong Sun <[email protected]>

src/proto/h1/io.rs

@@ -32,6 +32,7 @@ const MAX_BUF_LIST_BUFFERS: usize = 16;
 pub(crate) struct Buffered<T, B> {
     flush_pipeline: bool,
     io: T,
+    partial_len: Option<usize>,
     read_blocked: bool,
     read_buf: BytesMut,
     read_buf_strategy: ReadStrategy,
@@ -65,6 +66,7 @@ where
         Buffered {
             flush_pipeline: false,
             io,
+            partial_len: None,
             read_blocked: false,
             read_buf: BytesMut::with_capacity(0),
             read_buf_strategy: ReadStrategy::default(),
@@ -176,6 +178,7 @@ where
         loop {
             match super::role::parse_headers::<S>(
                 &mut self.read_buf,
+                self.partial_len,
                 ParseContext {
                     cached_headers: parse_ctx.cached_headers,
                     req_method: parse_ctx.req_method,
@@ -191,14 +194,17 @@ where
             )? {
                 Some(msg) => {
                     debug!("parsed {} headers", msg.head.headers.len());
+                    self.partial_len = None;
                     return Poll::Ready(Ok(msg));
                 }
                 None => {
                     let max = self.read_buf_strategy.max();
-                    if self.read_buf.len() >= max {
+                    let curr_len = self.read_buf.len();
+                    if curr_len >= max {
                         debug!("max_buf_size ({}) reached, closing", max);
                         return Poll::Ready(Err(crate::Error::new_too_large()));
                     }
+                    self.partial_len = Some(curr_len);
                 }
             }
             if ready!(self.poll_read_from_io(cx)).map_err(crate::Error::new_io)? == 0 {

src/proto/h1/role.rs

@@ -66,6 +66,7 @@ macro_rules! maybe_panic {
 
 pub(super) fn parse_headers<T>(
     bytes: &mut BytesMut,
+    prev_len: Option<usize>,
     ctx: ParseContext<'_>,
 ) -> ParseResult<T::Incoming>
 where
@@ -78,9 +79,37 @@ where
 
     let _entered = trace_span!("parse_headers");
 
+    if let Some(prev_len) = prev_len {
+        if !is_complete_fast(bytes, prev_len) {
+            return Ok(None);
+        }
+    }
+
     T::parse(bytes, ctx)
 }
 
+/// A fast scan for the end of a message.
+/// Used when there was a partial read, to skip full parsing on a
+/// a slow connection.
+fn is_complete_fast(bytes: &[u8], prev_len: usize) -> bool {
+    let start = if prev_len < 3 { 0 } else { prev_len - 3 };
+    let bytes = &bytes[start..];
+
+    for (i, b) in bytes.iter().copied().enumerate() {
+        if b == b'\r' {
+            if bytes[i + 1..].chunks(3).next() == Some(&b"\n\r\n"[..]) {
+                return true;
+            }
+        } else if b == b'\n' {
+            if bytes.get(i + 1) == Some(&b'\n') {
+                return true;
+            }
+        }
+    }
+
+    false
+}
+
 pub(super) fn encode_headers<T>(
     enc: Encode<'_, T::Outgoing>,
     dst: &mut Vec<u8>,
@@ -2827,6 +2856,28 @@ mod tests {
         parse(Some(200), 210, false);
     }
 
+    #[test]
+    fn test_is_complete_fast() {
+        let s = b"GET / HTTP/1.1\r\na: b\r\n\r\n";
+        for n in 0..s.len() {
+            assert!(is_complete_fast(s, n), "{:?}; {}", s, n);
+        }
+        let s = b"GET / HTTP/1.1\na: b\n\n";
+        for n in 0..s.len() {
+            assert!(is_complete_fast(s, n));
+        }
+
+        // Not
+        let s = b"GET / HTTP/1.1\r\na: b\r\n\r";
+        for n in 0..s.len() {
+            assert!(!is_complete_fast(s, n));
+        }
+        let s = b"GET / HTTP/1.1\na: b\n";
+        for n in 0..s.len() {
+            assert!(!is_complete_fast(s, n));
+        }
+    }
+
     #[test]
     fn test_write_headers_orig_case_empty_value() {
         let mut headers = HeaderMap::new();