perf: make BufList::remaining() O(1) by caching total bytes

Add a  field to BufList that tracks the total number
of bytes across all buffers. This avoids iterating the entire
VecDeque on every call to remaining(), which is invoked from
hot paths like poll_flush, can_buffer, and advance.

Previously, remaining() was O(n) where n is the number of
buffers in the queue (up to 16 in Queue write strategy).
Now it is O(1) — a simple field read.

Author: DioCrafts

src/common/buf.rs

@@ -5,18 +5,21 @@ use bytes::{Buf, BufMut, Bytes, BytesMut};
 
 pub(crate) struct BufList<T> {
     bufs: VecDeque<T>,
+    remaining: usize,
 }
 
 impl<T: Buf> BufList<T> {
     pub(crate) fn new() -> BufList<T> {
         BufList {
             bufs: VecDeque::new(),
+            remaining: 0,
         }
     }
 
     #[inline]
     pub(crate) fn push(&mut self, buf: T) {
         debug_assert!(buf.has_remaining());
+        self.remaining += buf.remaining();
         self.bufs.push_back(buf);
     }
 
@@ -29,7 +32,7 @@ impl<T: Buf> BufList<T> {
 impl<T: Buf> Buf for BufList<T> {
     #[inline]
     fn remaining(&self) -> usize {
-        self.bufs.iter().map(|buf| buf.remaining()).sum()
+        self.remaining
     }
 
     #[inline]
@@ -39,6 +42,7 @@ impl<T: Buf> Buf for BufList<T> {
 
     #[inline]
     fn advance(&mut self, mut cnt: usize) {
+        self.remaining -= cnt;
         while cnt > 0 {
             {
                 let front = &mut self.bufs[0];
@@ -78,12 +82,18 @@ impl<T: Buf> Buf for BufList<T> {
             Some(front) if front.remaining() == len => {
                 let b = front.copy_to_bytes(len);
                 self.bufs.pop_front();
+                self.remaining -= len;
                 b
             }
-            Some(front) if front.remaining() > len => front.copy_to_bytes(len),
+            Some(front) if front.remaining() > len => {
+                self.remaining -= len;
+                front.copy_to_bytes(len)
+            }
             _ => {
-                assert!(len <= self.remaining(), "`len` greater than remaining");
+                assert!(len <= self.remaining, "`len` greater than remaining");
                 let mut bm = BytesMut::with_capacity(len);
+                // Note: `self.take(len)` calls `self.advance()` internally,
+                // which already decrements `self.remaining`.
                 bm.put(self.take(len));
                 bm.freeze()
             }
@@ -100,6 +110,7 @@ mod tests {
     fn hello_world_buf() -> BufList<Bytes> {
         BufList {
             bufs: vec![Bytes::from("Hello"), Bytes::from(" "), Bytes::from("World")].into(),
+            remaining: 11,
         }
     }