[web] use actual vectored write

Since IoSlice::advance isn't stable I had to hack my own. I will need to
confirm that my implementation is safe.

Signed-off-by: Ben Aaron Goldberg <ben@benaaron.dev>

web/src/main.rs

@@ -2,6 +2,7 @@ pub mod ogp_images;
 pub mod contrast;
 pub mod statics;
 pub mod configuration;
+mod write_vectored_all;
 
 use crate::configuration::Conf;
 use std::io::IoSlice;
@@ -16,6 +17,7 @@ use percent_encoding::{percent_decode_str, percent_encode, NON_ALPHANUMERIC};
 use pronouns_today::user_preferences::ParseError;
 use pronouns_today::UserPreferences;
 use configuration::ConfigError;
+use write_vectored_all::AsyncWriteAllVectored;
 
 use std::net::SocketAddr;
 use std::process::exit;
@@ -135,16 +137,11 @@ async fn handle_request(
 
     let response = route_request(&req)
         .generate_response(conf);
-    let io_slices = response.into_io_slices();
+    let mut io_slices = response.into_io_slices();
 
-    for slice in io_slices {
-        if let Err(e) = stream.write_all(&slice).await {
-            log::warn!(
-                "Encountered an IO error while sending response: {}", e
-            );
-            break;
-        }
-    }
+	if let Err(e) = stream.write_all_vectored(&mut io_slices).await {
+		log::warn!("Encountered an IO error while sending response: {}", e);
+	}
 	if let Err(e) = stream.close().await {
 		log::warn!(
 			"Encountered an IO error while closing connection to server: {}", e

web/src/write_vectored_all.rs

@@ -0,0 +1,89 @@
+use std::slice;
+use std::future::Future;
+use std::io::{Error, ErrorKind, IoSlice, Result};
+use std::mem::replace;
+use std::pin::Pin;
+use std::task::Poll;
+
+use futures_lite::AsyncWrite;
+use futures_lite::ready;
+
+pub trait AsyncWriteAllVectored: AsyncWrite {
+    fn write_all_vectored<'a>(&'a mut self, bufs: &'a mut [IoSlice<'a>]) -> WriteAllVectoredFuture<'a, Self>
+    where
+        Self: Unpin,
+    {
+        WriteAllVectoredFuture { writer: self, bufs }
+    }
+}
+
+impl <T: AsyncWrite> AsyncWriteAllVectored for T {}
+
+pub struct WriteAllVectoredFuture<'a, W: Unpin + ?Sized> {
+    writer: &'a mut W,
+    bufs: &'a mut [IoSlice<'a>],
+}
+
+impl<W: Unpin + ?Sized> Unpin for WriteAllVectoredFuture<'_, W> {}
+
+impl<W: AsyncWrite + Unpin + ?Sized> Future for WriteAllVectoredFuture<'_, W> {
+    type Output = Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
+        let Self { writer, bufs } = &mut *self;
+
+		// Guarantee that bufs is empty if it contains no data,
+        // to avoid calling write_vectored if there is no data to be written.
+		advance_slices(bufs, 0);
+		while !bufs.is_empty() {
+            match ready!(Pin::new(&mut ** writer).poll_write_vectored(cx, bufs)) {
+                Ok(0) => {
+                    return Poll::Ready(Err(Error::new(
+                        ErrorKind::WriteZero,
+                        "failed to write whole buffer",
+                    )));
+                }
+                Ok(n) => advance_slices(bufs, n),
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+                Err(e) => return Poll::Ready(Err(e)),
+            }
+        }
+
+        Poll::Ready(Ok(()))
+    }
+}
+
+fn advance_slices(bufs: &mut &mut [IoSlice<'_>], n: usize) {
+    // Number of buffers to remove.
+    let mut remove = 0;
+    // Total length of all the to be removed buffers.
+    let mut accumulated_len = 0;
+    for buf in bufs.iter() {
+        if accumulated_len + buf.len() > n {
+            break;
+        } else {
+            accumulated_len += buf.len();
+            remove += 1;
+        }
+    }
+
+    *bufs = &mut replace(bufs, &mut [])[remove..];
+    if !bufs.is_empty() {
+        advance(&mut bufs[0], n - accumulated_len);
+    }
+}
+
+fn advance<'a>(buf: &mut IoSlice<'a>, n: usize) {
+	if buf.len() < n {
+	    panic!("advancing IoSlice beyond its length");
+	}
+	// SAFTEY: hopefully
+	unsafe {
+		let mut ptr = buf.as_ptr() as *mut u8;
+		ptr = ptr.add(n);
+		let len = buf.len() - n;
+		let new_slice: &'a [u8] = slice::from_raw_parts(ptr, len);
+		*buf = IoSlice::new(new_slice);
+	}
+}
+