Buffer IMAP client writes

async-imap does not do its own buffering, but calls flush() after
sending each command. Using BufWriter reduces the number of write()
system calls used to send a single command.

Note that BufWriter is set up on top of TLS streams, because
we can't guarantee that TLS libraries flush the stream before
waiting for response.

src/net.rs

@@ -0,0 +1,26 @@
+///! # Common network utilities.
+use std::pin::Pin;
+use std::time::Duration;
+
+use anyhow::{Context as _, Result};
+use tokio::net::{TcpStream, ToSocketAddrs};
+use tokio::time::timeout;
+use tokio_io_timeout::TimeoutStream;
+
+/// Returns a TCP connection with read/write timeouts set.
+pub(crate) async fn connect_tcp(
+    addr: impl ToSocketAddrs,
+    timeout_val: Duration,
+) -> Result<Pin<Box<TimeoutStream<TcpStream>>>> {
+    let tcp_stream = timeout(timeout_val, TcpStream::connect(addr))
+        .await
+        .context("connection timeout")?
+        .context("connection failure")?;
+
+    let mut timeout_stream = TimeoutStream::new(tcp_stream);
+    timeout_stream.set_write_timeout(Some(timeout_val));
+    timeout_stream.set_read_timeout(Some(timeout_val));
+    let pinned_stream = Box::pin(timeout_stream);
+
+    Ok(pinned_stream)
+}