Add timeout example

examples/README.md

@@ -7,3 +7,4 @@ Examples:
   * basic - This is a very basic example of using the client.
   * gmail_oauth2 - This is an example using oauth2 for logging into gmail as a secure appplication.
   * rustls - This demonstrates how to use Rustls instead of Openssl for secure connections (helpful for cross compilation).
+  * timeout - This demonstrates how to use timeouts while connecting to an IMAP server.

examples/timeout.rs

@@ -0,0 +1,92 @@
+extern crate imap;
+extern crate native_tls;
+
+use imap::Client;
+use native_tls::TlsConnector;
+use native_tls::TlsStream;
+use std::env;
+use std::error::Error;
+use std::fmt;
+use std::net::{SocketAddr, TcpStream, ToSocketAddrs};
+use std::time::Duration;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let server = env::var("IMAP_SERVER")?;
+    let port = env::var("IMAP_PORT").unwrap_or_else(|_| String::from("993"));
+    let port = port.parse()?;
+
+    let username = env::var("IMAP_USER")?;
+    let password = env::var("IMAP_PASSWORD")?;
+
+    let timeout = env::var("IMAP_TIMEOUT").unwrap_or_else(|_| String::from("1"));
+    let timeout = timeout.parse()?;
+    let timeout = Duration::from_secs(timeout);
+
+    let tls = TlsConnector::builder().build()?;
+
+    let client = connect_all_timeout((server.as_str(), port), server.as_str(), &tls, timeout)?;
+
+    let mut session = client.login(&username, &password).map_err(|e| e.0)?;
+
+    // do something productive with session
+
+    session.logout()?;
+
+    Ok(())
+}
+
+// connect to an IMAP host with a `Duration` timeout; note that this accepts only a single
+// `SocketAddr` while `connect_all_timeout` does resolve the DNS entry and try to connect to all;
+// this is necessary due to the difference of the function signatures of `TcpStream::connect` and
+// `TcpStream::connect_timeout`
+fn connect_timeout<S: AsRef<str>>(
+    addr: &SocketAddr,
+    domain: S,
+    ssl_connector: &TlsConnector,
+    timeout: Duration,
+) -> Result<Client<TlsStream<TcpStream>>, Box<dyn Error>> {
+    // the timeout is actually used with the initial TcpStream
+    let tcp_stream = TcpStream::connect_timeout(addr, timeout)?;
+
+    let tls_stream = TlsConnector::connect(ssl_connector, domain.as_ref(), tcp_stream)?;
+
+    let mut client = Client::new(tls_stream);
+
+    // don't forget to wait for the IMAP protocol server greeting ;)
+    client.read_greeting()?;
+
+    Ok(client)
+}
+
+// resolve address and try to connect to all in order; note that this function is required to fully
+// mimic `imap::connect` with the usage of `ToSocketAddrs`
+fn connect_all_timeout<A: ToSocketAddrs, S: AsRef<str>>(
+    addr: A,
+    domain: S,
+    ssl_connector: &TlsConnector,
+    timeout: Duration,
+) -> Result<Client<TlsStream<TcpStream>>, Box<dyn Error>> {
+    let addrs = addr.to_socket_addrs()?;
+
+    for addr in addrs {
+        match connect_timeout(&addr, &domain, ssl_connector, timeout) {
+            Ok(client) => return Ok(client),
+            Err(error) => eprintln!("couldn't connect to {}: {}", addr, error),
+        }
+    }
+
+    Err(Box::new(TimeoutError))
+}
+
+// very simple timeout error; instead of printing the errors immediately like in
+// `connect_all_timeout`, you may want to collect and return them
+#[derive(Debug)]
+struct TimeoutError;
+
+impl fmt::Display for TimeoutError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "all addresses failed to connect")
+    }
+}
+
+impl Error for TimeoutError {}