Added an example for the "multi" api (#431)

Author: reduf

Cargo.toml

@@ -71,3 +71,8 @@ required-features = ["ssl"]
 name = "aws_sigv4"
 path = "examples/aws_sigv4.rs"
 required-features = ["static-curl", "ssl"]
+
+[[example]]
+name = "multi-dl"
+path = "examples/multi-dl.rs"
+required-features = ["ssl"]

examples/multi-dl.rs

@@ -0,0 +1,88 @@
+use std::collections::HashMap;
+use std::time::Duration;
+
+use anyhow::Result;
+
+use curl::easy::{Easy2, Handler, WriteError};
+use curl::multi::{Easy2Handle, Multi};
+
+const URLS: &[&str] = &[
+    "https://www.microsoft.com",
+    "https://www.google.com",
+    "https://www.amazon.com",
+    "https://www.apple.com",
+];
+
+struct Collector(Vec<u8>);
+impl Handler for Collector {
+    fn write(&mut self, data: &[u8]) -> Result<usize, WriteError> {
+        self.0.extend_from_slice(data);
+        Ok(data.len())
+    }
+}
+
+fn download(multi: &mut Multi, token: usize, url: &str) -> Result<Easy2Handle<Collector>> {
+    let version = curl::Version::get();
+    let mut request = Easy2::new(Collector(Vec::new()));
+    request.url(&url)?;
+    request.useragent(&format!("curl/{}", version.version()))?;
+
+    let mut handle = multi.add2(request)?;
+    handle.set_token(token)?;
+    Ok(handle)
+}
+
+fn main() -> Result<()> {
+    let mut multi = Multi::new();
+    let mut handles = URLS
+        .iter()
+        .enumerate()
+        .map(|(token, url)| Ok((token, download(&mut multi, token, url)?)))
+        .collect::<Result<HashMap<_, _>>>()?;
+
+    let mut still_alive = true;
+    while still_alive {
+        // We still need to process the last messages when
+        // `Multi::perform` returns "0".
+        if multi.perform()? == 0 {
+            still_alive = false;
+        }
+
+        multi.messages(|message| {
+            let token = message.token().expect("failed to get the token");
+            let handle = handles
+                .get_mut(&token)
+                .expect("the download value should exist in the HashMap");
+
+            match message
+                .result_for2(&handle)
+                .expect("token mismatch with the `EasyHandle`")
+            {
+                Ok(()) => {
+                    let http_status = handle
+                        .response_code()
+                        .expect("HTTP request finished without status code");
+
+                    println!(
+                        "R: Transfer succeeded (Status: {}) {} (Download length: {})",
+                        http_status,
+                        URLS[token],
+                        handle.get_ref().0.len()
+                    );
+                }
+                Err(error) => {
+                    println!("E: {} - <{}>", error, URLS[token]);
+                }
+            }
+        });
+
+        if still_alive {
+            // The sleeping time could be reduced to allow other processing.
+            // For instance, a thread could check a condition signalling the
+            // thread shutdown.
+            multi.wait(&mut [], Duration::from_secs(60))?;
+        }
+    }
+
+    Ok(())
+}