download_sysext: retry to download 10 times at max

If HTTP client fails to download, retry to download once in 500 msec,
up to 10 times.

src/download.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context, Result, bail};
+use anyhow::{anyhow, Context, Result, bail};
 use std::io::{BufReader, Read, Seek, SeekFrom, Write};
 use std::io;
 use std::fs::File;
@@ -10,6 +10,8 @@ use reqwest::StatusCode;
 
 use sha2::{Sha256, Digest};
 
+const MAX_DOWNLOAD_RETRY: u32 = 10;
+
 pub struct DownloadResult<W: std::io::Write> {
     pub hash: omaha::Hash<omaha::Sha256>,
     pub data: W,
@@ -65,11 +67,15 @@ where
 {
     let client_url = url.clone();
 
-    #[rustfmt::skip]
-    let mut res = client.get(url)
-        .send()
-        .await
-        .context(format!("client get and send({:?}) failed", client_url.as_str()))?;
+    let mut res = crate::retry_loop(
+        || {
+            let res = client.get(url.clone()).send();
+            Ok(res)
+        },
+        MAX_DOWNLOAD_RETRY,
+    )
+    .map_err(|err: anyhow::Error| anyhow!("client get & send failed with err {:?}", err))?
+    .await?;
 
     // Redirect was already handled at this point, so there is no need to touch
     // response or url again. Simply print info and continue.

src/lib.rs

@@ -3,4 +3,7 @@ pub use download::DownloadResult;
 pub use download::download_and_hash;
 pub use download::hash_on_disk_sha256;
 
+mod util;
+pub use util::retry_loop;
+
 pub mod request;

src/util.rs

@@ -0,0 +1,23 @@
+use tokio::runtime::Handle;
+use tokio::time::{sleep, Duration};
+
+pub fn retry_loop<F, T, E>(mut func: F, max_tries: u32) -> Result<T, E>
+where
+    F: FnMut() -> Result<T, E>,
+{
+    let mut tries = 0;
+
+    loop {
+        match func() {
+            ok @ Ok(_) => return ok,
+            err @ Err(_) => {
+                tries += 1;
+
+                if tries >= max_tries {
+                    return err;
+                }
+                Handle::current().block_on(sleep(Duration::from_millis(500)));
+            }
+        }
+    }
+}