Update download.rs to match non-CNB operation

libherokubuildpack/src/download.rs

@@ -1,16 +1,23 @@
-use std::{fs, io};
+/// Unfortunately the rust-ification of buildpacks introduces a wealth of
+/// incompatibilities and unexpected behavior. In original buildpacks a web
+/// download [was a curl command with reasonable options](https://github.com/heroku/heroku-buildpack-nodejs/blob/76b36fcf5daa1b1aed17b3616a57f99d2ef05a29/lib/binaries.sh#L98C1-L99C1):
+/// curl "$url" -L --silent --fail --retry 5 --retry-max-time 15 --retry-connrefused --connect-timeout 5 -o target
+/// Unfortunately, the new rust-implementation dropped the retry, max-time and whatnot
+/// and therefore things [like NodeJS downloads fail to install](https://github.com/heroku/buildpacks-nodejs/issues/868). 
+/// But on the other hand, Rust provides memory safety and cross-platform compile targets.
+/// This is a copilot suggestion to bring parity.
+use std::{fs, io, thread, time::Duration};
 
 #[derive(thiserror::Error, Debug)]
 pub enum DownloadError {
-    // Boxed to prevent `large_enum_variant` errors since `ureq::Error` is massive.
     #[error("HTTP error while downloading file: {0}")]
     HttpError(#[from] Box<ureq::Error>),
 
     #[error("I/O error while downloading file: {0}")]
     IoError(#[from] std::io::Error),
 }
 
-/// Downloads a file via HTTP(S) to a local path
+/// Downloads a file via HTTP(S) to a local path with retry logic
 ///
 /// # Examples
 /// ```
@@ -31,7 +38,34 @@ pub fn download_file(
     uri: impl AsRef<str>,
     destination: impl AsRef<std::path::Path>,
 ) -> Result<(), DownloadError> {
-    let response = ureq::get(uri.as_ref()).call().map_err(Box::new)?;
+    let max_retries = 5;
+    let retry_delay = Duration::from_secs(2);
+    let mut attempts = 0;
+
+    while attempts < max_retries {
+        attempts += 1;
+        match attempt_download(&uri, &destination) {
+            Ok(_) => return Ok(()),
+            Err(e) if attempts < max_retries => {
+                eprintln!("Attempt {}/{} failed: {}. Retrying in {:?}...", attempts, max_retries, e, retry_delay);
+                thread::sleep(retry_delay);
+            }
+            Err(e) => return Err(e),
+        }
+    }
+
+    Err(DownloadError::HttpError(Box::new(ureq::Error::new("Max retries reached"))))
+}
+
+fn attempt_download(
+    uri: &impl AsRef<str>,
+    destination: &impl AsRef<std::path::Path>,
+) -> Result<(), DownloadError> {
+    let response = ureq::get(uri.as_ref())
+        .timeout_connect(5_000) // 5 seconds timeout
+        .timeout_read(15_000) // 15 seconds max time
+        .call()
+        .map_err(Box::new)?;
     let mut reader = response.into_reader();
     let mut file = fs::File::create(destination.as_ref())?;
     io::copy(&mut reader, &mut file)?;