downloader: Refactor retry logic and improve errors

Author: cyberphantom52

src/downloader.rs

@@ -1,13 +1,12 @@
+use crate::{error::DownloadError, Error};
 use reqwest::{Client, Url};
-use std::{path::PathBuf, sync::Arc};
+use std::{path::PathBuf, sync::Arc, time::Duration};
 use tokio::{
     fs::File,
     io::AsyncWriteExt,
     sync::{mpsc, oneshot, watch, Semaphore},
 };
 
-use crate::Error;
-
 const QUEUE_SIZE: usize = 100;
 const MAX_RETRIES: usize = 3;
 
@@ -67,7 +66,6 @@ pub enum Status {
     InProgress(DownloadProgress),
     Completed,
     Retrying,
-    Cancelled,
     Failed,
 }
 
@@ -139,7 +137,7 @@ async fn dispatcher_thread(
     mut rx: mpsc::Receiver<DownloadRequest>,
     sem: Arc<Semaphore>,
 ) {
-    while let Some(mut request) = rx.recv().await {
+    while let Some(request) = rx.recv().await {
         let permit = match sem.clone().acquire_owned().await {
             Ok(permit) => permit,
             Err(_) => break,
@@ -148,40 +146,81 @@ async fn dispatcher_thread(
         tokio::spawn(async move {
             // Move the permit into the worker thread so it's automatically released when the thread finishes
             let _permit = permit;
-            loop {
-                match download_thread(client.clone(), &mut request).await {
-                    Ok(file) => {
-                        let _ = request.status.send(Status::Completed);
-                        let _ = request.result.send(Ok(file));
-                        break;
-                    }
-                    Err(e) => {
-                        if request.remaining_retries > 0 {
-                            let _ = request.status.send(Status::Retrying);
-                            request.remaining_retries -= 1;
-                        } else {
-                            let status = match e {
-                                Error::Io(ref io_err) => {
-                                    if io_err.kind() == std::io::ErrorKind::Interrupted {
-                                        Status::Cancelled
-                                    } else {
-                                        Status::Failed
-                                    }
-                                }
-                                _ => Status::Failed,
-                            };
-                            let _ = request.status.send(status);
-                            let _ = request.result.send(Err(e));
-                            break;
-                        }
-                    }
+            download_thread(client.clone(), request).await;
+        });
+    }
+}
+
+async fn download_thread(client: Client, mut req: DownloadRequest) {
+    fn should_retry(e: &Error) -> bool {
+        match e {
+            Error::Reqwest(network_err) => {
+                network_err.is_timeout()
+                    || network_err.is_connect()
+                    || network_err.is_request()
+                    || network_err
+                        .status()
+                        .map(|status_code| status_code.is_server_error())
+                        .unwrap_or(true)
+            }
+            Error::Download(DownloadError::Cancelled) | Error::Io(_) => false,
+            _ => false,
+        }
+    }
+
+    let mut last_error = None;
+    for attempt in 0..=(MAX_RETRIES) {
+        if attempt > MAX_RETRIES {
+            req.status.send(Status::Failed).ok();
+            req.result
+                .send(Err(Error::Download(DownloadError::RetriesExhausted {
+                    last_error_msg: last_error
+                        .as_ref()
+                        .map(|e: &crate::Error| e.to_string())
+                        .unwrap_or_else(|| "Unknown Error".to_string()),
+                })))
+                .ok();
+            return;
+        }
+
+        if attempt > 0 {
+            req.status.send(Status::Retrying).ok();
+            // Basic exponential backoff
+            let delay_ms = 1000 * 2u64.pow(attempt as u32 - 1);
+            let delay = Duration::from_millis(delay_ms);
+
+            tokio::select! {
+                _ = tokio::time::sleep(delay) => {},
+                _ = &mut req.cancel => {
+                    req.status.send(Status::Failed).ok();
+                    req.result.send(Err(Error::Download(DownloadError::Cancelled))).ok();
+                    return;
                 }
             }
-        });
+        }
+    }
+
+    loop {
+        match download(client.clone(), &mut req).await {
+            Ok(file) => {
+                req.status.send(Status::Completed).ok();
+                req.result.send(Ok(file)).ok();
+                return;
+            }
+            Err(e) => {
+                if should_retry(&e) {
+                    last_error = Some(e);
+                    continue;
+                }
+                req.status.send(Status::Failed).ok();
+                req.result.send(Err(e)).ok();
+                return;
+            }
+        }
     }
 }
 
-async fn download_thread(client: Client, req: &mut DownloadRequest) -> Result<File, Error> {
+async fn download(client: Client, req: &mut DownloadRequest) -> Result<File, Error> {
     let update_progress = |bytes_downloaded: u64, total_bytes: Option<u64>| {
         req.status
             .send(Status::InProgress(DownloadProgress {
@@ -211,10 +250,7 @@ async fn download_thread(client: Client, req: &mut DownloadRequest) -> Result<Fi
             _ = &mut req.cancel => {
                 drop(file); // Manually drop the file handle to ensure that deletion doesn't fail
                 tokio::fs::remove_file(&req.destination).await?;
-                return Err(Error::Io(std::io::Error::new(
-                    std::io::ErrorKind::Interrupted,
-                    "Download cancelled",
-                )));
+                return Err(Error::Download(DownloadError::Cancelled));
             }
             chunk = response.chunk() => {
                 match chunk {
@@ -230,7 +266,6 @@ async fn download_thread(client: Client, req: &mut DownloadRequest) -> Result<Fi
                         return Err(Error::Reqwest(e))
                     },
                 }
-
             }
         }
     }

src/error.rs

@@ -10,4 +10,14 @@ pub enum Error {
     Reqwest(#[from] reqwest::Error),
     #[error("Oneshot: {0}")]
     Oneshot(#[from] tokio::sync::oneshot::error::RecvError),
+    #[error("Download: {0}")]
+    Download(#[from] DownloadError),
+}
+
+#[derive(Error, Debug, Clone)]
+pub enum DownloadError {
+    #[error("Download was cancelled")]
+    Cancelled,
+    #[error("Retry limit exceeded")]
+    RetriesExhausted { last_error_msg: String },
 }