feat(downloads): interleave the downloads with their installations

Even though downloads are done concurrently, the installations are done
sequentially. This means that, as downloads complete, they are in a
queue (an mpsc channel) waiting to be consumed by the future responsible
for the (sequential) installations.

Author: FranciscoTGouveia

src/dist/manifestation.rs

@@ -7,9 +7,9 @@ mod tests;
 use std::path::Path;
 
 use anyhow::{Context, Result, anyhow, bail};
-use futures_util::stream::StreamExt;
+use futures_util::{future::join, stream::StreamExt};
 use std::sync::Arc;
-use tokio::sync::Semaphore;
+use tokio::sync::{Semaphore, mpsc};
 use tracing::info;
 
 use crate::dist::component::{
@@ -153,7 +153,6 @@ impl Manifestation {
         let altered = tmp_cx.dist_server != DEFAULT_DIST_SERVER;
 
         // Download component packages and validate hashes
-        let mut things_to_install: Vec<(Component, CompressionKind, File)> = Vec::new();
         let mut things_downloaded: Vec<String> = Vec::new();
         let components = update.components_urls_and_hashes(new_manifest)?;
         let components_len = components.len();
@@ -170,49 +169,7 @@ impl Manifestation {
             .and_then(|s| s.parse().ok())
             .unwrap_or(DEFAULT_MAX_RETRIES);
 
-        info!("downloading component(s)");
-        for (component, _, url, _) in components.clone() {
-            (download_cfg.notify_handler)(Notification::DownloadingComponent(
-                &component.short_name(new_manifest),
-                &self.target_triple,
-                component.target.as_ref(),
-                &url,
-            ));
-        }
-
-        let semaphore = Arc::new(Semaphore::new(num_channels));
-        let component_stream =
-            tokio_stream::iter(components.into_iter()).map(|(component, format, url, hash)| {
-                let sem = semaphore.clone();
-                async move {
-                    let _permit = sem.acquire().await.unwrap();
-                    self.download_component(
-                        component,
-                        format,
-                        url,
-                        hash,
-                        altered,
-                        tmp_cx,
-                        download_cfg,
-                        max_retries,
-                        new_manifest,
-                    )
-                    .await
-                }
-            });
-        if num_channels > 0 {
-            let results = component_stream
-                .buffered(components_len)
-                .collect::<Vec<_>>()
-                .await;
-            for result in results {
-                let (component, format, downloaded_file, hash) = result?;
-                things_downloaded.push(hash);
-                things_to_install.push((component, format, downloaded_file));
-            }
-        }
-
-        // Begin transaction
+        // Begin transaction before the downloads, as installations are interleaved with those
         let mut tx = Transaction::new(
             prefix.clone(),
             tmp_cx,
@@ -224,6 +181,16 @@ impl Manifestation {
         // to uninstall it first.
         tx = self.maybe_handle_v2_upgrade(&config, tx, download_cfg.process)?;
 
+        info!("downloading component(s)");
+        for (component, _, url, _) in components.clone() {
+            (download_cfg.notify_handler)(Notification::DownloadingComponent(
+                &component.short_name(new_manifest),
+                &self.target_triple,
+                component.target.as_ref(),
+                &url,
+            ));
+        }
+
         // Uninstall components
         for component in &update.components_to_uninstall {
             let notification = if implicit_modify {
@@ -246,17 +213,94 @@ impl Manifestation {
             )?;
         }
 
-        // Install components
-        for (component, format, installer_file) in things_to_install {
-              tx = self.install_component(
-                component,
-                format,
-                installer_file,
-                tmp_cx,
-                download_cfg,
-                new_manifest,
-                tx,
-            )?;
+        if num_channels > 0 {
+            // Create a channel to communicate whenever a download is done and the component can be installed
+            // The `mpsc` channel was used as we need to send many messages from one producer (download's thread) to one consumer (install's thread)
+            // This is recommended in the official docs: https://docs.rs/tokio/latest/tokio/sync/index.html#mpsc-channel
+            let total_components = components.len();
+            let (download_tx, mut download_rx) =
+                mpsc::channel::<Result<(Component, CompressionKind, File)>>(total_components);
+
+            let semaphore = Arc::new(Semaphore::new(num_channels));
+            let component_stream =
+                tokio_stream::iter(components.into_iter()).map(|(component, format, url, hash)| {
+                    let sem = semaphore.clone();
+                    let download_tx_cloned = download_tx.clone();
+                    async move {
+                        let _permit = sem.acquire().await.unwrap();
+                        self.download_component(
+                            component,
+                            format,
+                            url,
+                            hash,
+                            altered,
+                            tmp_cx,
+                            download_cfg,
+                            max_retries,
+                            new_manifest,
+                            download_tx_cloned,
+                        )
+                        .await
+                    }
+                });
+
+            let mut stream = component_stream.buffered(num_channels);
+            let (download_results, install_result) = join(
+                async {
+                    let mut hashes = Vec::new();
+                    while let Some(result) = stream.next().await {
+                        match result {
+                            Ok(hash) => {
+                                hashes.push(hash);
+                            }
+                            Err(e) => {
+                                let _ = download_tx.send(Err(e)).await;
+                            }
+                        }
+                    }
+                    hashes
+                },
+                async {
+                    let mut current_tx = tx;
+                    let mut counter = 0;
+                    loop {
+                        if counter >= total_components {
+                            break;
+                        }
+                        if let Some(message) = download_rx.recv().await {
+                            match message {
+                                Ok((component, format, installer_file)) => {
+                                    match self.install_component(
+                                        component,
+                                        format,
+                                        installer_file,
+                                        tmp_cx,
+                                        download_cfg,
+                                        new_manifest,
+                                        current_tx,
+                                    ) {
+                                        Ok(new_tx) => {
+                                            current_tx = new_tx;
+                                        }
+                                        Err(e) => {
+                                            return Err(e);
+                                        }
+                                    }
+                                }
+                                Err(e) => {
+                                    return Err(e);
+                                }
+                            }
+                            counter += 1;
+                        }
+                    }
+                    Ok(current_tx)
+                },
+            )
+            .await;
+
+            things_downloaded = download_results;
+            tx = install_result?;
         }
 
         // Install new distribution manifest
@@ -508,7 +552,8 @@ impl Manifestation {
         download_cfg: &DownloadCfg<'_>,
         max_retries: usize,
         new_manifest: &Manifest,
-    ) -> Result<(Component, CompressionKind, File, String)> {
+        notification_tx: mpsc::Sender<Result<(Component, CompressionKind, File)>>,
+    ) -> Result<String> {
         use tokio_retry::{RetryIf, strategy::FixedInterval};
 
         let url = if altered {
@@ -537,9 +582,13 @@ impl Manifestation {
         .await
         .with_context(|| RustupError::ComponentDownloadFailed(component.name(new_manifest)))?;
 
-        Ok((component, format, downloaded_file, hash))
+        let _ = notification_tx
+            .send(Ok((component.clone(), format, downloaded_file)))
+            .await;
+        Ok(hash)
     }
 
+    #[allow(clippy::too_many_arguments)]
     fn install_component<'a>(
         &self,
         component: Component,