feat(*): init Rust port

This is a Rust port of the original Perl script, legacy cruft is removed
and it focuses on a modern Hydra deployment.

Nonetheless, it knows how to perform migrations based on the channel
versions.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

.gitignore

@@ -1 +1,6 @@
 result
+
+
+# Added by cargo
+
+/target

Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "nixos-channel-scripts"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+clap = { version = "4.5.13", features = [ "derive" ] }
+indicatif = { version = "0.17.8", features = ["futures", "tokio"] }
+log = "0.4.22"
+object_store = { version = "0.10.2", features = [ "aws" ] }
+pretty_env_logger = "0.5.0"
+regex = "1.10.6"
+reqwest = { version = "0.12.5", features = ["brotli"] }
+serde = "1.0.204"
+tempdir = "0.3.7"
+textwrap = "0.16.1"
+tokio = { version = "1.39.2", features = ["full"] }
+toml = "0.8.19"
+nix-compat = { git = "https://github.com/tvlfyi/tvix" }
+futures = "0.3.30"
+anyhow = "1.0.86"
+bytes = "1.7.1"
+async-compression = { version = "0.4.12", features = ["tokio", "zstd"] }
+tokio-util = { version = "0.7.11", features = ["io"] }

default.nix

@@ -1,3 +1,11 @@
-(import (fetchTarball https://github.com/edolstra/flake-compat/archive/master.tar.gz) {
+{ pkgs ? import <nixpkgs> {} }:
-  src = builtins.fetchGit ./.;
+{
-}).defaultNix
+  shell = pkgs.mkShell {
+    buildInputs = [
+      pkgs.cargo
+      pkgs.rustc
+      pkgs.openssl
+      pkgs.pkg-config
+    ];
+  };
+}

forkos.toml

@@ -0,0 +1,5 @@
+hydra_uri = "https://hydra.forkos.org"
+binary_cache_uri = "https://cache.forkos.org"
+nixpkgs_dir = "/var/lib/nixpkgs"
+s3_release_bucket_uri = "s3://cache.forkos.org/release"
+s3_channel_bucket_uri = "s3://cache.forkos.org/channel"

shell.nix

@@ -1,3 +0,0 @@
-(import (fetchTarball https://github.com/edolstra/flake-compat/archive/master.tar.gz) {
-  src = builtins.fetchGit ./.;
-}).shellNix

src/actions.rs

@@ -0,0 +1,257 @@
+use std::path::PathBuf;
+use anyhow::Context;
+use async_compression::tokio::write::ZstdEncoder;
+use futures::future::join_all;
+use indicatif::MultiProgress;
+use indicatif::ProgressBar;
+use indicatif::ProgressDrawTarget;
+use log::debug;
+use tempdir::TempDir;
+use tokio::fs::File;
+use crate::hydra;
+use crate::config;
+
+#[derive(Debug, Copy, Clone)]
+pub enum HydraProductType {
+    BrotliJson,
+    SourceDistribution,
+}
+
+#[derive(Debug)]
+pub enum Compression {
+    Zstd,
+    Brotli,
+    None
+}
+
+impl Compression {
+    pub fn http_encoding(&self) -> Option<&str> {
+        Some(match self {
+            Self::Zstd => "zstd",
+            Self::Brotli => "br",
+            Self::None => return None,
+        })
+    }
+
+    pub fn file_extension(&self) -> Option<&str> {
+        Some(match self {
+            Self::Zstd => "zst",
+            Self::Brotli => "br",
+            Self::None => return None,
+        })
+    }
+}
+
+impl ToString for HydraProductType {
+    fn to_string(&self) -> String {
+        match self {
+            Self::BrotliJson => "json-br".to_string(),
+            Self::SourceDistribution => "source-dist".to_string(),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub enum Action {
+    WriteFile {
+        dst_path: PathBuf,
+        contents: String
+    },
+    WriteHydraProduct {
+        product_name: String,
+        dst_path: Option<PathBuf>,
+        product_type: Option<HydraProductType>,
+    },
+    WriteCompressedStorePaths {
+        compression: Compression
+    }
+}
+
+#[derive(Debug)]
+pub struct Plan {
+    pub actions: Vec<Action>,
+    pub release: hydra::Release,
+    pub channel_name: String,
+    pub target_bucket: String
+}
+
+macro_rules! plan_write_file {
+    ($plan_container:ident, $dst_path:literal, $contents:expr) => {
+        $plan_container.push(Action::WriteFile {
+            dst_path: $dst_path.into(),
+            contents: $contents
+        });
+    }
+}
+
+macro_rules! plan_write_hydra_product {
+    ($plan_container:ident, $pname:expr) => {
+        $plan_container.push(Action::WriteHydraProduct {
+            product_name: $pname.into(),
+            dst_path: None,
+            product_type: None
+        });
+    };
+
+    ($plan_container:ident, $dst_path:literal, $pname:expr) => {
+        $plan_container.push(Action::WriteHydraProduct {
+            dst_path: Some($dst_path.into()),
+            product_name: $pname.into(),
+            product_type: None
+        });
+    };
+
+    ($plan_container:ident, $dst_path:literal, $pname:expr, $ptype:expr) => {
+        $plan_container.push(Action::WriteHydraProduct {
+            dst_path: Some($dst_path.into()),
+            product_name: $pname.into(),
+            product_type: Some($ptype)
+        });
+    };
+}
+
+macro_rules! plan_iso {
+    ($plan_container:ident, $iso_name:literal, $arch:literal) => {
+        plan_write_hydra_product!($plan_container, format!("nixos.iso_{}.{}", $iso_name, $arch));
+    }
+}
+
+pub async fn generate_plan(config: &config::MirrorConfig, channel: &hydra::Channel, release: &hydra::Release, evaluation: hydra::Evaluation) -> Vec<Action> {
+    let mut plan = vec![];
+
+    /*plan.push(Action::WriteCompressedStorePaths {
+        compression: Compression::Zstd
+    });*/
+
+    if let hydra::Input::Git { revision, .. } = evaluation.jobset_eval_inputs.get("nixpkgs").unwrap() {
+        plan_write_file!(plan, "src-url", release.evaluation_url(&config.hydra_uri));
+        plan_write_file!(plan, "git-revision", revision.clone());
+        plan_write_file!(plan, "binary-cache-url", config.binary_cache_uri.clone());
+
+        if matches!(channel.r#type(), hydra::ChannelType::NixOS) {
+            plan_write_hydra_product!(plan, "nixexprs.tar.xz", "nixos.channel");
+            plan_write_hydra_product!(plan, "packages.json.br", "nixpkgs.tarball",
+                HydraProductType::BrotliJson
+            );
+            plan_write_hydra_product!(plan, "options.json.br", "nixos.options",
+                HydraProductType::BrotliJson
+            );
+
+            // TODO: we don't have aarch64-linux yet
+            //plan_iso!(plan, "minimal", "aarch64-linux");
+            plan_iso!(plan, "minimal", "x86_64-linux");
+
+            if !matches!(channel.variant(), hydra::ChannelVariant::Small) {
+                // TODO: for some reason, those ISOs are not present in ForkOS hydra.
+                // TODO: implement error recovery, skipping or failure?
+                // maybe encode this in the plan (fallible, etc.)
+                // TODO: compat with nixos-20, 21, 22, 23 using plasma5?
+                // should we have plasma_latest?
+                //plan_iso!(plan, "plasma6", "aarch64-linux");
+                //plan_iso!(plan, "plasma6", "x86_64-linux");
+
+                //plan_iso!(plan, "gnome", "aarch64-linux");
+                //plan_iso!(plan, "gnome", "x86_64-linux");
+
+                // TODO: i686-linux ISO compat? for nixos ≤ 23.11
+                
+                // FIXME: weird OVA appliance here, should investigate what to do
+                //plan_write_hydra_product!(plan, "nixos.ova.x86_64-linux");
+            }
+        } else {
+            plan_write_hydra_product!(plan, "nixexprs.tar.xz", "tarball",
+                HydraProductType::SourceDistribution);
+            plan_write_hydra_product!(plan, "packages.json.br", "tarball",
+                HydraProductType::BrotliJson);
+        }
+    }
+
+    // TODO: take into account the SQLite database & debuginfo
+
+    plan
+}
+
+/// Interets the plan as a list of action.
+///
+/// It creates a staging directory where all actions are prepared.
+/// No irreversible action is committed.
+///
+/// At the end, it returns the staging directory for further post-processing, including possible
+/// uploads.
+///
+/// Enabling streaming will also directly stream the contents of the staging directory to the
+/// target bucket in a temporary suffix of the target prefix.
+/// Caller is responsible to promote that upload to a finalized upload.
+pub async fn interpret_plan(hydra_client: &hydra::HydraClient<'_>, plan: Plan, streaming: bool) -> TempDir {
+    if streaming {
+        // TODO: streaming impl is just replacing the async write to go directly to the bucket
+        // returning a tempdir is useless now as all the contents is already remote?
+        // what should become the return type?
+        todo!("Streaming the plan is not supported yet");
+    }
+
+    // Prepare a staging directory
+    let staging_directory = TempDir::new(format!("staging-{}", &plan.channel_name).as_str()).expect("Failed to create temporary directory for staging directory");
+    debug!("Preparing the update in {}", staging_directory.path().display());
+
+    // Initialize multiple progress bars
+    let multi_pbar = MultiProgress::with_draw_target(ProgressDrawTarget::stdout());
+    let mut pending = Vec::new();
+
+    for action in plan.actions {
+        let pbar = ProgressBar::new(0);
+        let pbar = multi_pbar.add(pbar.clone());
+        let task = async {
+            match action {
+                Action::WriteHydraProduct { product_name, dst_path, product_type } => {
+                    // spawn two pbs with bytes styles, attach it to the mb
+                    // perform an io copy from the async read of the remote download of hydra to the
+                    // async write of the IO file behind dst_path
+                    // and wrap_async_read + wrap_async_write
+                    let dst_path = dst_path.unwrap_or_else(|| product_name.clone().into());
+                    assert!(dst_path.is_relative(), "Unexpected absolute path in a channel script plan, all paths needs to be relative in order for them to be prefixed by the staging directory");
+                    let mut tgt_file = tokio::fs::File::create(staging_directory.path().join(dst_path.clone())).await.unwrap();
+                    let copied = hydra_client.copy_hydra_product(&plan.release, &product_name, product_type, pbar, &mut tgt_file).await
+                        .context(format!("while writing the hydra product {} ({:?}) to {}", product_name, product_type, dst_path.display()))
+                        .expect("Failed to copy an Hydra product");
+
+                    debug!("Copied {} bytes", copied);
+                },
+                Action::WriteFile { dst_path, contents } => {
+                    // We don't need a progress bar here.
+                    multi_pbar.remove(&pbar);
+                    assert!(dst_path.is_relative(), "Unexpected absolute path in a channel script plan, all paths needs to be relative in order for them to be prefixed by the staging directory");
+                    let size = contents.len();
+                    tokio::fs::write(staging_directory.path().join(dst_path), contents).await.expect("Failed to write a file");
+                    debug!("Written {} bytes", size);
+                },
+                Action::WriteCompressedStorePaths { compression } => {
+                    // spawn a pb with remote download from hydra
+                    // spawn a pb with write + compression to some compressing writer
+                    let mut writer;
+                    let tgt_file = tokio::fs::File::create(staging_directory.path().join("store-paths.zst")).await.unwrap();
+                    match compression {
+                        Compression::Zstd => {
+                            writer = ZstdEncoder::new(tgt_file);
+                        },
+                        Compression::Brotli => todo!(),
+                        Compression::None => todo!(),
+                    }
+                    let written = hydra_client.copy_hydra_product(&plan.release, "store-paths", None, pbar, &mut writer).await
+                        .context("while writing compressed store paths")
+                        .expect("Failed to write compressed store paths from Hydra");
+                    debug!("Written {} bytes of compressed store paths", written);
+                },
+            };
+        };
+
+        pending.push(task);
+    }
+
+    join_all(pending).await;
+
+    // join all the futures
+    // returns the staging directory
+    staging_directory
+}
+

src/config.rs

@@ -0,0 +1,33 @@
+use object_store::aws::{AmazonS3, AmazonS3Builder};
+use serde::Deserialize;
+use std::path::PathBuf;
+
+#[derive(Debug, Deserialize)]
+pub struct MirrorConfig {
+    /// URI to Hydra instance
+    pub hydra_uri: String,
+    /// URI to the binary cache
+    pub binary_cache_uri: String,
+    /// A path to a checkout of nixpkgs
+    nixpkgs_dir: PathBuf,
+    /// S3 releases bucket URL
+    s3_release_bucket_uri: String,
+    /// S3 channels bucket URL
+    s3_channel_bucket_uri: String,
+}
+
+impl MirrorConfig {
+    pub fn release_bucket(&self) -> AmazonS3 {
+        AmazonS3Builder::from_env()
+            .with_bucket_name(&self.s3_release_bucket_uri)
+            .build()
+            .expect("Failed to connect to the S3 release bucket")
+    }
+
+    pub fn channel_bucket(&self) -> AmazonS3 {
+        AmazonS3Builder::from_env()
+            .with_bucket_name(&self.s3_channel_bucket_uri)
+            .build()
+            .expect("Failed to connect to the S3 channel bucket")
+    }
+}

src/git.rs

@@ -0,0 +1 @@
+

src/hydra.rs

@@ -0,0 +1,276 @@
+use std::{collections::HashMap, path::PathBuf};
+use anyhow::Context;
+use indicatif::ProgressBar;
+use nix_compat::store_path::StorePathRef;
+use regex::Regex;
+
+use serde::{de::Error, Deserialize, Deserializer};
+use tokio::io::AsyncWrite;
+
+use crate::{actions::HydraProductType, config::MirrorConfig};
+
+pub type ReleaseId = u64;
+pub type EvaluationId = u64;
+pub type BuildId = u64;
+
+fn deser_bool_as_int<'de, D>(deserializer: D) -> Result<bool, D::Error>
+    where D: Deserializer<'de>
+{
+    u8::deserialize(deserializer).map(|b| if b == 1 { true } else { false })
+}
+
+fn deser_bool_as_string<'de, D>(deserializer: D) -> Result<bool, D::Error>
+where D: Deserializer<'de>
+{
+    let s: &str = Deserialize::deserialize(deserializer)?;
+
+    match s {
+        "false" => Ok(false),
+        "true" => Ok(true),
+        _ => Err(Error::unknown_variant(s, &["false", "true"]))
+    }
+}
+
+#[derive(Debug, Deserialize)]
+pub struct OutputPath {
+    #[serde(rename="path")]
+    store_path: String
+}
+
+#[derive(Debug, Deserialize)]
+pub struct BuildProduct {
+    #[serde(rename="path")]
+    store_path: String,
+    name: String,
+    subtype: String,
+    sha256hash: String,
+    r#type: String,
+    #[serde(rename="filesize")]
+    file_size: u64
+}
+
+#[derive(Debug, Deserialize)]
+pub struct BuildInfo {
+    jobset: String,
+    job: String,
+    project: String,
+    #[serde(deserialize_with = "deser_bool_as_int", rename="buildstatus")]
+    finished: bool,
+    #[serde(rename="releasename")]
+    release_name: Option<String>,
+    #[serde(rename="stoptime")]
+    stop_time: u64,
+    #[serde(rename="starttime")]
+    start_time: u64,
+    system: String,
+    id: BuildId,
+    #[serde(rename="buildoutputs")]
+    build_outputs: HashMap<String, OutputPath>,
+    #[serde(rename="buildproducts")]
+    build_products: HashMap<String, BuildProduct>,
+    #[serde(rename="nixname")]
+    nix_name: String,
+    #[serde(rename="drvpath")]
+    drv_path: String,
+}
+
+#[derive(Debug)]
+pub struct Channel {
+    pub name: String
+}
+
+#[non_exhaustive]
+pub enum ChannelType {
+    NixOS,
+    Nixpkgs,
+}
+
+#[non_exhaustive]
+pub enum ChannelVariant {
+    Normal,
+    Small
+}
+
+impl Channel {
+    pub fn version(&self) -> String {
+        let re = Regex::new("([a-z]+)-(?<ver>.*)").unwrap();
+        let caps = re.captures(&self.name).expect("Failed to parse the channel name");
+
+        caps["ver"].to_string()
+    }
+
+    pub fn prefix(&self) -> String {
+        let re = Regex::new("(?<name>[a-z]+)-(.*)").unwrap();
+        let caps = re.captures(&self.name).expect("Failed to parse the channel name");
+
+        caps["name"].to_string()
+    }
+
+    // TODO: just regex match?
+    pub fn r#type(&self) -> ChannelType {
+        ChannelType::NixOS
+    }
+
+    pub fn variant(&self) -> ChannelVariant {
+        ChannelVariant::Normal
+    }
+}
+
+#[derive(Deserialize, Debug)]
+pub struct Release {
+    pub id: ReleaseId,
+    job: String,
+    #[serde(rename = "releasename")]
+    release_name: Option<String>,
+    #[serde(rename = "starttime")]
+    start_time: u64,
+    #[serde(rename = "stoptime")]
+    stop_time: u64,
+    #[serde(rename = "nixname")]
+    pub nix_name: String,
+    #[serde(rename = "jobsetevals")]
+    jobset_evals: Vec<EvaluationId>,
+    jobset: String,
+    #[serde(deserialize_with = "deser_bool_as_int")]
+    finished: bool,
+    priority: u64,
+    system: String,
+    timestamp: u64,
+    project: String,
+    #[serde(rename = "drvpath")]
+    derivation_path: String,
+    // ignored: buildproducts, buildoutputs, buildmetrics, buildstatus
+}
+
+#[derive(Debug)]
+pub struct GitInput {
+    uri: String,
+    revision: String
+}
+
+// FIXME(Raito): for some reason, #[serde(tag = "type"), rename_all = "lowercase"] doesn't behave
+// correctly, and causes deserialization failures in practice. `untagged` is suboptimal but works
+// because of the way responses works...
+#[derive(Debug, Deserialize)]
+#[serde(untagged, expecting = "An valid jobset input")]
+pub enum Input {
+    Boolean {
+        #[serde(deserialize_with = "deser_bool_as_string")]
+        value: bool
+    },
+    Git { uri: String, revision: String },
+    Nix { value: String },
+}
+
+#[derive(Deserialize, Debug)]
+pub struct Evaluation {
+    pub id: EvaluationId,
+    #[serde(rename="checkouttime")]
+    checkout_time: u64,
+    #[serde(rename="evaltime")]
+    eval_time: u64,
+    flake: Option<String>,
+    #[serde(rename="jobsetevalinputs", default)]
+    pub jobset_eval_inputs: HashMap<String, Input>,
+    timestamp: u64,
+    builds: Vec<BuildId>,
+}
+
+impl Release {
+    pub fn version(&self) -> String {
+        let re = Regex::new(".+-(?<ver>[0-9].+)").unwrap();
+        let caps = re.captures(&self.nix_name).expect("Failed to parse the release name");
+
+        caps["ver"].to_string()
+    }
+
+    pub fn evaluation_url(&self, hydra_base_uri: &str) -> String {
+        let eval_id = self.jobset_evals.first().expect("Failed to obtain the corresponding evaluation, malformed release?");
+        format!("{}/eval/{}", hydra_base_uri, eval_id)
+    }
+
+    pub fn job_url(&self, hydra_base_uri: &str, job_name: &str) -> String {
+        let eval_id = self.jobset_evals.first().expect("Failed to obtain the corresponding evaluation, malformed release?");
+        format!("{}/eval/{}/job/{}", hydra_base_uri, eval_id, job_name)
+    }
+
+    /// Directory related to this release.
+    fn directory(&self, channel: &Channel) -> String {
+        match channel.name.as_str() {
+            "nixpkgs-unstable" => "nixpkgs".to_string(),
+            _ => format!("{}/{}", channel.prefix(), channel.version())
+        }
+    }
+
+    pub fn prefix(&self, channel: &Channel) -> object_store::path::Path {
+        format!("{}/{}", self.directory(channel), self.nix_name).into()
+    }
+}
+
+pub fn release_uri(hydra_uri: &str, job_name: &str) -> String {
+    format!("{}/job/{}/latest", hydra_uri, job_name)
+}
+
+pub struct HydraClient<'a> {
+    pub config: &'a MirrorConfig,
+}
+
+impl HydraClient<'_> {
+    pub async fn fetch_release(&self, job_name: &str) -> reqwest::Result<Release> {
+        let client = reqwest::Client::new();
+        let resp = client.get(release_uri(&self.config.hydra_uri, job_name))
+            .header("Accept", "application/json")
+            // TODO: put a proper version
+            .header("User-Agent", "nixos-channel-scripts (rust)")
+            .send()
+            .await?;
+
+        resp.json().await
+    }
+
+    pub async fn fetch_evaluation(&self, release: &Release) -> reqwest::Result<Evaluation> {
+        let client = reqwest::Client::new();
+        let resp = client.get(release.evaluation_url(&self.config.hydra_uri))
+            .header("Accept", "application/json")
+            .header("User-Agent", "nixos-channel-scripts (rust)")
+            .send()
+            .await?;
+
+        resp.json().await
+    }
+
+    pub async fn copy_hydra_product<W: AsyncWrite + Unpin>(&self, release: &Release, job_name: &str, product_type: Option<HydraProductType>, pbar: ProgressBar, out: &mut W) -> anyhow::Result<u64> {
+        let client = reqwest::Client::new();
+        let build_info: BuildInfo = client.get(release.job_url(&self.config.hydra_uri, job_name))
+            .header("Accept", "application/json")
+            .header("User-Agent", "nixos-channel-scripts (rust)")
+            .send()
+            .await?
+            .json()
+            .await
+            .context(format!("while downloading build information from {}", release.job_url(&self.config.hydra_uri, job_name)))?;
+
+        let mut products_by_subtype: HashMap<String, BuildProduct> = HashMap::new();
+        for (_, product) in build_info.build_products {
+            if products_by_subtype.contains_key(&product.subtype) {
+                todo!("Job {} has multiple products of the same subtype {:?}. This is not supported yet.", job_name, product_type);
+            }
+
+            products_by_subtype.insert(product.subtype.clone(), product);
+        }
+
+        if products_by_subtype.len() > 1 && product_type.is_none() {
+            panic!("Job {} has {} build products. Select the right product via the subtypes.", job_name, products_by_subtype.len());
+        }
+
+        let product: BuildProduct = if let Some(ptype) = product_type {
+            products_by_subtype.remove(&ptype.to_string()).expect(&format!("Expected product type '{}' but not found in the list of products", &ptype.to_string()))
+        } else {
+            products_by_subtype.into_iter().last().expect(&format!("Expected at least one build product in job {}, found zero", job_name)).1
+        };
+
+        // 3. FIXME: validate sha256hash during read?
+        let (store_path, rel_path) = StorePathRef::from_absolute_path_full(&product.store_path).expect("Failed to parse the product's store path");
+        crate::nar::copy_file_in_nar(&self.config.binary_cache_uri, &nix_compat::nixbase32::encode(store_path.digest()), rel_path.to_str().unwrap(), pbar, out).await.context("while copying the NAR to the target")
+    }
+}

src/main.rs

@@ -0,0 +1,160 @@
+mod config;
+mod actions;
+mod hydra;
+mod git;
+mod nar;
+
+use std::path::PathBuf;
+
+use actions::{interpret_plan, Plan};
+use clap::{Subcommand, Parser, Args};
+use hydra::{Channel, Evaluation, HydraClient, Release};
+use log::{info, trace, warn};
+use object_store::{aws::{AmazonS3, AmazonS3Builder}, ObjectStore};
+
+#[derive(Debug, Args)]
+struct ChannelArgs {
+    /// Channel name to update
+    channel_name: String,
+    /// Job name to fetch from the Hydra instance configured
+    job_name: String,
+}
+
+#[derive(Debug, Args)]
+struct GlobalOpts {
+    /// TOML configuration file for channel updates
+    #[arg(short, long)]
+    config_file: PathBuf,
+    /// Whether to execute no remote side effects (S3 uploads, redirections), etc.
+    #[arg(short, long, default_value_t = false)]
+    dry_run: bool,
+    /// Whether to bypass all preflight checks
+    #[arg(short, long, default_value_t = false)]
+    bypass_preflight_checks: bool
+}
+
+#[derive(Debug, Parser)]
+#[command(version, about, long_about = None)]
+struct App {
+    #[command(flatten)]
+    global_opts: GlobalOpts,
+    #[command(subcommand)]
+    command: Commands
+}
+
+#[derive(Debug, Subcommand)]
+enum Commands {
+    /// Print the plan for the given channel name and job name
+    Plan(ChannelArgs),
+    /// Apply the plan that would be generated for the given channel name and job name
+    Apply(ChannelArgs),
+}
+
+#[derive(Debug)]
+struct PreflightCheckContext<'a> {
+    target_channel: &'a Channel,
+    new_release: &'a Release,
+    new_evaluation: &'a Evaluation,
+    current_release_path: Option<object_store::path::Path>,
+}
+
+async fn run_preflight_checks(channel: &Channel, release: &Release, evaluation: &Evaluation, channel_bucket: AmazonS3) -> bool {
+    info!("Running pre-flight checks...");
+    let channel_name = object_store::path::Path::parse(&channel.name).expect("Channel name should be a valid S3 path");
+    let mut context = PreflightCheckContext {
+        target_channel: channel,
+        new_release: release,
+        new_evaluation: evaluation,
+        current_release_path: None
+    };
+    match channel_bucket.get_opts(&channel_name, object_store::GetOptions { head: true, ..Default::default() }).await {
+        Ok(object_store::GetResult { attributes, meta, .. }) => {
+            info!("Release found: {:?}", meta);
+            trace!("Attributes: {:#?}", attributes);
+            if let Some(redirection_target) = attributes.get(&object_store::Attribute::Metadata("x-amz-website-redirect-location".into())) {
+                context.current_release_path = Some(redirection_target.to_string().into());
+            }
+        },
+        Err(err) => {
+            warn!("Error while asking the channel bucket: {}", err);
+            todo!();
+        }
+    }
+
+    trace!("Preflight context check assembled: {:?}", context);
+    // TODO: run anti rollback protection
+    true
+}
+
+
+#[tokio::main]
+async fn main() -> std::io::Result<()> {
+    pretty_env_logger::init();
+
+    let args = App::parse();
+
+    let config: config::MirrorConfig = toml::from_str(&std::fs::read_to_string(args.global_opts.config_file)
+        .expect("Failed to read the configuration file"))
+        .expect("Failed to deserialize the configuration file");
+
+    println!("config: {:?}", config);
+
+    let hydra_client: HydraClient = HydraClient {
+        config: &config
+    };
+
+    match args.command {
+        Commands::Plan(channel) => {
+            let chan = Channel {
+                name: channel.channel_name,
+            };
+            info!("Planning for channel {} using job {}", chan.name, channel.job_name);
+            let release = hydra_client.fetch_release(&channel.job_name)
+                .await.expect("Failed to fetch release");
+            trace!("{:#?}", release);
+            let evaluation = hydra_client.fetch_evaluation(&release)
+                .await.expect("Failed to fetch evaluation");
+            trace!("{:?}", evaluation.jobset_eval_inputs);
+
+            if let hydra::Input::Git { revision, .. } = evaluation.jobset_eval_inputs.get("nixpkgs").expect("Expected a nixpkgs repository") {
+                info!("Release information:\n- Release is: {} (build {})\n- Eval is: {}\n- Prefix is: {}\n- Git commit is {}\n", 
+                    release.nix_name, release.id, evaluation.id, release.prefix(&chan), revision);
+
+                if args.global_opts.bypass_preflight_checks {
+                    let actions = actions::generate_plan(&config, &chan, &release, evaluation).await;
+                    log::debug!("Action plan: {:#?}", actions);
+                    
+                    let staging_dir = interpret_plan(&hydra_client, Plan {
+                        actions,
+                        release,
+                        channel_name: chan.name.clone(),
+                        target_bucket: "something".to_string()
+                    }, false).await;
+                    info!("Plan interpreted in {}", staging_dir.path().display());
+                } else {
+                    let release_bucket = config.release_bucket();
+
+                    // If the release already exists, skip it.
+                    if release_bucket.head(&release.prefix(&chan)).await.is_ok() {
+                        log::warn!("Release already exists, skipping");
+                        return Ok(());
+                    }
+
+                    // Runs the preflight checks.
+                    let channel_bucket = config.channel_bucket();
+                    if run_preflight_checks(&chan, &release, &evaluation, channel_bucket).await {
+                        info!("Preflight checks passed"); 
+                    } else {
+                        log::error!("Preflight check failed, cannot continue, pass `--force` if you want to bypass preflight checks");
+                    }
+
+                }
+            } else {
+                panic!("Nixpkgs input is not of type Git");
+            }
+        },
+        Commands::Apply(_) => todo!(),
+    }
+
+    Ok(())
+}

src/nar.rs

@@ -0,0 +1,106 @@
+use std::borrow::{Borrow, BorrowMut};
+
+use anyhow::{bail, Context};
+use async_compression::tokio::bufread::ZstdDecoder;
+use futures::{Stream, StreamExt, TryStreamExt};
+use indicatif::{ProgressBar, ProgressStyle};
+use log::debug;
+use nix_compat::nar::listing::{Listing, ListingEntry};
+use nix_compat::narinfo::NarInfo;
+use tokio::io::{AsyncReadExt, AsyncWrite, AsyncWriteExt};
+use tokio_util::io::StreamReader;
+
+/// Returns a NarInfo from a HTTP binary cache.
+pub async fn get_narinfo(binary_cache_uri: &str, narinfo_hash: &str) -> reqwest::Result<String> {
+    let client = reqwest::Client::new();
+    let resp = client.get(format!("{}/{}.narinfo", binary_cache_uri, narinfo_hash))
+        .header("User-Agent", "nixos-channel-scripts (rust)")
+        .send()
+        .await?;
+
+    resp.text().await
+}
+
+/// Returns a listing from a HTTP binary cache.
+pub async fn get_listing(binary_cache_uri: &str, narinfo_hash: &str) -> reqwest::Result<Listing> {
+    let client = reqwest::Client::builder().brotli(true).build().unwrap();
+    let resp = client.get(format!("{}/{}.ls", binary_cache_uri, narinfo_hash))
+        .header("User-Agent", "nixos-channel-scripts (rust)")
+        .send()
+        .await?;
+
+    resp.json().await
+}
+
+/// Returns a `Content-Range`-aware HTTP body reader of the NAR.
+pub async fn ranged_nar_reader(binary_cache_uri: &str, narinfo: NarInfo<'_>, start: u64, end: u64) -> reqwest::Result<impl Stream<Item=reqwest::Result<bytes::Bytes>>> {
+    let client = reqwest::Client::new();
+    // TODO: handle compression?
+    let resp = client.get(format!("{}/{}", binary_cache_uri, narinfo.url))
+        .header("User-Agent", "nixos-channel-scripts (rust)")
+        .header("Content-Range", format!("bytes {}-{}/*", start, end))
+        .send()
+        .await?;
+
+    Ok(resp.bytes_stream())
+}
+
+pub async fn nar_reader(binary_cache_uri: &str, narinfo: NarInfo<'_>) -> reqwest::Result<impl Stream<Item=reqwest::Result<bytes::Bytes>>> {
+    let client = reqwest::Client::new();
+    // TODO: handle compression?
+    let resp = client.get(format!("{}/{}", binary_cache_uri, narinfo.url))
+        .header("User-Agent", "nixos-channel-scripts (rust)")
+        .send()
+        .await?;
+
+    Ok(resp.bytes_stream())
+}
+
+/// Hits the binary cache with a narinfo request
+/// and copies a specific file to the provided writer
+/// asynchronously.
+pub async fn copy_file_in_nar<W: AsyncWrite + Unpin>(binary_cache_uri: &str, narinfo_hash: &str, path: &str, pbar: ProgressBar, out: &mut W) -> anyhow::Result<u64> {
+    let narinfo_str: String = get_narinfo(binary_cache_uri, narinfo_hash).await?;
+    let narinfo: NarInfo = NarInfo::parse(narinfo_str.as_ref()).context("while parsing the narinfo").expect("Failed to parse narinfo from HTTP binary cache server");
+    let listing: Listing = get_listing(binary_cache_uri, narinfo_hash).await.context("while parsing the listing of that NAR")?;
+    match listing {
+        Listing::V1 { root, .. } => {
+            let entry = root.locate(path)
+                .expect("Invalid relative path to the NAR")
+                .ok_or(tokio::io::Error::new(tokio::io::ErrorKind::NotFound, format!("Entry {} not found in the NAR listing", path)))?;
+
+            if let ListingEntry::Regular {
+                size, nar_offset, ..
+            } = entry {
+                pbar.set_length(*size);
+                pbar.set_style(ProgressStyle::with_template("[{elapsed_precise}] {bar:40.cyan/blue} {bytes:>7}/{total_bytes:>7} @ {bytes_per_sec:>7} {msg}").unwrap().progress_chars("##-"));
+                pbar.set_message(format!("Downloading and decompressing '{}' @ '{}' to the staging directory", narinfo_hash, path));
+
+                // TODO: this is ugly.
+                let mut file_reader = ZstdDecoder::new(
+                    StreamReader::new(nar_reader(binary_cache_uri, narinfo).await?.map_err(std::io::Error::other))
+                );
+
+
+                // Discard *nar_offset bytes
+                let discarded = tokio::io::copy(&mut file_reader.borrow_mut().take(*nar_offset), &mut pbar.wrap_async_write(tokio::io::sink())).await
+                    .context("while discarding the start of the NAR").unwrap();
+
+                debug!("discarded {} bytes", discarded);
+
+                // Copy at most `*size` bytes now.
+                let copied = tokio::io::copy(&mut file_reader.take(*size), &mut pbar.wrap_async_write(out)).await
+                    .context("while copying the file in the NAR").unwrap();
+
+                assert!(copied == *size, "mismatch in the copy sizes");
+                pbar.finish();
+
+                return Ok(copied)
+            } else {
+                bail!("Expected a file, obtained either a symlink or a directory");
+            }
+
+        },
+        _ => bail!("Unsupported listing version")
+    }
+}