Make flake resolution (slightly) less terrible

Instead of using `path:` which always copies the entire directory,
we now try to resolve the Flake URI using `nix flake metadata` which
may give us a `git+file:`.

src/command/apply.rs

@@ -115,7 +115,7 @@ pub fn subcommand() -> App<'static, 'static> {
 }
 
 pub async fn run(_global_args: &ArgMatches<'_>, local_args: &ArgMatches<'_>) {
-    let hive = util::hive_from_args(local_args).unwrap();
+    let hive = util::hive_from_args(local_args).await.unwrap();
 
     log::info!("Enumerating nodes...");
     let all_nodes = hive.deployment_info().await.unwrap();

src/command/apply_local.rs

@@ -89,7 +89,7 @@ pub async fn run(_global_args: &ArgMatches<'_>, local_args: &ArgMatches<'_>) {
         }
     }
 
-    let hive = util::hive_from_args(local_args).unwrap();
+    let hive = util::hive_from_args(local_args).await.unwrap();
     let hostname = if local_args.is_present("node") {
         local_args.value_of("node").unwrap().to_owned()
     } else {

src/command/exec.rs

@@ -55,7 +55,7 @@ It's recommended to use -- to separate Colmena options from the command to run.
 }
 
 pub async fn run(_global_args: &ArgMatches<'_>, local_args: &ArgMatches<'_>) {
-    let hive = util::hive_from_args(local_args).unwrap();
+    let hive = util::hive_from_args(local_args).await.unwrap();
 
     log::info!("Enumerating nodes...");
     let all_nodes = hive.deployment_info().await.unwrap();

src/command/introspect.rs

@@ -28,7 +28,7 @@ For example, to retrieve the configuration of one node, you may write something
 }
 
 pub async fn run(_global_args: &ArgMatches<'_>, local_args: &ArgMatches<'_>) {
-    let hive = util::hive_from_args(local_args).unwrap();
+    let hive = util::hive_from_args(local_args).await.unwrap();
 
     if !(local_args.is_present("expression") ^ local_args.is_present("expression_file")) {
         log::error!("Either an expression (-E) or a .nix file containing an expression should be specified, not both.");

src/nix/flake.rs

@@ -0,0 +1,92 @@
+//! Nix Flake utilities.
+
+use std::convert::AsRef;
+use std::path::{Path, PathBuf};
+use std::process::Stdio;
+
+use serde::Deserialize;
+use tokio::process::Command;
+
+use super::{NixCheck, NixError, NixResult};
+
+/// A Nix Flake.
+#[derive(Debug)]
+pub struct Flake {
+    /// The Flake URI.
+    uri: String,
+
+    /// The directory the flake lives in, if it's a local flake.
+    local_dir: Option<PathBuf>,
+}
+
+impl Flake {
+    /// Creates a flake from the given directory.
+    ///
+    /// This will try to retrieve the resolved URL of the local flake
+    /// in the specified directory.
+    pub async fn from_dir<P: AsRef<Path>>(dir: P) -> NixResult<Self> {
+        NixCheck::require_flake_support().await?;
+
+        let flake = dir.as_ref().as_os_str().to_str()
+            .expect("Flake directory path contains non-UTF-8 characters");
+
+        let info = FlakeMetadata::resolve(flake).await?;
+
+        Ok(Self {
+            uri: info.resolved_url,
+            local_dir: Some(dir.as_ref().to_owned()),
+        })
+    }
+
+    /// Creates a flake from a Flake URI.
+    pub async fn from_uri(uri: String) -> NixResult<Self> {
+        NixCheck::require_flake_support().await?;
+
+        Ok(Self {
+            uri,
+            local_dir: None,
+        })
+    }
+
+    /// Returns the URI.
+    pub fn uri(&self) -> &str {
+        &self.uri
+    }
+
+    /// Returns the local directory, if it exists.
+    pub fn local_dir(&self) -> Option<&Path> {
+        self.local_dir.as_ref().map(|d| d.as_path())
+    }
+}
+
+/// A `nix flake metadata --json` invocation.
+#[derive(Deserialize, Debug)]
+struct FlakeMetadata {
+    /// The resolved URL of the flake.
+    #[serde(rename = "resolvedUrl")]
+    resolved_url: String,
+}
+
+impl FlakeMetadata {
+    /// Resolves a flake.
+    async fn resolve(flake: &str) -> NixResult<Self> {
+        let child = Command::new("nix")
+            .args(&["flake", "metadata", "--json"])
+            .args(&["--experimental-features", "nix-command flakes"])
+            .arg(flake)
+            .stdout(Stdio::piped())
+            .spawn()?;
+
+        let output = child.wait_with_output().await?;
+
+        if !output.status.success() {
+            return Err(output.status.into());
+        }
+
+        serde_json::from_slice::<FlakeMetadata>(&output.stdout)
+            .map_err(|_| {
+                let output = String::from_utf8_lossy(&output.stdout).to_string();
+                NixError::BadOutput { output }
+            })
+    }
+}

src/nix/hive.rs

@@ -10,13 +10,13 @@ use serde::Serialize;
 use validator::Validate;
 
 use super::{
+    Flake,
     StoreDerivation,
     NixResult,
-    NixError,
     NodeConfig,
     ProfileMap,
 };
-use super::{NixCommand, NixCheck};
+use super::NixCommand;
 use crate::util::CommandExecution;
 use crate::progress::TaskProgress;
 
@@ -24,49 +24,38 @@ const HIVE_EVAL: &'static [u8] = include_bytes!("eval.nix");
 
 #[derive(Debug)]
 pub enum HivePath {
-    /// A Nix Flake URI.
+    /// A Nix Flake.
     ///
     /// The flake must contain the `colmena` output.
-    Flake(String),
+    Flake(Flake),
 
     /// A regular .nix file
     Legacy(PathBuf),
 }
 
 impl HivePath {
-    pub fn from_path<P: AsRef<Path>>(path: P) -> Self {
+    pub async fn from_path<P: AsRef<Path>>(path: P) -> NixResult<Self> {
         let path = path.as_ref();
 
         if let Some(osstr) = path.file_name() {
             if osstr == "flake.nix" {
-                let parent = path.parent().unwrap().canonicalize().unwrap();
-                let parent = parent.to_str().unwrap();
-                let uri = format!("path:{}", parent);
-
-                return Self::Flake(uri);
+                let parent = path.parent().unwrap();
+                let flake = Flake::from_dir(parent).await?;
+                return Ok(Self::Flake(flake));
             }
         }
 
-        Self::Legacy(path.to_owned())
+        Ok(Self::Legacy(path.to_owned()))
     }
 
     fn context_dir(&self) -> Option<PathBuf> {
         match self {
             Self::Legacy(p) => {
-                if let Some(parent) = p.parent() {
-                    return Some(parent.to_owned());
-                }
-                None
+                p.parent().map(|d| d.to_owned())
+            }
+            Self::Flake(flake) => {
+                flake.local_dir().map(|d| d.to_owned())
             }
-            _ => None,
-        }
-    }
-
-    fn is_flake(&self) -> bool {
-        if let Self::Flake(_) = self {
-            true
-        } else {
-            false
         }
     }
 }
@@ -128,13 +117,6 @@ impl Hive {
 
     /// Retrieve deployment info for all nodes.
     pub async fn deployment_info(&self) -> NixResult<HashMap<String, NodeConfig>> {
-        let nix_check = NixCheck::detect().await;
-
-        if self.path.is_flake() && !nix_check.flakes_supported() {
-            nix_check.print_flakes_info(true);
-            return Err(NixError::NoFlakesSupport);
-        }
-
         // FIXME: Really ugly :(
         let s: String = self.nix_instantiate("hive.deploymentConfigJson").eval_with_builders().await?
             .capture_json().await?;
@@ -281,7 +263,7 @@ impl<'hive> NixInstantiate<'hive> {
                         self.expression,
                     ));
             }
-            HivePath::Flake(uri) => {
+            HivePath::Flake(flake) => {
                 command
                     .args(&["--experimental-features", "flakes"])
                     .arg("--no-gc-warning")
@@ -289,7 +271,7 @@ impl<'hive> NixInstantiate<'hive> {
                     .arg(format!(
                         "with builtins; let eval = import {}; hive = eval {{ flakeUri = \"{}\"; }}; in {}",
                         self.hive.eval_nix.to_str().unwrap(),
-                        &uri,
+                        flake.uri(),
                         self.expression,
                     ));
             }

src/nix/info.rs

@@ -3,9 +3,10 @@ use std::process::Stdio;
 
 use log::Level;
 use regex::Regex;
-
 use tokio::process::Command;
 
+use super::{NixError, NixResult};
+
 struct NixVersion {
     major: usize,
     minor: usize,
@@ -88,6 +89,17 @@ impl NixCheck {
         }
     }
 
+    pub async fn require_flake_support() -> NixResult<()> {
+        let check = Self::detect().await;
+
+        if !check.flakes_supported() {
+            check.print_flakes_info(true);
+            Err(NixError::NoFlakesSupport)
+        } else {
+            Ok(())
+        }
+    }
+
     pub fn print_version_info(&self) {
         if let Some(v) = &self.version {
             log::info!("Nix Version: {}", v);

src/nix/mod.rs

@@ -36,6 +36,9 @@ pub use deployment::{Goal, Target, Deployment};
 pub mod info;
 pub use info::NixCheck;
 
+pub mod flake;
+pub use flake::Flake;
+
 #[cfg(test)]
 mod tests;
 

src/nix/tests/mod.rs

@@ -34,7 +34,7 @@ impl TempHive {
         let mut temp_file = NamedTempFile::new().unwrap();
         temp_file.write_all(text.as_bytes()).unwrap();
 
-        let hive_path = HivePath::from_path(temp_file.path());
+        let hive_path = block_on(HivePath::from_path(temp_file.path())).unwrap();
         let hive = Hive::new(hive_path).unwrap();
 
         Self {
@@ -149,11 +149,10 @@ fn test_parse_simple() {
 
 #[test]
 fn test_parse_flake() {
-    let flake_path = {
-        let p = PathBuf::from("./src/nix/tests/simple-flake");
-        p.canonicalize().unwrap()
-    };
-    let hive_path = HivePath::Flake(format!("path:{}", flake_path.to_str().unwrap()));
+    let flake_dir = PathBuf::from("./src/nix/tests/simple-flake");
+    let flake = block_on(Flake::from_dir(flake_dir)).unwrap();
+
+    let hive_path = HivePath::Flake(flake);
     let mut hive = Hive::new(hive_path).unwrap();
 
     hive.set_show_trace(true);

src/util.rs

@@ -9,7 +9,7 @@ use glob::Pattern as GlobPattern;
 use tokio::io::{AsyncRead, AsyncBufReadExt, BufReader};
 use tokio::process::Command;
 
-use super::nix::{NodeConfig, Hive, HivePath, NixResult};
+use super::nix::{Flake, NodeConfig, Hive, HivePath, NixResult};
 use super::progress::TaskProgress;
 
 enum NodeFilter {
@@ -79,7 +79,7 @@ impl CommandExecution {
     }
 }
 
-pub fn hive_from_args(args: &ArgMatches<'_>) -> NixResult<Hive> {
+pub async fn hive_from_args(args: &ArgMatches<'_>) -> NixResult<Hive> {
     let path = match args.occurrences_of("config") {
         0 => {
             // traverse upwards until we find hive.nix
@@ -142,7 +142,8 @@ pub fn hive_from_args(args: &ArgMatches<'_>) -> NixResult<Hive> {
 
             if !fpath.exists() && path.contains(":") {
                 // Treat as flake URI
-                let hive_path = HivePath::Flake(path);
+                let flake = Flake::from_uri(path).await?;
+                let hive_path = HivePath::Flake(flake);
                 let mut hive = Hive::new(hive_path)?;
 
                 if args.is_present("show-trace") {
@@ -156,7 +157,7 @@ pub fn hive_from_args(args: &ArgMatches<'_>) -> NixResult<Hive> {
         }
     };
 
-    let hive_path = HivePath::from_path(path);
+    let hive_path = HivePath::from_path(path).await?;
     let mut hive = Hive::new(hive_path)?;
 
     if args.is_present("show-trace") {