Move the systemsConfigs key so that every entry defines its system. Determine flake attrs automatically.

flake.nix

@@ -100,17 +100,11 @@
       };
     in
     {
-      systemConfig = self.lib.makeSystemConfig {
-        inherit system;
-        modules = [
-          ./nix/modules
-        ];
-      };
-
       packages = {
         inherit system-manager;
         default = self.packages.${system}.system-manager;
       };
+
       devShells.default = pkgs.devshell.mkShell {
         packages = with pkgs; [
           llvm.clang
@@ -174,6 +168,7 @@
           };
         }).shellHook;
       };
+
       checks = {
         inherit
           # Build the crate as part of `nix flake check` for convenience
@@ -186,5 +181,12 @@
       lib = import ./nix/lib.nix {
         inherit nixpkgs self;
       };
+
+      systemConfigs.default = self.lib.makeSystemConfig {
+        system = flake-utils.lib.system.x86_64-linux;
+        modules = [
+          ./nix/modules
+        ];
+      };
     };
 }

src/generate.rs

@@ -3,6 +3,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::fs::DirBuilder;
 use std::path::Path;
+use std::process::ExitStatus;
 use std::{fs, process, str};
 
 use super::{create_store_link, StorePath, FLAKE_ATTR, GCROOT_PATH, PROFILE_DIR, PROFILE_NAME};
@@ -52,8 +53,7 @@ fn create_gcroot(gcroot_path: &str, profile_path: &Path) -> Result<()> {
 }
 
 pub fn build(flake_uri: &str) -> Result<StorePath> {
-    // FIXME: we should not hard-code the system here
-    let flake_attr = format!("{FLAKE_ATTR}.x86_64-linux");
+    let flake_attr = find_flake_attr(flake_uri)?;
 
     log::info!("Building new system-manager generation...");
     log::info!("Running nix build...");
@@ -62,6 +62,34 @@ pub fn build(flake_uri: &str) -> Result<StorePath> {
     Ok(store_path)
 }
 
+fn find_flake_attr(flake_uri: &str) -> Result<String> {
+    let hostname = nix::unistd::gethostname()?;
+    let flake_attr = format!("{FLAKE_ATTR}.{}", hostname.to_string_lossy());
+
+    let status = try_flake_attr(flake_uri, &flake_attr)?;
+    if status {
+        return Ok(flake_attr);
+    } else {
+        let flake_attr = format!("{FLAKE_ATTR}.default");
+        let status = try_flake_attr(flake_uri, &flake_attr)?;
+        if status {
+            return Ok(flake_attr);
+        };
+    };
+    anyhow::bail!("No suitable flake attribute found, giving up.");
+}
+
+fn try_flake_attr(flake_uri: &str, flake_attr: &str) -> Result<bool> {
+    log::info!("Trying flake attribute: {flake_uri}#{flake_attr}...");
+    let status = try_nix_eval(flake_uri, flake_attr)?;
+    if status.success() {
+        log::info!("Success, using {flake_uri}#{flake_attr}");
+    } else {
+        log::info!("Attribute {flake_uri}#{flake_attr} not found in flake.");
+    };
+    Ok(status.success())
+}
+
 fn get_store_path(nix_build_result: process::Output) -> Result<StorePath> {
     if nix_build_result.status.success() {
         String::from_utf8(nix_build_result.stdout)
@@ -103,3 +131,14 @@ fn run_nix_build(flake_uri: &str, flake_attr: &str) -> Result<process::Output> {
         .output()?;
     Ok(output)
 }
+
+fn try_nix_eval(flake_uri: &str, flake_attr: &str) -> Result<process::ExitStatus> {
+    let status = process::Command::new("nix")
+        .arg("eval")
+        .arg(format!("{flake_uri}#{flake_attr}"))
+        .arg("--json")
+        .stdout(process::Stdio::null())
+        .stderr(process::Stdio::null())
+        .status()?;
+    Ok(status)
+}

src/lib.rs

@@ -8,7 +8,7 @@ use std::os::unix;
 use std::path::{Path, PathBuf};
 use std::{fs, str};
 
-const FLAKE_ATTR: &str = "systemConfig";
+const FLAKE_ATTR: &str = "systemConfigs";
 const PROFILE_DIR: &str = "/nix/var/nix/profiles/system-manager-profiles";
 const PROFILE_NAME: &str = "system-manager";
 const GCROOT_PATH: &str = "/nix/var/nix/gcroots/system-manager-current";