Support update builds.

src/nix_util/high_level.rs

@@ -8,6 +8,7 @@ use tokio::process::Command;
 use crate::Result;
 use crate::database::db_handle::DbHandle;
 
+use super::RunningUpdate;
 use super::running_build::RunningBuild;
 
 pub(crate) async fn nixos_build_target<B, F, A, TN>(
@@ -59,3 +60,29 @@ where
 
     Ok(())
 }
+
+pub(crate) async fn nix_flake_update<F>(flake_path: F) -> Result<()>
+where
+    F: AsRef<Path>,
+{
+    let mut command = Command::new("nix");
+    command.current_dir(flake_path);
+    command.stdout(Stdio::piped());
+    command.stderr(Stdio::piped());
+    command.stdin(Stdio::null());
+    command.args([
+        "flake",
+        "update",
+        "--log-format",
+        "internal-json",
+        "-vvvvvvvvvvv",
+    ]);
+    command.kill_on_drop(true);
+
+    let child = command.spawn()?;
+
+    let mut running_update = RunningUpdate::new()?;
+    running_update.run_to_completion(child).await?;
+
+    Ok(())
+}