fix 'failed to start transient scope unit' in daemon start

Reviewed By: blackm00n

Differential Revision: D71620633

fbshipit-source-id: 42149beaa9ec4ee792bc9f563cd012f533d8c15c

Author: Yury Samkevich

app/buck2_client_ctx/src/daemon/client/connect.rs

@@ -396,6 +396,9 @@ impl<'a> BuckdLifecycle<'a> {
                 ParentSlice::Root(slice_name.clone()),
             ))?;
         let mut cmd = if let Some(systemd_runner) = &systemd_runner {
+            systemd_runner
+                .ensure_scope_stopped(&format!("{}.scope", &slice_name))
+                .await?;
             systemd_runner
                 .background_command_linux(daemon_exe, &slice_name, &project_dir.root())
                 .into()

app/buck2_common/src/systemd.rs

@@ -15,6 +15,7 @@ use std::sync::OnceLock;
 
 use buck2_core::fs::paths::abs_norm_path::AbsNormPath;
 use buck2_util::process;
+use tracing::warn;
 
 use crate::init::ResourceControlConfig;
 use crate::init::ResourceControlStatus;
@@ -219,6 +220,28 @@ impl SystemdRunner {
         }
         Ok(())
     }
+
+    pub async fn ensure_scope_stopped(&self, scope: &str) -> buck2_error::Result<()> {
+        let mut cmd = process::async_background_command("systemctl");
+        cmd.arg("is-active").arg("--quiet").arg("--user").arg(scope);
+        // systemctl returns no error if scope is active
+        let is_active = cmd.status().await?.success();
+        if is_active {
+            warn!(
+                "Transient scope unit {} is already active. Stopping before start a new one.",
+                scope
+            );
+            self.stop_scope(scope).await?;
+        }
+        Ok(())
+    }
+
+    async fn stop_scope(&self, scope: &str) -> buck2_error::Result<()> {
+        let mut cmd = process::async_background_command("systemctl");
+        cmd.arg("stop").arg("--user").arg(scope);
+        cmd.spawn()?.wait().await?;
+        Ok(())
+    }
 }
 
 // Helper function to replace a special characters in a cgroup unit name