cli: log startup and shutdown, don't clear service logs on restart

Fixes microsoft#183696

Author: connor4312

cli/src/commands/tunnels.rs

@@ -345,6 +345,13 @@ async fn serve_with_csa(
 	log = log.tee(log_broadcast.clone());
 	log::install_global_logger(log.clone()); // re-install so that library logs are captured
 
+	debug!(
+		log,
+		"Starting tunnel with `{} {}`",
+		APPLICATION_NAME,
+		std::env::args().collect::<Vec<_>>().join(" ")
+	);
+
 	// Intentionally read before starting the server. If the server updated and
 	// respawn is requested, the old binary will get renamed, and then
 	// current_exe will point to the wrong path.
@@ -435,7 +442,10 @@ async fn serve_with_csa(
 
 				return Ok(exit.code().unwrap_or(1));
 			}
-			Next::Exit => return Ok(0),
+			Next::Exit => {
+				debug!(log, "Tunnel shut down");
+				return Ok(0);
+			}
 			Next::Restart => continue,
 		}
 	}

cli/src/log.rs

@@ -159,9 +159,21 @@ pub struct FileLogSink {
 	file: Arc<std::sync::Mutex<std::fs::File>>,
 }
 
+const FILE_LOG_SIZE_LIMIT: u64 = 1024 * 1024 * 10; // 10MB
+
 impl FileLogSink {
 	pub fn new(level: Level, path: &Path) -> std::io::Result<Self> {
-		let file = std::fs::File::create(path)?;
+		// Truncate the service log occasionally to avoid growing infinitely
+		if matches!(path.metadata(), Ok(m) if m.len() > FILE_LOG_SIZE_LIMIT) {
+			// ignore errors, can happen if another process is writing right now
+			let _ = std::fs::remove_file(path);
+		}
+
+		let file = std::fs::OpenOptions::new()
+			.append(true)
+			.create(true)
+			.open(path)?;
+
 		Ok(Self {
 			level,
 			file: Arc::new(std::sync::Mutex::new(file)),