appender: prune old files at startup

The prune_old_logs function only gets called when we rollover the
appender. The problem is that at startup we create an initial log file
without rolling over, if e.g., the executable then exits before rolling
over for the first time, we never call prune_old_logs.

E.g.: If we have a tracing-appender that rolls over every minute and we
have a max_files parameter of 2. Running the program (which prints a
single log line and exits) every minute, will result in a new log file
everytime without deleting the old ones.

Fixes: tokio-rs#2937

tracing-appender/src/rolling.rs

@@ -543,6 +543,11 @@ impl Inner {
             rotation,
             max_files,
         };
+
+        if let Some(max_files) = max_files {
+            inner.prune_old_logs(max_files);
+        }
+
         let filename = inner.join_date(&now);
         let writer = RwLock::new(create_writer(inner.log_directory.as_ref(), &filename)?);
         Ok((inner, writer))