A0-2244: Remove old junk backups (#1490)

### Description
Changed the backup removal code to remove everything from the backup
stash folder except the subfolder corresponding to the current session.

### Testing
Tested manually - ran for many short sessions, verified finalization,
verified backup folder contents

finality-aleph/src/party/backup.rs

@@ -7,7 +7,7 @@ use std::{
     str::FromStr,
 };
 
-use log::{debug, warn};
+use log::debug;
 
 const BACKUP_FILE_EXTENSION: &str = ".abfts";
 
@@ -118,27 +118,32 @@ pub fn rotate(
     Ok((backup_saver, backup_loader))
 }
 
-/// Removes the backup directory for a session.
+/// Removes the backup directory for all old sessions except the current session.
 ///
 /// `backup_path` is the path to the backup directory (i.e. the argument to `--backup-saving-path`).
 /// If it is `None`, nothing is done.
 ///
-/// Any filesystem errors are logged and dropped.
+/// Any filesystem errors are returned.
 ///
-/// This should be done after the end of the session.
-pub fn remove(path: Option<PathBuf>, session_id: u32) {
-    let path = match path {
-        Some(path) => path.join(session_id.to_string()),
-        None => return,
-    };
-    match fs::remove_dir_all(path) {
-        Ok(()) => {
-            debug!(target: "aleph-party", "Removed backup for session {}", session_id);
-        }
-        Err(err) => {
-            if err.kind() != io::ErrorKind::NotFound {
-                warn!(target: "aleph-party", "Error cleaning up backup for session {}: {}", session_id, err);
-            }
+/// This should be done at the beginning of the new session.
+pub fn remove_old_backups(path: Option<PathBuf>, current_session: u32) -> io::Result<()> {
+    if let Some(path) = path {
+        for read_dir in fs::read_dir(path)? {
+            let item = read_dir?;
+            match item.file_name().to_str() {
+                Some(name) => match name.parse::<u32>() {
+                    Ok(session_id) => {
+                        if session_id < current_session {
+                            fs::remove_dir_all(item.path())?;
+                        }
+                    }
+                    Err(_) => {
+                        debug!(target: "aleph-party", "backup directory contains unexpected data.")
+                    }
+                },
+                None => debug!(target: "aleph-party", "backup directory contains unexpected data."),
+            };
         }
     }
+    Ok(())
 }

finality-aleph/src/party/mod.rs

@@ -73,9 +73,13 @@ where
 
     async fn run_session(&mut self, session_id: SessionId) {
         let last_block = self.session_info.last_block_of_session(session_id);
-        if let Some(previous_session_id) = session_id.0.checked_sub(1) {
+        if session_id.0.checked_sub(1).is_some() {
             let backup_saving_path = self.backup_saving_path.clone();
-            spawn_blocking(move || backup::remove(backup_saving_path, previous_session_id));
+            spawn_blocking(move || {
+                if let Err(e) = backup::remove_old_backups(backup_saving_path, session_id.0) {
+                    warn!(target: "aleph-party", "Error when clearing old backups: {}", e);
+                }
+            });
         }
 
         // Early skip attempt -- this will trigger during catching up (initial sync).