backup: ensure base snapshots are still available after backup

This should never trigger if everything else works correctly, but it is
still a very cheap check to avoid wrongly marking a backup as "OK" when
in fact some chunks might be missing.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/api2/backup.rs

@@ -660,6 +660,7 @@ fn download_previous(
             };
             if let Some(index) = index {
                 env.log(format!("register chunks in '{}' from previous backup.", archive_name));
+                env.register_base_snapshot(last_backup.backup_dir.clone());
 
                 for pos in 0..index.index_count() {
                     let info = index.chunk_info(pos).unwrap();

src/api2/backup/environment.rs

@@ -1,6 +1,6 @@
 use anyhow::{bail, Error};
 use std::sync::{Arc, Mutex};
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 
 use serde_json::{json, Value};
 
@@ -57,6 +57,7 @@ struct SharedBackupState {
     dynamic_writers: HashMap<usize, DynamicWriterState>,
     fixed_writers: HashMap<usize, FixedWriterState>,
     known_chunks: HashMap<[u8;32], u32>,
+    base_snapshots: HashSet<BackupDir>,
 }
 
 impl SharedBackupState {
@@ -108,6 +109,7 @@ impl BackupEnvironment {
             dynamic_writers: HashMap::new(),
             fixed_writers: HashMap::new(),
             known_chunks: HashMap::new(),
+            base_snapshots: HashSet::new(),
         };
 
         Self {
@@ -124,6 +126,13 @@ impl BackupEnvironment {
         }
     }
 
+    /// Register a snapshot as a predecessor of the current backup.
+    /// It's existance will be ensured on finishing.
+    pub fn register_base_snapshot(&self, snap: BackupDir) {
+        let mut state = self.state.lock().unwrap();
+        state.base_snapshots.insert(snap);
+    }
+
     /// Register a Chunk with associated length.
     ///
     /// We do not fully trust clients, so a client may only use registered
@@ -445,6 +454,16 @@ impl BackupEnvironment {
             bail!("backup does not contain valid files (file count == 0)");
         }
 
+        for snap in &state.base_snapshots {
+            let path = self.datastore.snapshot_path(snap);
+            if !path.exists() {
+                bail!(
+                    "base snapshot {} was removed during backup, cannot finish as chunks might be missing",
+                    snap
+                );
+            }
+        }
+
         state.finished = true;
 
         Ok(())

src/backup/backup_info.rs

@@ -217,7 +217,7 @@ impl std::str::FromStr for BackupGroup {
 /// Uniquely identify a Backup (relative to data store)
 ///
 /// We also call this a backup snaphost.
-#[derive(Debug, Eq, PartialEq, Clone)]
+#[derive(Debug, Eq, PartialEq, Hash, Clone)]
 pub struct BackupDir {
     /// Backup group
     group: BackupGroup,