[pbs-devel] [PATCH v2 proxmox-backup 1/4] gc: avoid race between phase1 and forget/prune

[Stefan Reiter <s.reiter@proxmox.com>]

...by saving all forgotten chunks into a HashSet when we detect a GC
phase1 currently running. If GC then hits a NotFound, it can check if
the snapshot was simply forgotten behind its back, or if an actual error
occurred and it needs to abort (since it might delete still referenced
chunks, if the error is transient and the index file is still there).

We have to attach the error message in {fixed,dynamic}_index via the
.context() method, otherwise the original std::io::Error gets lost and
we can't check for NotFound.

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

v2:
* use File::open directly and catch std::io::Error that way, avoid Error.context

 src/backup/datastore.rs | 62 ++++++++++++++++++++++++++++++++++++-----
 1 file changed, 55 insertions(+), 7 deletions(-)

diff --git a/src/backup/datastore.rs b/src/backup/datastore.rs
index 7dd2624c..ca8ca438 100644
--- a/src/backup/datastore.rs
+++ b/src/backup/datastore.rs
@@ -36,6 +36,9 @@ pub struct DataStore {
     chunk_store: Arc<ChunkStore>,
     gc_mutex: Mutex<bool>,
     last_gc_status: Mutex<GarbageCollectionStatus>,
+
+    // bool indicates if phase1 is currently active
+    removed_during_gc: Mutex<(bool, HashSet<PathBuf>)>,
 }
 
 impl DataStore {
@@ -82,6 +85,7 @@ impl DataStore {
             chunk_store: Arc::new(chunk_store),
             gc_mutex: Mutex::new(false),
             last_gc_status: Mutex::new(gc_status),
+            removed_during_gc: Mutex::new((false, HashSet::new())),
         })
     }
 
@@ -236,6 +240,14 @@ impl DataStore {
             _guard = lock_dir_noblock(&full_path, "snapshot", "possibly running or in use")?;
         }
 
+        // Acquire lock and keep it during remove operation, so there's no
+        // chance for a race between adding to the hash and actually removing
+        // the dir (phase1 might otherwise start in-between)
+        let mut removed_guard = self.removed_during_gc.lock().unwrap();
+        if removed_guard.0 {
+            removed_guard.1.insert(self.snapshot_path(&backup_dir));
+        }
+
         log::info!("removing backup snapshot {:?}", full_path);
         std::fs::remove_dir_all(&full_path)
             .map_err(|err| {
@@ -461,12 +473,35 @@ impl DataStore {
             tools::fail_on_shutdown()?;
 
             if let Ok(archive_type) = archive_type(&path) {
-                if archive_type == ArchiveType::FixedIndex {
-                    let index = self.open_fixed_reader(&path)?;
-                    self.index_mark_used_chunks(index, &path, status, worker)?;
-                } else if archive_type == ArchiveType::DynamicIndex {
-                    let index = self.open_dynamic_reader(&path)?;
-                    self.index_mark_used_chunks(index, &path, status, worker)?;
+                let full_path =  self.chunk_store.relative_path(&path);
+
+                match std::fs::File::open(&full_path) {
+                    Ok(file) => {
+                        if archive_type == ArchiveType::FixedIndex {
+                            let index = FixedIndexReader::new(file)?;
+                            self.index_mark_used_chunks(index, &path, status, worker)?;
+                        } else if archive_type == ArchiveType::DynamicIndex {
+                            let index = DynamicIndexReader::new(file)?;
+                            self.index_mark_used_chunks(index, &path, status, worker)?;
+                        }
+                    },
+                    Err(err) => {
+                        if err.kind() == std::io::ErrorKind::NotFound {
+                            let (_, removed_hash) = &*self.removed_during_gc.lock().unwrap();
+                            let backup_dir = path.parent();
+                            if backup_dir.is_some() && removed_hash.contains(backup_dir.unwrap()) {
+                                // index file not found, but we know that it was deleted by
+                                // a concurrent 'forget', so we can safely ignore
+                            } else {
+                                bail!(
+                                    "index file not found but hasn't been removed by forget/prune, aborting GC - {}",
+                                    err
+                                )
+                            }
+                        } else {
+                            bail!(err)
+                        }
+                    }
                 }
             }
             done += 1;
@@ -512,7 +547,20 @@ impl DataStore {
 
             crate::task_log!(worker, "Start GC phase1 (mark used chunks)");
 
-            self.mark_used_chunks(&mut gc_status, worker)?;
+            {
+                let mut guard = self.removed_during_gc.lock().unwrap();
+                guard.0 = true;
+            }
+            let mark_res = self.mark_used_chunks(&mut gc_status, worker);
+            {
+                let mut guard = self.removed_during_gc.lock().unwrap();
+                guard.0 = false;
+                guard.1.clear();
+            }
+
+            if let Err(err) = mark_res {
+                bail!(err);
+            }
 
             crate::task_log!(worker, "Start GC phase2 (sweep unused chunks)");
             self.chunk_store.sweep_unused_chunks(
-- 
2.20.1