[pbs-devel] [PATCH proxmox-backup v3 3/5] fix #3935: datastore/api/backup: move datastore locking to '/run'

[Stefan Sterz <s.sterz@proxmox.com>]

to avoid issues when removing a group or snapshot directory where two
threads hold a lock to the same directory, move locking to the tmpfs
backed '/run' directory and use a flat file structure. also adds
double stat'ing to make it possible to remove locks without certain
race condition issues.

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>
---
 pbs-config/src/lib.rs                |   7 ++
 pbs-datastore/src/backup_info.rs     | 121 +++++++++++++++++++++++----
 pbs-datastore/src/datastore.rs       |   7 +-
 pbs-datastore/src/snapshot_reader.rs |  17 +++-
 src/api2/backup/environment.rs       |   5 +-
 src/backup/verify.rs                 |   4 +-
 6 files changed, 133 insertions(+), 28 deletions(-)

diff --git a/pbs-config/src/lib.rs b/pbs-config/src/lib.rs
index f9b63922..81f106c4 100644
--- a/pbs-config/src/lib.rs
+++ b/pbs-config/src/lib.rs
@@ -21,6 +21,7 @@ pub use config_version_cache::ConfigVersionCache;
 
 use anyhow::{format_err, Error};
 use nix::unistd::{Gid, Group, Uid, User};
+use std::os::unix::prelude::AsRawFd;
 
 pub use pbs_buildcfg::{BACKUP_GROUP_NAME, BACKUP_USER_NAME};
 
@@ -46,6 +47,12 @@ pub fn backup_group() -> Result<nix::unistd::Group, Error> {
 
 pub struct BackupLockGuard(Option<std::fs::File>);
 
+impl AsRawFd for BackupLockGuard {
+    fn as_raw_fd(&self) -> i32 {
+        self.0.as_ref().map_or(-1, |f| f.as_raw_fd())
+    }
+}
+
 #[doc(hidden)]
 /// Note: do not use for production code, this is only intended for tests
 pub unsafe fn create_mocked_lock() -> BackupLockGuard {
diff --git a/pbs-datastore/src/backup_info.rs b/pbs-datastore/src/backup_info.rs
index 0711fcfa..9bbe68ac 100644
--- a/pbs-datastore/src/backup_info.rs
+++ b/pbs-datastore/src/backup_info.rs
@@ -1,14 +1,15 @@
 use std::convert::TryFrom;
 use std::fmt;
-use std::os::unix::io::RawFd;
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::path::Path;
 use std::path::PathBuf;
 use std::sync::Arc;
+use std::time::Duration;
 
 use anyhow::{bail, format_err, Error};
 
-use proxmox_sys::fs::{
-    lock_dir_noblock, lock_dir_noblock_shared, replace_file, CreateOptions, DirLockGuard,
-};
+use proxmox_sys::fs::{replace_file, CreateOptions};
+use proxmox_sys::systemd::escape_unit;
 
 use pbs_api_types::{
     Authid, BackupNamespace, BackupType, GroupFilter, BACKUP_DATE_REGEX, BACKUP_FILE_REGEX,
@@ -20,6 +21,8 @@ use crate::manifest::{
 };
 use crate::{DataBlob, DataStore};
 
+pub const DATASTORE_LOCKS_DIR: &str = "/run/proxmox-backup/locks";
+
 /// BackupGroup is a directory containing a list of BackupDir
 #[derive(Clone)]
 pub struct BackupGroup {
@@ -216,6 +219,8 @@ impl BackupGroup {
             })?;
         }
 
+        let _ = std::fs::remove_file(self.lock_path());
+
         Ok(removed_all_snaps)
     }
 
@@ -232,13 +237,29 @@ impl BackupGroup {
             .set_owner(&self.ns, self.as_ref(), auth_id, force)
     }
 
-    /// Lock a group exclusively
-    pub fn lock(&self) -> Result<DirLockGuard, Error> {
-        lock_dir_noblock(
-            &self.full_group_path(),
-            "backup group",
-            "possible running backup",
-        )
+    /// Returns a file name for locking a group.
+    ///
+    /// The lock file will be located in:
+    /// `${DATASTORE_LOCKS_DIR}/${datastore name}/${lock_file_name_helper(rpath)}`
+    /// where `rpath` is the relative path of the group.
+    ///
+    /// If the group's relative path contains non-Unicode sequences they will be replaced via
+    /// [std::ffi::OsStr::to_string_lossy()].
+    fn lock_path(&self) -> PathBuf {
+        let path = Path::new(DATASTORE_LOCKS_DIR).join(self.store.name());
+
+        let rpath = self.relative_group_path();
+        let rpath = rpath.as_os_str().to_string_lossy();
+
+        path.join(lock_file_name_helper(&rpath))
+    }
+
+    /// Locks a group exclusively.
+    pub fn lock(&self) -> Result<BackupLockGuard, Error> {
+        lock_helper(self.store.name(), &self.lock_path(), |p| {
+            open_backup_lockfile(p, Some(Duration::from_secs(0)), true)
+                .map_err(|err| format_err!("unable to acquire backup group lock {p:?} - {err}"))
+        })
     }
 }
 
@@ -443,14 +464,37 @@ impl BackupDir {
             .map_err(|err| format_err!("unable to acquire manifest lock {:?} - {}", &path, err))
     }
 
-    /// Lock this snapshot exclusively
-    pub fn lock(&self) -> Result<DirLockGuard, Error> {
-        lock_dir_noblock(&self.full_path(), "snapshot", "possibly running or in use")
+    /// Returns a file name for locking a snapshot.
+    ///
+    /// The lock file will be located in:
+    /// `${DATASTORE_LOCKS_DIR}/${datastore name}/${lock_file_name_helper(rpath)}`
+    /// where `rpath` is the relative path of the snapshot.
+    ///
+    /// If the snapshot's relative path contains non-Unicode sequences they will be replaced via
+    /// [std::ffi::OsStr::to_string_lossy()].
+    fn lock_path(&self) -> PathBuf {
+        let path = Path::new(DATASTORE_LOCKS_DIR).join(self.store.name());
+
+        let rpath = self.relative_path();
+        let rpath = rpath.as_os_str().to_string_lossy();
+
+        path.join(lock_file_name_helper(&rpath))
+    }
+
+    /// Locks a snapshot exclusively.
+    pub fn lock(&self) -> Result<BackupLockGuard, Error> {
+        lock_helper(self.store.name(), &self.lock_path(), |p| {
+            open_backup_lockfile(p, Some(Duration::from_secs(0)), true)
+                .map_err(|err| format_err!("unable to acquire snapshot lock {p:?} - {err}"))
+        })
     }
 
-    /// Acquire a shared lock on this snapshot
-    pub fn lock_shared(&self) -> Result<DirLockGuard, Error> {
-        lock_dir_noblock_shared(&self.full_path(), "snapshot", "possibly running or in use")
+    /// Acquires a shared lock on a snapshot.
+    pub fn lock_shared(&self) -> Result<BackupLockGuard, Error> {
+        lock_helper(self.store.name(), &self.lock_path(), |p| {
+            open_backup_lockfile(p, Some(Duration::from_secs(0)), false)
+                .map_err(|err| format_err!("unable to acquire shared snapshot lock {p:?} - {err}"))
+        })
     }
 
     /// Destroy the whole snapshot, bails if it's protected
@@ -473,11 +517,13 @@ impl BackupDir {
             format_err!("removing backup snapshot {:?} failed - {}", full_path, err,)
         })?;
 
-        // the manifest doesn't exist anymore, no need to keep the lock (already done by guard?)
+        // remove no longer needed lock files
         if let Ok(path) = self.manifest_lock_path() {
             let _ = std::fs::remove_file(path); // ignore errors
         }
 
+        let _ = std::fs::remove_file(self.lock_path()); // ignore errors
+
         Ok(())
     }
 
@@ -664,3 +710,42 @@ fn list_backup_files<P: ?Sized + nix::NixPath>(
 
     Ok(files)
 }
+
+/// Encodes a string so it can be used as a lock file name.
+///
+/// The first 64 characters will be the sha256 hash of `path` then a hyphen followed by up to 100
+/// characters of the unit encoded version of `path`.
+fn lock_file_name_helper(path: &str) -> String {
+    let enc = escape_unit(path, true);
+    let from = enc.len().checked_sub(100).unwrap_or(0);
+    let enc = enc[from..].to_string();
+    let hash = hex::encode(openssl::sha::sha256(path.as_bytes()));
+
+    format!("{hash}-{enc}")
+}
+
+/// Helps implement the double stat'ing procedure. It avoids certain race conditions upon lock
+/// deletion.
+///
+/// It also creates the base directory for lock files.
+fn lock_helper<F>(
+    store_name: &str,
+    path: &std::path::Path,
+    lock_fn: F,
+) -> Result<BackupLockGuard, Error>
+where
+    F: Fn(&std::path::Path) -> Result<BackupLockGuard, Error>,
+{
+    let lock_dir = Path::new(DATASTORE_LOCKS_DIR).join(store_name);
+    std::fs::create_dir_all(&lock_dir)?;
+
+    let lock = lock_fn(path)?;
+
+    let stat = nix::sys::stat::fstat(lock.as_raw_fd())?.st_ino;
+
+    if nix::sys::stat::stat(path).map_or(true, |st| stat != st.st_ino) {
+        bail!("could not acquire lock, another thread modified the lock file");
+    }
+
+    Ok(lock)
+}
diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
index f74ea04b..1fc7efa7 100644
--- a/pbs-datastore/src/datastore.rs
+++ b/pbs-datastore/src/datastore.rs
@@ -11,7 +11,6 @@ use nix::unistd::{unlinkat, UnlinkatFlags};
 
 use proxmox_schema::ApiType;
 
-use proxmox_sys::fs::DirLockGuard;
 use proxmox_sys::fs::{file_read_optional_string, replace_file, CreateOptions};
 use proxmox_sys::process_locker::ProcessLockSharedGuard;
 use proxmox_sys::WorkerTaskContext;
@@ -21,7 +20,7 @@ use pbs_api_types::{
     Authid, BackupNamespace, BackupType, ChunkOrder, DataStoreConfig, DatastoreTuning,
     GarbageCollectionStatus, HumanByte, Operation, UPID,
 };
-use pbs_config::ConfigVersionCache;
+use pbs_config::{BackupLockGuard, ConfigVersionCache};
 
 use crate::backup_info::{BackupDir, BackupGroup};
 use crate::chunk_store::ChunkStore;
@@ -615,7 +614,7 @@ impl DataStore {
         ns: &BackupNamespace,
         backup_group: &pbs_api_types::BackupGroup,
         auth_id: &Authid,
-    ) -> Result<(Authid, DirLockGuard), Error> {
+    ) -> Result<(Authid, BackupLockGuard), Error> {
         let backup_group =
             BackupGroup::new(Arc::new(self.clone()), ns.clone(), backup_group.clone());
 
@@ -650,7 +649,7 @@ impl DataStore {
         &self,
         ns: &BackupNamespace,
         backup_dir: &pbs_api_types::BackupDir,
-    ) -> Result<(PathBuf, bool, DirLockGuard), Error> {
+    ) -> Result<(PathBuf, bool, BackupLockGuard), Error> {
         let backup_dir = Arc::new(self.clone()).backup_dir(ns.clone(), backup_dir.clone())?;
         let relative_path = backup_dir.relative_path();
 
diff --git a/pbs-datastore/src/snapshot_reader.rs b/pbs-datastore/src/snapshot_reader.rs
index 8c7eefe1..40083900 100644
--- a/pbs-datastore/src/snapshot_reader.rs
+++ b/pbs-datastore/src/snapshot_reader.rs
@@ -3,8 +3,11 @@ use std::os::unix::io::{AsRawFd, FromRawFd};
 use std::path::Path;
 use std::sync::Arc;
 
-use anyhow::{bail, Error};
+use anyhow::{bail, format_err, Error};
 use nix::dir::Dir;
+use nix::fcntl::OFlag;
+use nix::sys::stat::Mode;
+use pbs_config::BackupLockGuard;
 
 use pbs_api_types::{BackupNamespace, DatastoreWithNamespace, Operation};
 
@@ -23,6 +26,10 @@ pub struct SnapshotReader {
     datastore_name: String,
     file_list: Vec<String>,
     locked_dir: Dir,
+
+    // while this is never read, the lock needs to be kept until the
+    // reader is dropped to ensure valid locking semantics
+    _lock: BackupLockGuard,
 }
 
 impl SnapshotReader {
@@ -42,7 +49,12 @@ impl SnapshotReader {
             ns: snapshot.backup_ns().clone(),
         };
 
-        let locked_dir = snapshot.lock_shared()?;
+        let lock = snapshot.lock_shared()?;
+
+        let path = snapshot.full_path();
+
+        let locked_dir = Dir::open(&path, OFlag::O_RDONLY, Mode::empty())
+            .map_err(|err| format_err!("unable to open snapshot directory {path:?} - {err}"))?;
 
         let datastore_name = datastore.name().to_string();
         let manifest = match snapshot.load_manifest() {
@@ -74,6 +86,7 @@ impl SnapshotReader {
             datastore_name,
             file_list,
             locked_dir,
+            _lock: lock,
         })
     }
 
diff --git a/src/api2/backup/environment.rs b/src/api2/backup/environment.rs
index 8c1c42db..c4280b3f 100644
--- a/src/api2/backup/environment.rs
+++ b/src/api2/backup/environment.rs
@@ -1,5 +1,6 @@
 use anyhow::{bail, format_err, Error};
-use nix::dir::Dir;
+
+use pbs_config::BackupLockGuard;
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
 
@@ -632,7 +633,7 @@ impl BackupEnvironment {
     /// If verify-new is set on the datastore, this will run a new verify task
     /// for the backup. If not, this will return and also drop the passed lock
     /// immediately.
-    pub fn verify_after_complete(&self, snap_lock: Dir) -> Result<(), Error> {
+    pub fn verify_after_complete(&self, snap_lock: BackupLockGuard) -> Result<(), Error> {
         self.ensure_finished()?;
 
         if !self.datastore.verify_new() {
diff --git a/src/backup/verify.rs b/src/backup/verify.rs
index dfe1dc56..2a89ab87 100644
--- a/src/backup/verify.rs
+++ b/src/backup/verify.rs
@@ -1,4 +1,4 @@
-use nix::dir::Dir;
+use pbs_config::BackupLockGuard;
 use std::collections::HashSet;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
@@ -351,7 +351,7 @@ pub fn verify_backup_dir_with_lock(
     backup_dir: &BackupDir,
     upid: UPID,
     filter: Option<&dyn Fn(&BackupManifest) -> bool>,
-    _snap_lock: Dir,
+    _snap_lock: BackupLockGuard,
 ) -> Result<bool, Error> {
     let manifest = match backup_dir.load_manifest() {
         Ok((manifest, _)) => manifest,
-- 
2.30.2