[pbs-devel] [PATCH proxmox-backup 2/2] api: backup: never hold mutex guard when doing manifest update

[Christian Ebner]

An manifest update with s3 backend will call async code, which must
be avoided because of possible deadlocks [0]. Therefore, perform all
changes on the shared backup state and drop the guard before
updating the manifest, which performs the backend specific update.
Dropping the guard prematurely is fine, as the state has already been
set to be finished, so no other api calls belonging to the same
backup task cannot perform further changes anyways.

[0] https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html#which-kind-of-mutex-should-you-use

Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
---
 src/api2/backup/environment.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/api2/backup/environment.rs b/src/api2/backup/environment.rs
index e535891a4..073027c51 100644
--- a/src/api2/backup/environment.rs
+++ b/src/api2/backup/environment.rs
@@ -682,8 +682,15 @@ impl BackupEnvironment {
             }
         }
 
-        // check for valid manifest and store stats
         let stats = serde_json::to_value(state.backup_stat)?;
+
+        // marks the backup state as finished, so no other api calls can modify its state anymore
+        state.finished = true;
+
+        // never hold mutex guard during s3 upload due to possible deadlocks
+        drop(state);
+
+        // check for valid manifest and store stats
         self.backup_dir
             .update_manifest(&self.backend, |manifest| {
                 manifest.unprotected["chunk_upload_stats"] = stats;
@@ -692,9 +699,6 @@ impl BackupEnvironment {
 
         self.datastore.try_ensure_sync_level()?;
 
-        // marks the backup as successful
-        state.finished = true;
-
         Ok(())
     }
 
-- 
2.47.3