[pbs-devel] [RFC PATCH 0/5] fix #3935: refactor datastore locking to use tmpfs

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

This RFC overhauls the locking mechanism for snapshots and backup
groups in pbs-datastore and wherever groups and snapshots are
accessed. Sending this as an RFC in case I missed places where
locking occurs on groups, snapshots or manifests.

The first patch in this series moves most of the locking into new
functions that are part of the `datastore` trait. These functions
create a mirror of the directory structure under
`/run/proxmox-backup/locks` that is used for locking. The second
patch adds one more locking function that enables shared locks on
groups. This functionality has not been needed so far, but might be
useful in the future. I am unsure whether it should be included.

The third patch moves the `/run/proxmox-backup/locks` path into a
constant in order to be able to switch the lock location more easily
latter. Sending this as its own patch as it also affects manifest
locking in a minor way, unlike the previous patches.

Lastly, patch four and five fix issues that are locking related and
currently present in the codebase. For patch four, see my comment
there. Patch five resolves a race condition when two clients try to
change the owner of a data store via the API. Patch five depends on
patch one, but patch four could be applied independently.

Stefan Sterz (5):
  fix #3935: datastore/api2/backup: move datastore locking to '/run'
  fix #3935: datastore: add shared group lock to datastore
  fix #3935: datastore/config: add a constant for the locking directory
  fix #3935: datastore: keep manifest locks, avoid similar locking issue
  fix: api: avoid race condition in set_backup_owner

 pbs-datastore/src/datastore.rs       | 95 +++++++++++++++++++++-------
 pbs-datastore/src/snapshot_reader.rs | 12 +---
 src/api2/admin/datastore.rs          |  9 ++-
 src/api2/backup/mod.rs               |  5 +-
 src/api2/reader/mod.rs               |  7 +-
 src/backup/verify.rs                 |  8 +--
 6 files changed, 88 insertions(+), 48 deletions(-)

-- 
2.30.2