[pbs-devel] [PATCH v2 proxmox 1/2] process_locker: use ofd locking

[Gabriel Goller]

Instead of using normal fcntl locking, use the OFD (open file
descriptor) flags [0].
This blocks the file on the file-descriptor level, not on the
process-level.

This solves two problems:

-  If a process closes any file descriptor referring to a file,
  then all of the process's locks on that file are released,
  regardless of the file descriptor(s) on which the locks were
  obtained.  This is bad: it means that a process can lose its
  locks on a file such as /etc/passwd or /etc/mtab when for some
  reason a library function decides to open, read, and close the
  same file.

-  The threads in a process share locks.  In other words, a
  multithreaded program can't use record locking to ensure that
  threads don't simultaneously access the same region of a file.

(Copied from [0])

Added a `create_dir_all()` call to create the folders if they are not
existing.

[0]: https://man7.org/linux/man-pages/man2/fcntl.2.html

Signed-off-by: Gabriel Goller <g.goller at proxmox.com>
---
 proxmox-sys/src/process_locker.rs | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/proxmox-sys/src/process_locker.rs b/proxmox-sys/src/process_locker.rs
index 4874da8..58deacb 100644
--- a/proxmox-sys/src/process_locker.rs
+++ b/proxmox-sys/src/process_locker.rs
@@ -1,7 +1,7 @@
 //! Inter-process reader-writer lock builder.
 //!
 //! This implementation uses fcntl record locks with non-blocking
-//! F_SETLK command (never blocks).
+//! F_OFD_SETLK command (never blocks).
 //!
 //! We maintain a map of shared locks with time stamps, so you can get
 //! the timestamp for the oldest open lock with
@@ -13,8 +13,6 @@ use std::sync::{Arc, Mutex};
 
 use anyhow::{bail, Error};
 
-// fixme: use F_OFD_ locks when implemented with nix::fcntl
-
 // Note: flock lock conversion is not atomic, so we need to use fcntl
 
 /// Inter-process reader-writer lock
@@ -53,9 +51,10 @@ impl Drop for ProcessLockSharedGuard {
                 l_pid: 0,
             };
 
-            if let Err(err) =
-                nix::fcntl::fcntl(data.file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op))
-            {
+            if let Err(err) = nix::fcntl::fcntl(
+                data.file.as_raw_fd(),
+                nix::fcntl::FcntlArg::F_OFD_SETLKW(&op),
+            ) {
                 panic!("unable to drop writer lock - {}", err);
             }
         }
@@ -93,9 +92,10 @@ impl Drop for ProcessLockExclusiveGuard {
             l_pid: 0,
         };
 
-        if let Err(err) =
-            nix::fcntl::fcntl(data.file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op))
-        {
+        if let Err(err) = nix::fcntl::fcntl(
+            data.file.as_raw_fd(),
+            nix::fcntl::FcntlArg::F_OFD_SETLKW(&op),
+        ) {
             panic!("unable to drop exclusive lock - {}", err);
         }
 
@@ -108,6 +108,12 @@ impl ProcessLocker {
     ///
     /// This simply creates the file if it does not exist.
     pub fn new<P: AsRef<std::path::Path>>(lockfile: P) -> Result<Arc<Mutex<Self>>, Error> {
+        let parent = lockfile
+            .as_ref()
+            .parent()
+            .ok_or(anyhow::anyhow!("Failed getting parent dir of locking file"))?;
+        std::fs::create_dir_all(parent)?;
+
         let file = std::fs::OpenOptions::new()
             .create(true)
             .read(true)
@@ -132,7 +138,7 @@ impl ProcessLocker {
             l_pid: 0,
         };
 
-        nix::fcntl::fcntl(file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLK(&op))?;
+        nix::fcntl::fcntl(file.as_raw_fd(), nix::fcntl::FcntlArg::F_OFD_SETLK(&op))?;
 
         Ok(())
     }
-- 
2.39.2