[pbs-devel] [PATCH proxmox] sys: open process_locker lockfile lazy

[Gabriel Goller <g.goller@proxmox.com>]

When setting a datastore in maintenance mode (offline or read-only) we
should be able to unmount it. This isn't possible because the
`ChunkReader` has a `ProcessLocker` instance that holds an open
file descriptor to (f.e.) `/mnt/datastore/test1/.lock`.

The `ChunkReader` is created at startup, so if the datastore is not set
to a maintenance mode at startup, we always have the lockfile open.
Now we create/open the lockfile lazy, when a shared lock or a exclusive
lock is wanted. Like this, we can set a datastore to 'offline' and
unmount it without restarting the proxmox-backup service.

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
---
 proxmox-sys/src/process_locker.rs | 79 +++++++++++++++++++++++--------
 1 file changed, 58 insertions(+), 21 deletions(-)

diff --git a/proxmox-sys/src/process_locker.rs b/proxmox-sys/src/process_locker.rs
index 4874da8..b612317 100644
--- a/proxmox-sys/src/process_locker.rs
+++ b/proxmox-sys/src/process_locker.rs
@@ -8,7 +8,9 @@
 //! `oldest_shared_lock()`.
 
 use std::collections::HashMap;
+use std::fs::File;
 use std::os::unix::io::AsRawFd;
+use std::path::PathBuf;
 use std::sync::{Arc, Mutex};
 
 use anyhow::{bail, Error};
@@ -19,7 +21,8 @@ use anyhow::{bail, Error};
 
 /// Inter-process reader-writer lock
 pub struct ProcessLocker {
-    file: std::fs::File,
+    path: PathBuf,
+    file_descriptor: Option<File>,
     exclusive: bool,
     writers: usize,
     next_guard_id: u64,
@@ -53,11 +56,16 @@ 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))
-            {
-                panic!("unable to drop writer lock - {}", err);
+            if let Some(file) = &data.file_descriptor {
+                if let Err(err) =
+                    nix::fcntl::fcntl(file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op))
+                {
+                    panic!("unable to drop writer lock - {}", err);
+                }
+            } else {
+                panic!("file descriptor has been closed while locking");
             }
+            data.file_descriptor = None;
         }
         if data.writers > 0 {
             data.writers -= 1;
@@ -93,29 +101,31 @@ 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))
-        {
-            panic!("unable to drop exclusive lock - {}", err);
+        if let Some(file) = &data.file_descriptor {
+            if let Err(err) =
+                nix::fcntl::fcntl(file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op))
+            {
+                panic!("unable to drop exclusive lock - {}", err);
+            }
+        } else {
+            panic!("file descriptor has been closed while locking");
         }
 
+        println!("closing exclusive file lock");
+        data.file_descriptor = None;
         data.exclusive = false;
     }
 }
 
 impl ProcessLocker {
-    /// Create a new instance for the specified file.
+    /// Create a new instance of the 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 file = std::fs::OpenOptions::new()
-            .create(true)
-            .read(true)
-            .write(true)
-            .open(lockfile)?;
-
+    /// Does not check if the lockfile exists. The lockfile gets opened/created
+    /// when using [Self::try_shared_lock()] or [Self::try_exclusive_lock()].
+    pub fn new(lockfile: PathBuf) -> Result<Arc<Mutex<Self>>, Error> {
         Ok(Arc::new(Mutex::new(Self {
-            file,
+            path: lockfile,
+            file_descriptor: None,
             exclusive: false,
             writers: 0,
             next_guard_id: 0,
@@ -144,7 +154,20 @@ impl ProcessLocker {
         let mut data = locker.lock().unwrap();
 
         if data.writers == 0 && !data.exclusive {
-            if let Err(err) = Self::try_lock(&data.file, libc::F_RDLCK) {
+            if data.file_descriptor.is_none() {
+                let file = std::fs::OpenOptions::new()
+                    .create(true)
+                    .read(true)
+                    .write(true)
+                    .open(&data.path)?;
+                data.file_descriptor = Some(file);
+            }
+            if let Err(err) = Self::try_lock(
+                data.file_descriptor
+                    .as_ref()
+                    .expect("unable to open lockfile"),
+                libc::F_RDLCK,
+            ) {
                 bail!("unable to get shared lock - {}", err);
             }
         }
@@ -187,7 +210,21 @@ impl ProcessLocker {
             bail!("already locked exclusively");
         }
 
-        if let Err(err) = Self::try_lock(&data.file, libc::F_WRLCK) {
+        if data.file_descriptor.is_none() {
+            let file = std::fs::OpenOptions::new()
+                .create(true)
+                .read(true)
+                .write(true)
+                .open(&data.path)?;
+            data.file_descriptor = Some(file);
+        }
+        println!("locking exclusive lock: {:?}", &data.file_descriptor);
+        if let Err(err) = Self::try_lock(
+            data.file_descriptor
+                .as_ref()
+                .expect("unable to open lockfile"),
+            libc::F_WRLCK,
+        ) {
             bail!("unable to get exclusive lock - {}", err);
         }
 
-- 
2.39.2