Implement very simple advisory lock mechanism

Blocking versions can't be implemented because they would require
spawning a new thread to avoid jamming up the background thread used for
other syscalls (the jamming up can generate a deadlock which is what
testing found). The blocking version would require spawning a new thread
with a standalone executor but that's horibbly expensive AND runs into
DataDog#448.

glommio/src/io/dma_file.rs

@@ -24,6 +24,7 @@ use std::{
     os::unix::io::{AsRawFd, RawFd},
     path::Path,
     rc::Rc,
+    sync::{Arc, Weak as AWeak},
 };
 
 use super::{
@@ -51,6 +52,66 @@ pub(crate) fn align_down(v: u64, align: u64) -> u64 {
     v & !(align - 1)
 }
 
+/// This guard represents that an advisory lock is being held over some file. See for more information about how
+/// advisory locks work. This is obtained through [DmaFile::try_lock_exclusive] / [DmaFile::try_lock_shared]. If not
+/// explicitly [unlocked](Self::unlock), the advisory lock is released synchronously on Drop.
+#[derive(Debug, Clone)]
+pub struct AdvisoryLockGuard(Option<Arc<OwnedGlommioFile>>);
+
+impl Drop for AdvisoryLockGuard {
+    fn drop(&mut self) {
+        if let Some(mut locked) = self.0.take().and_then(Arc::into_inner) {
+            unsafe { locked.funlock_immediately() }
+        }
+    }
+}
+
+impl AdvisoryLockGuard {
+    /// Explicitly releases the lock. The file can be successfully closed after calling this.
+    pub async fn unlock(mut self) -> Result<()> {
+        let inner = self.0.take().unwrap();
+
+        match Arc::try_unwrap(inner).map(GlommioFile::from) {
+            Ok(lock) => unsafe { lock.funlock() }.await,
+            Err(still_locked) => Err(crate::GlommioError::CanNotBeClosed(
+                crate::ResourceType::File(
+                    still_locked
+                        .path
+                        .as_ref()
+                        .map_or("path has already been taken!".to_string(), |p| {
+                            p.to_string_lossy().to_string()
+                        }),
+                ),
+                "Another clone of this file exists somewhere - cannot close fd",
+            )),
+        }
+    }
+
+    /// Downgrade the lock guard to a weak reference that won't keep the file open. The lock will be released when the
+    /// .
+    pub fn downgrade(&self) -> WeakAdvisoryLockGuard {
+        WeakAdvisoryLockGuard(Arc::downgrade(self.0.as_ref().unwrap()))
+    }
+}
+
+/// This holds a weak reference to an advisory lock. The advisory lock may be dropped while this reference is held
+/// in which case this instance will fail to [upgrade](Self::upgrade).
+#[derive(Default, Debug, Clone)]
+pub struct WeakAdvisoryLockGuard(AWeak<OwnedGlommioFile>);
+
+impl WeakAdvisoryLockGuard {
+    /// Returns a weak lock guard not pointing at anything.
+    pub fn new() -> Self {
+        Default::default()
+    }
+
+    /// Upgrades a weak handle to a held advisory lock back to a strong handle. Returns None if the advisory lock was
+    /// dropped.
+    pub fn upgrade(&self) -> Option<AdvisoryLockGuard> {
+        self.0.upgrade().map(|guard| AdvisoryLockGuard(Some(guard)))
+    }
+}
+
 #[derive(Debug, Clone)]
 /// An asynchronously accessed Direct Memory Access (DMA) file.
 ///
@@ -604,6 +665,96 @@ impl DmaFile {
         self.file.statx().await.map(Into::into)
     }
 
+    /// Tries to acquire a process-wide advisory shared lock on this file instance. If an exclusive advisory lock is
+    /// already held for the underlying file (by any process), this immediately returns with an error. Similarly,
+    /// if a shared lock is already held on this file instance, this returns an error. To acquire multiple shared locks
+    /// in-process on the same file, you must acquire them through files obtained through separate calls to open the
+    /// file.
+    ///
+    /// This is equivalent to [flock](https://linux.die.net/man/2/flock) with `LOCK_SH | LOCK_NB` and requires the
+    /// file have [read](OpenOptions::read) allowed on it.
+    ///
+    /// # File instances and advisory locks.
+    ///
+    /// For the purpose of advisory locks, clones and [dupes](Self::dup) count as being the same file instance (even
+    /// though dupes don't keep the file open). Advisory locks keep the underlying file that they were acquired on open
+    /// while they are held.
+    ///
+    /// # Safety
+    ///
+    /// The simple mechanism here only allows one advisory lock holder for the process for a given file instance
+    /// and that lock must either be [unlocked](AdvisoryLockGuard::unlock).
+    ///
+    /// The reason only a single advisory lock can be held on a given file instance at a time is for safety and
+    /// soundness - the kernel advisory lock is a status bit and any unlock request will unlock the file. That causes
+    /// some danger if you acquire two shared locks and then unlock one of them - the file is unlocked from then on. Or
+    /// worse, you unlock one of them, acquire an exclusive lock and then release what you thought was the shared lock
+    /// but end up releasing the exclusive lock. Similarly, acquiring a shared lock will downgrade if you're holding
+    /// an exclusive lock.
+    ///
+    /// # Usage
+    ///
+    /// The purpose of advisory locks is to synchronize access to a resource. A common use-case is that if you're
+    /// reading to a file, you acquire a shared lock so that any other processes can also continue reading it but you're
+    /// guaranteed that no one trying to write holding an exclusive lock is admitted.
+    ///
+    /// ```
+    ///     /// ```no_run
+    /// use glommio::{
+    ///   LocalExecutor,
+    ///   io::{
+    ///     OpenOptions,
+    ///     DmaBuffer,
+    ///   }
+    /// };
+    ///
+    /// let ex = LocalExecutor::default();
+    /// ex.run(async {
+    ///     // A new anonymous file is created within `some_directory/`.
+    ///     let file = OpenOptions::new()
+    ///       .create_new(true)
+    ///       .read(true)
+    ///       .write(true)
+    ///       .tmpfile(true)
+    ///       .dma_open("some_directory")
+    ///       .await
+    ///       .unwrap();
+    ///
+    ///     let guard = file.try_lock_shared().await.unwrap();
+    ///
+    ///     // Until the guard is dropped, no other process on the file system can acquire an advisory read lock
+    ///     // on the file.
+    ///
+    ///     // File won't close until you either unlock the guard, drop it, or give it back as part of closing.
+    ///     guard.unlock().await;
+    /// });
+    /// ```
+    ///
+    /// NOTE:
+    /// [Currently](https://github.com/axboe/liburing/issues/85) flock isn't implemented through io_uring so it requires
+    /// dispatching to a background thread.
+    pub async fn try_lock_shared(&self) -> Result<AdvisoryLockGuard> {
+        self.file
+            .try_lock_shared()
+            .await
+            .map(|f| AdvisoryLockGuard(Some(Arc::new(f))))
+    }
+
+    /// Tries to acquire an advisory exclusive lock on this file instance. If successful, then the OS advisory lock
+    /// is acquired. If failed, it's either because the OS advisory lock is held as shared OR because this file instance
+    /// already handed out a lock (or is in the process of handing one out) - either exclusive or shared.
+    ///
+    /// This is equivalent to [flock](https://linux.die.net/man/2/flock) with `LOCK_EX | LOCK_NB` and requires the
+    /// file have [write](OpenOptions::write) allowed on it.
+    ///
+    /// See [try_lock_shared](Self::try_lock_shared) for more details.
+    pub async fn try_lock_exclusive(&self) -> Result<AdvisoryLockGuard> {
+        self.file
+            .try_lock_exclusive()
+            .await
+            .map(|f| AdvisoryLockGuard(Some(Arc::new(f))))
+    }
+
     /// Attempt to confirm no other clones of this file exist. If no clones exist,
     /// Ok(self) is returned. If clones remain, Err(self) is returned. Do not use
     /// this unless you are implementing a polling mechanism to determine when it's
@@ -950,7 +1101,7 @@ impl WeakDmaFile {
     }
 
     /// The major ID of the device containing the filesystem where the file resides.
-    /// The device may be found by issuing a `readlink`` on `/sys/dev/block/<major>:<minor>`
+    /// The device may be found by issuing a `readlink` on `/sys/dev/block/<major>:<minor>`
     pub fn dev_major(&self) -> u32 {
         self.file.dev_major
     }
@@ -972,8 +1123,11 @@ impl WeakDmaFile {
 pub(crate) mod test {
     use super::*;
     use crate::{
-        enclose, test_utils::make_test_directories, ByteSliceMutExt, GlommioError, Latency,
-        LocalExecutor, ResourceType, Shares,
+        enclose,
+        sync::Semaphore,
+        test_utils::make_test_directories,
+        timer::{sleep, timeout},
+        ByteSliceMutExt, GlommioError, Latency, LocalExecutor, ResourceType, Shares,
     };
     use futures::join;
     use futures_lite::{stream, StreamExt};
@@ -2208,4 +2362,197 @@ pub(crate) mod test {
 
         assert_eq!(weak.strong_count(), 0);
     });
+
+    dma_file_test!(advisory_lock_exclusive_wait_for_exclusive, path, _k, {
+        let file = OpenOptions::new()
+            .create_new(true)
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let same_path_different_file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let duped = file.dup().unwrap();
+
+        let guard = file.try_lock_exclusive().await.unwrap();
+        let waiting_on_exclusive_lock_attempted = std::rc::Rc::new(Semaphore::new(0));
+
+        let notify_exclusive_lock_attempted = waiting_on_exclusive_lock_attempted.clone();
+        let acquire_exclusive = crate::spawn_local(timeout(Duration::from_millis(500), async move {
+            assert!(same_path_different_file.try_lock_exclusive().await.is_err(), "Shouldn't be able to acquire an exclusive lock on the same path while holding it through a different file");
+            notify_exclusive_lock_attempted.signal(1);
+            eprintln!("Starting to wait for exclusive lock");
+            loop {
+                if let Ok(locked) = same_path_different_file.try_lock_exclusive().await {
+                    return Ok(locked)
+                }
+                sleep(Duration::from_millis(5)).await;
+            }
+        })).detach();
+
+        duped.try_lock_shared().await.expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+        duped.try_lock_exclusive().await.expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+
+        let file = file.try_take_last_clone().expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+        let weak = file.downgrade();
+        std::mem::drop(file);
+        assert!(
+            weak.upgrade().is_some(),
+            "Guard should be keeping the file open"
+        );
+
+        eprintln!("Waiting for exclusive lock to have been attempted");
+        waiting_on_exclusive_lock_attempted
+            .acquire(1)
+            .await
+            .unwrap();
+        eprintln!("Dropping guard");
+        guard.unlock().await.unwrap();
+
+        assert!(
+            weak.upgrade().is_none(),
+            "Guard unlocking should have implicitly caused the file to close"
+        );
+
+        timeout(Duration::from_millis(15), async move {
+            acquire_exclusive.await.expect("Task not cancelled")
+        })
+        .await
+        .expect("Exclusive lock should have been automatically acquired");
+    });
+
+    dma_file_test!(advisory_lock_shared_wait_for_exclusive, path, _k, {
+        let file = OpenOptions::new()
+            .create_new(true)
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let same_path_different_file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let duped = file.dup().unwrap();
+
+        let guard = file.try_lock_exclusive().await.unwrap();
+        let waiting_on_shared_lock = std::rc::Rc::new(Semaphore::new(0));
+
+        let notify_waiting_on_shared_lock = waiting_on_shared_lock.clone();
+        let acquire_exclusive = crate::spawn_local(timeout(Duration::from_millis(500), async move {
+            assert!(same_path_different_file.try_lock_shared().await.is_err(), "Shouldn't be able to acquire a shared lock on the same path while holding an exclusive lock through a different file");
+            notify_waiting_on_shared_lock.signal(1);
+            loop {
+                if let Ok(locked) = same_path_different_file.try_lock_shared().await {
+                    return Ok(locked);
+                }
+                sleep(Duration::from_millis(5)).await;
+            }
+        })).detach();
+
+        duped.try_lock_shared().await.expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+        duped.try_lock_exclusive().await.expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+
+        guard.unlock().await.unwrap();
+
+        timeout(Duration::from_millis(15), async move {
+            acquire_exclusive.await.expect("Task not cancelled")
+        })
+        .await
+        .expect("Shared lock should have been automatically acquired");
+    });
+
+    dma_file_test!(advisory_lock_shared, path, _k, {
+        let file = OpenOptions::new()
+            .create_new(true)
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let same_path_different_file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let duped = file.dup().unwrap();
+
+        let guard = file.try_lock_shared().await.unwrap();
+
+        duped.try_lock_shared().await.expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+        duped.try_lock_exclusive().await.expect_err(
+            "Shouldn't be able to acquire a lock on a dupe while holding it on the parent",
+        );
+
+        let guard2 = same_path_different_file
+            .try_lock_shared()
+            .await
+            .expect("Should be able to acquire a shared lock through a different open instance");
+
+        guard.unlock().await.unwrap();
+
+        duped.try_lock_exclusive().await.expect_err(
+            "Shouldn't be able to acquire an exclusive lock while holding a shared lock",
+        );
+
+        guard2.unlock().await.unwrap();
+
+        duped
+            .try_lock_exclusive()
+            .await
+            .expect("Locked exclusively once the shared lock was released");
+    });
+
+    dma_file_test!(advisory_weak_lock, path, _k, {
+        let file = OpenOptions::new()
+            .create_new(true)
+            .read(true)
+            .write(true)
+            .dma_open(path.join("testfile"))
+            .await
+            .unwrap();
+
+        let exclusive = file.try_lock_exclusive().await.unwrap();
+        let exclusive_clone = exclusive.clone();
+        let weak_exclusive = exclusive.downgrade();
+
+        assert!(weak_exclusive.upgrade().is_some());
+
+        exclusive
+            .unlock()
+            .await
+            .expect_err("Shouldn't be able to unlock while multiple lock references");
+
+        assert!(weak_exclusive.upgrade().is_some());
+
+        exclusive_clone.unlock().await.unwrap();
+
+        assert!(weak_exclusive.upgrade().is_none());
+    });
 }