64 lines
2.0 KiB
Rust
64 lines
2.0 KiB
Rust
 
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//! Ownership lock for a state directory.
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//!
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//! One persistent state directory belongs to exactly one live agent instance.
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//! Checking whether a file exists is not enough — a stale file from a crashed
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//! process must not block a restart, and two concurrently starting agents must
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//! not both win. An advisory OS file lock gives both properties.
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use std::fs::{File, OpenOptions};
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use std::path::{Path, PathBuf};
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use fs4::{FileExt, TryLockError};
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use crate::error::{Error, Result};
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/// An exclusive lock held for the lifetime of an agent.
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///
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/// Dropping it releases the lock, so a cleanly stopped agent leaves the
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/// directory immediately reopenable.
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#[derive(Debug)]
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pub struct DirectoryLock {
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file: File,
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path: PathBuf,
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}
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impl DirectoryLock {
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/// Acquires the lock, failing fast if another live agent holds it.
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pub fn acquire(path: impl AsRef<Path>) -> Result<Self> {
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let path = path.as_ref().to_path_buf();
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let file = OpenOptions::new()
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.create(true)
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.read(true)
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.write(true)
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.truncate(false)
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.open(&path)
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.map_err(|source| Error::Io {
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path: path.clone(),
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source,
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})?;
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super::restrict_permissions(&file, &path)?;
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match FileExt::try_lock(&file) {
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Ok(()) => Ok(Self { file, path }),
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Err(TryLockError::WouldBlock) => Err(Error::StateLocked {
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path: path
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.parent()
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.map(Path::to_path_buf)
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.unwrap_or_else(|| path.clone()),
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}),
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Err(TryLockError::Error(source)) => Err(Error::Io { path, source }),
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}
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}
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/// The path of the lock file.
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pub fn path(&self) -> &Path {
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&self.path
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}
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}
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impl Drop for DirectoryLock {
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fn drop(&mut self) {
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// Best effort: the OS releases the lock when the descriptor closes anyway.
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let _ = FileExt::unlock(&self.file);
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}
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}
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