rhg: Initial repository locking
Initial Rust implementation of locking based on the `.hg/wlock` symlink (or file),
with lock breaking when the recorded pid and hostname show that a lock was
left by a local process that is not running anymore (as it might have been
killed).
Differential Revision: https://phab.mercurial-scm.org/D11835
--- a/rust/hg-core/src/lib.rs Mon Nov 29 17:37:08 2021 +0100
+++ b/rust/hg-core/src/lib.rs Mon Mar 22 09:07:10 2021 +0100
@@ -29,6 +29,7 @@
pub mod revlog;
pub use revlog::*;
pub mod config;
+pub mod lock;
pub mod logging;
pub mod operations;
pub mod revset;
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/lock.rs Mon Mar 22 09:07:10 2021 +0100
@@ -0,0 +1,187 @@
+//! Filesystem-based locks for local repositories
+
+use crate::errors::HgError;
+use crate::errors::HgResultExt;
+use crate::utils::StrExt;
+use crate::vfs::Vfs;
+use std::io;
+use std::io::ErrorKind;
+
+#[derive(derive_more::From)]
+pub enum LockError {
+ AlreadyHeld,
+ #[from]
+ Other(HgError),
+}
+
+/// Try to call `f` with the lock acquired, without waiting.
+///
+/// If the lock is aready held, `f` is not called and `LockError::AlreadyHeld`
+/// is returned. `LockError::Io` is returned for any unexpected I/O error
+/// accessing the lock file, including for removing it after `f` was called.
+/// The return value of `f` is dropped in that case. If all is successful, the
+/// return value of `f` is forwarded.
+pub fn try_with_lock_no_wait<R>(
+ hg_vfs: Vfs,
+ lock_filename: &str,
+ f: impl FnOnce() -> R,
+) -> Result<R, LockError> {
+ let our_lock_data = &*OUR_LOCK_DATA;
+ for _retry in 0..5 {
+ match make_lock(hg_vfs, lock_filename, our_lock_data) {
+ Ok(()) => {
+ let result = f();
+ unlock(hg_vfs, lock_filename)?;
+ return Ok(result);
+ }
+ Err(HgError::IoError { error, .. })
+ if error.kind() == ErrorKind::AlreadyExists =>
+ {
+ let lock_data = read_lock(hg_vfs, lock_filename)?;
+ if lock_data.is_none() {
+ // Lock was apparently just released, retry acquiring it
+ continue;
+ }
+ if !lock_should_be_broken(&lock_data) {
+ return Err(LockError::AlreadyHeld);
+ }
+ // The lock file is left over from a process not running
+ // anymore. Break it, but with another lock to
+ // avoid a race.
+ break_lock(hg_vfs, lock_filename)?;
+
+ // Retry acquiring
+ }
+ Err(error) => Err(error)?,
+ }
+ }
+ Err(LockError::AlreadyHeld)
+}
+
+fn break_lock(hg_vfs: Vfs, lock_filename: &str) -> Result<(), LockError> {
+ try_with_lock_no_wait(hg_vfs, &format!("{}.break", lock_filename), || {
+ // Check again in case some other process broke and
+ // acquired the lock in the meantime
+ let lock_data = read_lock(hg_vfs, lock_filename)?;
+ if !lock_should_be_broken(&lock_data) {
+ return Err(LockError::AlreadyHeld);
+ }
+ Ok(hg_vfs.remove_file(lock_filename)?)
+ })?
+}
+
+#[cfg(unix)]
+fn make_lock(
+ hg_vfs: Vfs,
+ lock_filename: &str,
+ data: &str,
+) -> Result<(), HgError> {
+ // Use a symbolic link because creating it is atomic.
+ // The link’s "target" contains data not representing any path.
+ let fake_symlink_target = data;
+ hg_vfs.create_symlink(lock_filename, fake_symlink_target)
+}
+
+fn read_lock(
+ hg_vfs: Vfs,
+ lock_filename: &str,
+) -> Result<Option<String>, HgError> {
+ let link_target =
+ hg_vfs.read_link(lock_filename).io_not_found_as_none()?;
+ if let Some(target) = link_target {
+ let data = target
+ .into_os_string()
+ .into_string()
+ .map_err(|_| HgError::corrupted("non-UTF-8 lock data"))?;
+ Ok(Some(data))
+ } else {
+ Ok(None)
+ }
+}
+
+fn unlock(hg_vfs: Vfs, lock_filename: &str) -> Result<(), HgError> {
+ hg_vfs.remove_file(lock_filename)
+}
+
+/// Return whether the process that is/was holding the lock is known not to be
+/// running anymore.
+fn lock_should_be_broken(data: &Option<String>) -> bool {
+ (|| -> Option<bool> {
+ let (prefix, pid) = data.as_ref()?.split_2(':')?;
+ if prefix != &*LOCK_PREFIX {
+ return Some(false);
+ }
+ let process_is_running;
+
+ #[cfg(unix)]
+ {
+ let pid: libc::pid_t = pid.parse().ok()?;
+ unsafe {
+ let signal = 0; // Test if we could send a signal, without sending
+ let result = libc::kill(pid, signal);
+ if result == 0 {
+ process_is_running = true
+ } else {
+ let errno =
+ io::Error::last_os_error().raw_os_error().unwrap();
+ process_is_running = errno != libc::ESRCH
+ }
+ }
+ }
+
+ Some(!process_is_running)
+ })()
+ .unwrap_or(false)
+}
+
+lazy_static::lazy_static! {
+ /// A string which is used to differentiate pid namespaces
+ ///
+ /// It's useful to detect "dead" processes and remove stale locks with
+ /// confidence. Typically it's just hostname. On modern linux, we include an
+ /// extra Linux-specific pid namespace identifier.
+ static ref LOCK_PREFIX: String = {
+ // Note: this must match the behavior of `_getlockprefix` in `mercurial/lock.py`
+
+ /// Same as https://github.com/python/cpython/blob/v3.10.0/Modules/socketmodule.c#L5414
+ const BUFFER_SIZE: usize = 1024;
+ let mut buffer = [0_i8; BUFFER_SIZE];
+ let hostname_bytes = unsafe {
+ let result = libc::gethostname(buffer.as_mut_ptr(), BUFFER_SIZE);
+ if result != 0 {
+ panic!("gethostname: {}", io::Error::last_os_error())
+ }
+ std::ffi::CStr::from_ptr(buffer.as_mut_ptr()).to_bytes()
+ };
+ let hostname =
+ std::str::from_utf8(hostname_bytes).expect("non-UTF-8 hostname");
+
+ #[cfg(target_os = "linux")]
+ {
+ use std::os::linux::fs::MetadataExt;
+ match std::fs::metadata("/proc/self/ns/pid") {
+ Ok(meta) => {
+ return format!("{}/{:x}", hostname, meta.st_ino())
+ }
+ Err(error) => {
+ // TODO: match on `error.kind()` when `NotADirectory`
+ // is available on all supported Rust versions:
+ // https://github.com/rust-lang/rust/issues/86442
+ use libc::{
+ ENOENT, // ErrorKind::NotFound
+ ENOTDIR, // ErrorKind::NotADirectory
+ EACCES, // ErrorKind::PermissionDenied
+ };
+ match error.raw_os_error() {
+ Some(ENOENT) | Some(ENOTDIR) | Some(EACCES) => {}
+ _ => panic!("stat /proc/self/ns/pid: {}", error),
+ }
+ }
+ }
+ }
+
+ hostname.to_owned()
+ };
+
+ static ref OUR_LOCK_DATA: String = format!("{}:{}", &*LOCK_PREFIX, std::process::id());
+}
--- a/rust/hg-core/src/repo.rs Mon Nov 29 17:37:08 2021 +0100
+++ b/rust/hg-core/src/repo.rs Mon Mar 22 09:07:10 2021 +0100
@@ -6,6 +6,7 @@
use crate::errors::HgError;
use crate::errors::HgResultExt;
use crate::exit_codes;
+use crate::lock::{try_with_lock_no_wait, LockError};
use crate::manifest::{Manifest, Manifestlog};
use crate::revlog::filelog::Filelog;
use crate::revlog::revlog::RevlogError;
@@ -243,6 +244,13 @@
}
}
+ pub fn try_with_wlock_no_wait<R>(
+ &self,
+ f: impl FnOnce() -> R,
+ ) -> Result<R, LockError> {
+ try_with_lock_no_wait(self.hg_vfs(), "wlock", f)
+ }
+
pub fn has_dirstate_v2(&self) -> bool {
self.requirements
.contains(requirements::DIRSTATE_V2_REQUIREMENT)
--- a/rust/hg-core/src/utils.rs Mon Nov 29 17:37:08 2021 +0100
+++ b/rust/hg-core/src/utils.rs Mon Mar 22 09:07:10 2021 +0100
@@ -145,6 +145,21 @@
}
}
+pub trait StrExt {
+ // TODO: Use https://doc.rust-lang.org/nightly/std/primitive.str.html#method.split_once
+ // once we require Rust 1.52+
+ fn split_2(&self, separator: char) -> Option<(&str, &str)>;
+}
+
+impl StrExt for str {
+ fn split_2(&self, separator: char) -> Option<(&str, &str)> {
+ let mut iter = self.splitn(2, separator);
+ let a = iter.next()?;
+ let b = iter.next()?;
+ Some((a, b))
+ }
+}
+
pub trait Escaped {
/// Return bytes escaped for display to the user
fn escaped_bytes(&self) -> Vec<u8>;
--- a/rust/hg-core/src/vfs.rs Mon Nov 29 17:37:08 2021 +0100
+++ b/rust/hg-core/src/vfs.rs Mon Mar 22 09:07:10 2021 +0100
@@ -87,6 +87,26 @@
std::fs::rename(&from, &to)
.with_context(|| IoErrorContext::RenamingFile { from, to })
}
+
+ pub fn remove_file(
+ &self,
+ relative_path: impl AsRef<Path>,
+ ) -> Result<(), HgError> {
+ let path = self.join(relative_path);
+ std::fs::remove_file(&path)
+ .with_context(|| IoErrorContext::RemovingFile(path))
+ }
+
+ #[cfg(unix)]
+ pub fn create_symlink(
+ &self,
+ relative_link_path: impl AsRef<Path>,
+ target_path: impl AsRef<Path>,
+ ) -> Result<(), HgError> {
+ let link_path = self.join(relative_link_path);
+ std::os::unix::fs::symlink(target_path, &link_path)
+ .with_context(|| IoErrorContext::WritingFile(link_path))
+ }
}
fn fs_metadata(