revlog: update the split + transaction test
We add section, increase the amount of comments and simplify some of the
constructs. We are about to build more on top this tests so lets do a small
cleanup first.
//! Filesystem-based locks for local repositories
use crate::errors::HgError;
use crate::errors::HgResultExt;
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_once(':')?;
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;
// This cast is *needed* for platforms with signed chars
#[allow(clippy::unnecessary_cast)]
let mut buffer = [0 as libc::c_char; 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());
}