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|
//! The state file (`--state-file`): what a node remembers across restarts, so it
//! rejoins its swarm on its own: the swarm ID and some members to reach it through.
//! Not the document: a restarted node still starts empty and syncs.
//!
//! Written rarely: once the node is ready, then when the remembered members change
//! (checked every few minutes, written at most once an hour), and at shutdown if they
//! changed since. Atomically: a temporary file next to it, flushed, then renamed over
//! it, so a crash leaves the old file or the new one.
use std::io::Write;
use std::path::{Path, PathBuf};
use std::sync::{Mutex, MutexGuard};
use std::time::Duration;
use serde::{Deserialize, Serialize};
use tokio::time::Instant;
use tracing::{error, info, warn};
use crate::state::AppState;
use crate::swarm::members;
/// Members remembered as seeds.
const MAX_SEEDS: usize = 16;
/// How often the remembered members are compared with the registry.
const CHECK_INTERVAL: Duration = Duration::from_secs(300);
/// Writes at least this far apart, but for the first one and at shutdown.
const MIN_WRITE_INTERVAL: Duration = Duration::from_secs(3600);
/// The file's content.
#[derive(Clone, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub(crate) struct Remembered {
pub(crate) swarm_id: String,
/// Public keys of members to join through (sorted).
#[serde(default)]
pub(crate) seeds: Vec<String>,
}
pub(crate) struct StateFile {
path: PathBuf,
// A std mutex: never held across an await.
inner: Mutex<Inner>,
}
#[derive(Default)]
struct Inner {
written: Option<Remembered>,
written_at: Option<Instant>,
checked_at: Option<Instant>,
}
impl StateFile {
/// Opens `path`, returning what it remembers. A missing file remembers nothing; an
/// unreadable one is moved aside (`.bad`) and logged, and remembers nothing either.
pub(crate) fn open(path: PathBuf) -> (Self, Option<Remembered>) {
let remembered = match std::fs::read(&path) {
Ok(bytes) => match serde_json::from_slice::<Remembered>(&bytes) {
Ok(remembered) if !remembered.swarm_id.is_empty() => Some(remembered),
Ok(_) => None,
Err(error) => {
let aside = with_suffix(&path, ".bad");
error!(
path = %path.display(),
moved_to = %aside.display(),
error = %error,
"Unreadable state file; moving it aside and remembering nothing"
);
if let Err(error) = std::fs::rename(&path, &aside) {
warn!(error = %error, "Couldn't move the state file aside");
}
None
}
},
Err(error) if error.kind() == std::io::ErrorKind::NotFound => None,
Err(error) => {
error!(path = %path.display(), error = %error, "Couldn't read the state file; remembering nothing");
None
}
};
let file = Self {
path,
inner: Mutex::new(Inner {
written: remembered.clone(),
..Inner::default()
}),
};
(file, remembered)
}
fn lock(&self) -> MutexGuard<'_, Inner> {
self.inner
.lock()
.unwrap_or_else(|poisoned| poisoned.into_inner())
}
fn write(&self, remembered: &Remembered) -> std::io::Result<()> {
let bytes = serde_json::to_vec_pretty(remembered)?;
let temporary = with_suffix(&self.path, ".tmp");
{
let mut file = std::fs::File::create(&temporary)?;
file.write_all(&bytes)?;
file.write_all(b"\n")?;
file.sync_all()?;
}
std::fs::rename(&temporary, &self.path)?;
// The rename itself, durably (best effort: not every platform syncs directories).
if let Some(dir) = self.path.parent().filter(|dir| !dir.as_os_str().is_empty()) {
if let Ok(dir) = std::fs::File::open(dir) {
let _ = dir.sync_all();
}
}
Ok(())
}
}
fn with_suffix(path: &Path, suffix: &str) -> PathBuf {
let mut name = path.as_os_str().to_owned();
name.push(suffix);
PathBuf::from(name)
}
/// Brings the state file up to date if it is due: `now` for the first write and at
/// shutdown, otherwise only every few minutes and at most once an hour. Called by the
/// peer worker on its maintenance tick.
pub(crate) async fn update(state: &AppState, now: bool) {
let Some(file) = state.state_file.clone() else {
return;
};
if !state.is_ready().await {
return;
}
let Some(swarm_id) = state.swarm_id().await else {
return;
};
{
let mut inner = file.lock();
if !now
&& inner
.checked_at
.is_some_and(|at| at.elapsed() < CHECK_INTERVAL)
{
return;
}
inner.checked_at = Some(Instant::now());
}
let seeds = {
let replica = state.doc.read().await;
seeds(&replica.doc, state.local_id(), state.electorate_window)
};
let remembered = Remembered { swarm_id, seeds };
{
let inner = file.lock();
if inner.written.as_ref() == Some(&remembered) {
return;
}
let first = inner.written_at.is_none();
if !now
&& !first
&& inner
.written_at
.is_some_and(|at| at.elapsed() < MIN_WRITE_INTERVAL)
{
return;
}
}
let writer = std::sync::Arc::clone(&file);
let content = remembered.clone();
match tokio::task::spawn_blocking(move || writer.write(&content)).await {
Ok(Ok(())) => {
info!(
path = %file.path.display(),
seeds = remembered.seeds.len(),
"Wrote the state file"
);
let mut inner = file.lock();
inner.written = Some(remembered);
inner.written_at = Some(Instant::now());
}
Ok(Err(error)) => {
error!(path = %file.path.display(), error = %error, "Couldn't write the state file");
}
Err(error) => error!(error = %error, "State file write task failed"),
}
}
/// Members to rejoin through: those alive within the electorate window, stable ones
/// first, at most `MAX_SEEDS`, sorted. Changes only when members join, leave or change
/// tier, so the file rarely does.
fn seeds(doc: &automerge::AutoCommit, me: &str, window: Duration) -> Vec<String> {
let now = crate::state::unix_millis();
let (mut stable, transient): (Vec<String>, Vec<String>) = members::electorate(doc, now, window)
.into_iter()
.filter(|pk| pk != me)
.partition(|pk| members::tier(doc, pk) == members::Tier::Stable);
stable.extend(transient);
stable.truncate(MAX_SEEDS);
stable.sort();
stable
}
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