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//! The `EventLog` lets us cheaply record and query behavior
//! in a concurrent system. It lets us reconstruct stories about
//! what happened to our data. It lets us write tests like:
//! 1. no keys are lost through tree structural modifications
//! 2. no nodes are made inaccessible through structural modifications
//! 3. no segments are zeroed and reused before all resident
//! pages have been relocated and stabilized.
//! 4. recovery does not skip active segments
//! 5. no page is double-allocated or double-freed
//! 6. pages before restart match pages after restart
//!
//! What does it mean for data to be accessible?
//! 1. key -> page
//! 2. page -> lid
//! 3. lid ranges get stabiized over time
//! 4. lid ranges get zeroed over time
//! 5. segment trailers get written over time
//! 6. if a page's old location is zeroed before
//! `io_bufs` segment trailers have been written,
//! we are vulnerable to data loss
//! 3. segments have lifespans from fsync to zero
//! 4.
#![allow(missing_docs)]
use std::collections::HashMap;
use crate::pagecache::DiskPtr;
use crate::*;
use crate::stack::{Iter as StackIter, Stack};
/// A thing that happens at a certain time.
#[derive(Debug, Clone)]
enum Event {
PagesOnShutdown { pages: HashMap<PageId, Vec<DiskPtr>> },
PagesOnRecovery { pages: HashMap<PageId, Vec<DiskPtr>> },
MetaOnShutdown { meta: Meta },
MetaOnRecovery { meta: Meta },
RecoveredLsn(Lsn),
Stabilized(Lsn),
}
/// A lock-free queue of Events.
#[derive(Default, Debug)]
pub struct EventLog {
inner: Stack<Event>,
}
impl EventLog {
pub(crate) fn reset(&self) {
self.verify();
let guard = pin();
while self.inner.pop(&guard).is_some() {}
}
fn iter<'a>(&self, guard: &'a Guard) -> StackIter<'a, Event> {
let head = self.inner.head(guard);
StackIter::from_ptr(head, guard)
}
pub(crate) fn verify(&self) {
let guard = pin();
let iter = self.iter(&guard);
// if we encounter a `PagesOnRecovery`, then we should
// compare it to any subsequent `PagesOnShutdown`
let mut recovered_pages = None;
let mut recovered_meta = None;
let mut minimum_lsn = None;
for event in iter {
match event {
Event::Stabilized(lsn) | Event::RecoveredLsn(lsn) => {
if let Some(later_lsn) = minimum_lsn {
assert!(
later_lsn >= lsn,
"lsn must never go down between recoveries \
or stabilizations. It was {} but later became {}. history: {:?}",
lsn,
later_lsn,
self.iter(&guard)
.filter(|e| matches!(e, Event::Stabilized(_))
|| matches!(e, Event::RecoveredLsn(_)))
.collect::<Vec<_>>(),
);
}
minimum_lsn = Some(lsn);
}
Event::PagesOnRecovery { pages } => {
recovered_pages = Some(pages.clone());
}
Event::PagesOnShutdown { pages } => {
if let Some(ref par) = recovered_pages {
let pids = par
.iter()
.map(|(pid, _frag_locations)| *pid)
.chain(
pages.iter().map(|(pid, _frag_locations)| *pid),
)
.collect::<std::collections::HashSet<_>>()
.into_iter();
for pid in pids {
let locations_before_restart = pages.get(&pid);
let locations_after_restart = par.get(&pid);
assert_eq!(
locations_before_restart,
locations_after_restart,
"page {} had frag locations {:?} before \
restart, but {:?} after restart",
pid,
locations_before_restart,
locations_after_restart
);
}
assert_eq!(pages, par);
}
}
Event::MetaOnRecovery { meta } => {
recovered_meta = Some(meta);
}
Event::MetaOnShutdown { meta } => {
if let Some(rec_meta) = recovered_meta {
assert_eq!(meta, rec_meta);
}
}
}
}
debug!("event log verified \u{2713}");
}
pub(crate) fn stabilized_lsn(&self, lsn: Lsn) {
let guard = pin();
self.inner.push(Event::Stabilized(lsn), &guard);
}
pub(crate) fn recovered_lsn(&self, lsn: Lsn) {
let guard = pin();
self.inner.push(Event::RecoveredLsn(lsn), &guard);
}
pub(crate) fn pages_before_restart(
&self,
pages: HashMap<PageId, Vec<DiskPtr>>,
) {
let guard = pin();
self.inner.push(Event::PagesOnShutdown { pages }, &guard);
}
pub(crate) fn pages_after_restart(
&self,
pages: HashMap<PageId, Vec<DiskPtr>>,
) {
let guard = pin();
self.inner.push(Event::PagesOnRecovery { pages }, &guard);
}
pub fn meta_before_restart(&self, meta: Meta) {
let guard = pin();
self.inner.push(Event::MetaOnShutdown { meta }, &guard);
}
pub fn meta_after_restart(&self, meta: Meta) {
let guard = pin();
self.inner.push(Event::MetaOnRecovery { meta }, &guard);
}
}