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use std::{cell::UnsafeCell, mem::transmute, sync::Arc};
use anyhow::Result;
use arc_swap::ArcSwap;
use smallvec::SmallVec;
use thread_local::ThreadLocal;
use crate::database::{
key_value_database::{KeySpace, KeyValueDatabase},
write_batch::{
BaseWriteBatch, ConcurrentWriteBatch, SerialWriteBatch, WriteBatch, WriteBuffer,
},
};
struct ThreadLocalReadTransactionsContainer<T: KeyValueDatabase + 'static>(
UnsafeCell<SmallVec<[T::ReadTransaction<'static>; 4]>>,
);
impl<T: KeyValueDatabase> ThreadLocalReadTransactionsContainer<T> {
unsafe fn pop(&self) -> Option<T::ReadTransaction<'static>> {
let vec = unsafe { &mut *self.0.get() };
vec.pop()
}
unsafe fn push(&self, tx: T::ReadTransaction<'static>) {
let vec = unsafe { &mut *self.0.get() };
vec.push(tx)
}
}
// Safety: It's safe to send RoTransaction between threads, but the types don't allow that.
unsafe impl<T: KeyValueDatabase> Send for ThreadLocalReadTransactionsContainer<T> {}
pub struct ReadTransactionCache<T: KeyValueDatabase + 'static> {
// Safety: `read_transactions_cache` need to be dropped before `database` since it will end the
// transactions.
read_transactions_cache: ArcSwap<ThreadLocal<ThreadLocalReadTransactionsContainer<T>>>,
database: T,
}
impl<T: KeyValueDatabase + 'static> ReadTransactionCache<T> {
pub fn new(database: T) -> Self {
Self {
read_transactions_cache: ArcSwap::new(Arc::new(ThreadLocal::new())),
database,
}
}
}
impl<T: KeyValueDatabase + 'static> KeyValueDatabase for ReadTransactionCache<T> {
type ReadTransaction<'l>
= CachedReadTransaction<'l, T>
where
T: 'l;
fn is_empty(&self) -> bool {
self.database.is_empty()
}
fn begin_read_transaction(&self) -> Result<Self::ReadTransaction<'_>> {
let guard = self.read_transactions_cache.load();
let container = guard
.get_or(|| ThreadLocalReadTransactionsContainer(UnsafeCell::new(Default::default())));
// Safety: Since it's a thread local it's safe to take from the container
let tx = if let Some(tx) = unsafe { container.pop() } {
unsafe { transmute::<T::ReadTransaction<'static>, T::ReadTransaction<'_>>(tx) }
} else {
self.database.begin_read_transaction()?
};
let thread_locals = guard.clone();
Ok(CachedReadTransaction::<T> {
tx: Some(tx),
thread_locals,
})
}
type ValueBuffer<'l> = T::ValueBuffer<'l>;
fn get<'l, 'db: 'l>(
&'l self,
transaction: &'l Self::ReadTransaction<'db>,
key_space: KeySpace,
key: &[u8],
) -> anyhow::Result<Option<Self::ValueBuffer<'l>>> {
self.database
.get(transaction.tx.as_ref().unwrap(), key_space, key)
}
type SerialWriteBatch<'l> = ReadTransactionCacheWriteBatch<'l, T, T::SerialWriteBatch<'l>>;
type ConcurrentWriteBatch<'l> =
ReadTransactionCacheWriteBatch<'l, T, T::ConcurrentWriteBatch<'l>>;
fn write_batch(
&self,
) -> Result<WriteBatch<'_, Self::SerialWriteBatch<'_>, Self::ConcurrentWriteBatch<'_>>> {
Ok(match self.database.write_batch()? {
WriteBatch::Serial(write_batch) => WriteBatch::serial(ReadTransactionCacheWriteBatch {
write_batch,
read_transactions_cache: &self.read_transactions_cache,
}),
WriteBatch::Concurrent(write_batch, _) => {
WriteBatch::concurrent(ReadTransactionCacheWriteBatch {
write_batch,
read_transactions_cache: &self.read_transactions_cache,
})
}
})
}
}
pub struct CachedReadTransaction<'l, T: KeyValueDatabase + 'static> {
tx: Option<T::ReadTransaction<'l>>,
thread_locals: Arc<ThreadLocal<ThreadLocalReadTransactionsContainer<T>>>,
}
impl<T: KeyValueDatabase> Drop for CachedReadTransaction<'_, T> {
fn drop(&mut self) {
let container = self
.thread_locals
.get_or(|| ThreadLocalReadTransactionsContainer(UnsafeCell::new(Default::default())));
// Safety: We cast it to 'static lifetime, but it will be casted back to 'env when
// taken. It's safe since this will not outlive the environment. We need to
// be careful with Drop, but `read_transactions_cache` is before `env` in the
// LmdbBackingStorage struct, so it's fine.
let tx = unsafe {
transmute::<T::ReadTransaction<'_>, T::ReadTransaction<'static>>(
self.tx.take().unwrap(),
)
};
// Safety: It's safe to put it back since it's a thread local
unsafe {
container.push(tx);
}
}
}
pub struct ReadTransactionCacheWriteBatch<'l, T: KeyValueDatabase + 'static, B> {
write_batch: B,
read_transactions_cache: &'l ArcSwap<ThreadLocal<ThreadLocalReadTransactionsContainer<T>>>,
}
impl<'a, T: KeyValueDatabase + 'static, B: BaseWriteBatch<'a>> BaseWriteBatch<'a>
for ReadTransactionCacheWriteBatch<'a, T, B>
{
fn commit(self) -> anyhow::Result<()> {
self.write_batch.commit()?;
let _span = tracing::trace_span!("swap read transactions").entered();
// This resets the thread local storage for read transactions, read transactions are
// eventually dropped, allowing DB to free up unused storage.
self.read_transactions_cache
.store(Arc::new(ThreadLocal::new()));
Ok(())
}
type ValueBuffer<'l>
= B::ValueBuffer<'l>
where
Self: 'l,
'a: 'l;
fn get<'l>(&'l self, key_space: KeySpace, key: &[u8]) -> Result<Option<Self::ValueBuffer<'l>>>
where
'a: 'l,
{
self.write_batch.get(key_space, key)
}
}
impl<'a, T: KeyValueDatabase, B: SerialWriteBatch<'a>> SerialWriteBatch<'a>
for ReadTransactionCacheWriteBatch<'a, T, B>
{
fn put(
&mut self,
key_space: KeySpace,
key: WriteBuffer<'_>,
value: WriteBuffer<'_>,
) -> Result<()> {
self.write_batch.put(key_space, key, value)
}
fn delete(&mut self, key_space: KeySpace, key: WriteBuffer<'_>) -> Result<()> {
self.write_batch.delete(key_space, key)
}
fn flush(&mut self, key_space: KeySpace) -> Result<()> {
self.write_batch.flush(key_space)
}
}
impl<'a, T: KeyValueDatabase, B: ConcurrentWriteBatch<'a>> ConcurrentWriteBatch<'a>
for ReadTransactionCacheWriteBatch<'a, T, B>
{
fn put(&self, key_space: KeySpace, key: WriteBuffer<'_>, value: WriteBuffer<'_>) -> Result<()> {
self.write_batch.put(key_space, key, value)
}
fn delete(&self, key_space: KeySpace, key: WriteBuffer<'_>) -> Result<()> {
self.write_batch.delete(key_space, key)
}
unsafe fn flush(&self, key_space: KeySpace) -> Result<()> {
unsafe { self.write_batch.flush(key_space) }
}
}
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