cnidarium/future.rs
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//! Concrete futures types used by the storage crate.
use anyhow::Result;
use futures::{
future::{Either, Ready},
stream::Peekable,
Stream,
};
use parking_lot::RwLock;
use pin_project::pin_project;
use smallvec::SmallVec;
use std::{
future::Future,
ops::Bound,
pin::Pin,
sync::Arc,
task::{ready, Context, Poll},
};
use crate::Cache;
/// Future representing a read from a state snapshot.
#[pin_project]
pub struct SnapshotFuture(#[pin] pub(crate) tokio::task::JoinHandle<Result<Option<Vec<u8>>>>);
impl Future for SnapshotFuture {
type Output = Result<Option<Vec<u8>>>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
match this.0.poll(cx) {
Poll::Ready(result) => {
Poll::Ready(result.expect("unrecoverable join error from tokio task"))
}
Poll::Pending => Poll::Pending,
}
}
}
/// Future representing a read from an in-memory cache over an underlying state.
#[pin_project]
pub struct CacheFuture<F> {
#[pin]
inner: Either<Ready<Result<Option<Vec<u8>>>>, F>,
}
impl<F> CacheFuture<F> {
pub(crate) fn hit(value: Option<Vec<u8>>) -> Self {
Self {
inner: Either::Left(futures::future::ready(Ok(value))),
}
}
pub(crate) fn miss(underlying: F) -> Self {
Self {
inner: Either::Right(underlying),
}
}
}
impl<F> Future for CacheFuture<F>
where
F: Future<Output = Result<Option<Vec<u8>>>>,
{
type Output = Result<Option<Vec<u8>>>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
this.inner.poll(cx)
}
}
#[pin_project]
pub struct StateDeltaNonconsensusPrefixRawStream<St>
where
St: Stream<Item = Result<(Vec<u8>, Vec<u8>)>>,
{
#[pin]
pub(crate) underlying: Peekable<St>,
pub(crate) layers: Vec<Arc<RwLock<Option<Cache>>>>,
pub(crate) leaf_cache: Arc<RwLock<Option<Cache>>>,
pub(crate) last_key: Option<Vec<u8>>,
pub(crate) prefix: Vec<u8>,
}
impl<St> Stream for StateDeltaNonconsensusPrefixRawStream<St>
where
St: Stream<Item = Result<(Vec<u8>, Vec<u8>)>>,
{
type Item = Result<(Vec<u8>, Vec<u8>)>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
// This implementation interleaves items from the underlying stream with
// items in cache layers. To do this, it tracks the last key it
// returned, then, for each item in the underlying stream, searches for
// cached keys that lie between the last-returned key and the item's key,
// checking whether the cached key represents a deletion requiring further
// scanning. This process is illustrated as follows:
//
// ◇ skip ◇ skip ▲ yield ▲ yield ▲ yield
// │ │ │ │ │
// ░ pick ──────────────▶ ░ pick ────────▶ █ pick ────────▶ █ pick ─────────▶ █ pick
// ▲ ▲ ▲ ▲ ▲
// ▲ │ ▲ │ ▲ │ ▲ │ ▲ │
// write│ │ │ │ │ │ │ │ │ │
// layer│ │ █ │ │ █ │ │█ │ █ │ █ │
// │ │ ░ │ ░ │ ░│ │ ░ │ ░ │
// │ ░ │ ░ │ ░ │ │ ░ │ ░ │
// │ █ │ █ │ █│ │ █ │ █ │
// │ █ █ │ █ █ │ █ │ █ │ █ █ │ █ █
// │ █ │ █ │ █ │ █ │ █
// ─┼(─────]────keys─▶ ─┼──(───]────▶ ─┼────(─]────▶ ─┼─────(]────▶ ─┼──────(──]─▶
// │ ▲ █ █ │ █ █ │ █ █ │ █ █ │ █ █
// │
// │search range of key-value pairs in cache layers that could
// │affect whether to yield the next item in the underlying stream
// Optimization: ensure we have a peekable item in the underlying stream before continuing.
let mut this = self.project();
ready!(this.underlying.as_mut().poll_peek(cx));
// Now that we're ready to interleave the next underlying item with any
// cache layers, lock them all for the duration of the method, using a
// SmallVec to (hopefully) store all the guards on the stack.
let mut layer_guards = SmallVec::<[_; 8]>::new();
for layer in this.layers.iter() {
layer_guards.push(layer.read());
}
// Tacking the leaf cache onto the list is important to not miss any values.
// It's stored separately so that the contents of the
layer_guards.push(this.leaf_cache.read());
loop {
// Obtain a reference to the next key-value pair from the underlying stream.
let peeked = match ready!(this.underlying.as_mut().poll_peek(cx)) {
// If we get an underlying error, bubble it up immediately.
Some(Err(_e)) => return this.underlying.poll_next(cx),
// Otherwise, pass through the peeked value.
Some(Ok(pair)) => Some(pair),
None => None,
};
// To determine whether or not we should return the peeked value, we
// need to search the cache layers for keys that are between the last
// key we returned (exclusive, so we make forward progress on the
// stream) and the peeked key (inclusive, because we need to find out
// whether or not there was a covering deletion).
let search_range = (
this.last_key
.as_ref()
.map(Bound::Excluded)
.unwrap_or(Bound::Included(this.prefix)),
peeked
.map(|(k, _)| Bound::Included(k))
.unwrap_or(Bound::Unbounded),
);
// It'd be slightly cleaner to initialize `leftmost_pair` with the
// peeked contents, but that would taint `leftmost_pair` with a
// `peeked` borrow, and we may need to mutate the underlying stream
// later. Instead, initialize it with `None` to only search the
// cache layers, and compare at the end.
let mut leftmost_pair = None;
for layer in layer_guards.iter() {
// Find this layer's leftmost key-value pair in the search range.
let found_pair = layer
.as_ref()
.expect("layer must not have been applied")
.nonverifiable_changes
.range::<Vec<u8>, _>(search_range)
.take_while(|(k, _v)| k.starts_with(this.prefix))
.next();
// Check whether the new pair, if any, is the new leftmost pair.
match (leftmost_pair, found_pair) {
// We want to replace the pair even when the key is equal,
// so that we always prefer a newer value over an older value.
(Some((leftmost_k, _)), Some((k, v))) if k <= leftmost_k => {
leftmost_pair = Some((k, v));
}
(None, Some((k, v))) => {
leftmost_pair = Some((k, v));
}
_ => {}
}
}
// Overwrite a Vec, attempting to reuse its existing allocation.
let overwrite_in_place = |dst: &mut Option<Vec<u8>>, src: &[u8]| {
if let Some(ref mut dst) = dst {
dst.clear();
dst.extend_from_slice(src);
} else {
*dst = Some(src.to_vec());
}
};
match (leftmost_pair, peeked) {
(Some((k, v)), peeked) => {
// Since we searched for cached keys less than or equal to
// the peeked key, we know that the cached pair takes
// priority over the peeked pair.
//
// If the keys are exactly equal, we advance the underlying stream.
if peeked.map(|(kp, _)| kp) == Some(k) {
let _ = this.underlying.as_mut().poll_next(cx);
}
overwrite_in_place(this.last_key, k);
if let Some(v) = v {
// If the value is Some, we have a key-value pair to yield.
return Poll::Ready(Some(Ok((k.clone(), v.clone()))));
} else {
// If the value is None, this pair represents a deletion,
// so continue looping until we find a non-deleted pair.
continue;
}
}
(None, Some(_)) => {
// There's no cache hit before the peeked pair, so we want
// to extract and return it from the underlying stream.
let Poll::Ready(Some(Ok((k, v)))) = this.underlying.as_mut().poll_next(cx)
else {
unreachable!("peeked stream must yield peeked item");
};
overwrite_in_place(this.last_key, &k);
return Poll::Ready(Some(Ok((k, v))));
}
(None, None) => {
// Terminate the stream, no more items are available.
return Poll::Ready(None);
}
}
}
}
}
// This implementation is almost exactly the same as the one above, but with
// minor tweaks to work with string keys and to read different fields from the cache.
// Update them together.
#[pin_project]
pub struct StateDeltaPrefixRawStream<St>
where
St: Stream<Item = Result<(String, Vec<u8>)>>,
{
#[pin]
pub(crate) underlying: Peekable<St>,
pub(crate) layers: Vec<Arc<RwLock<Option<Cache>>>>,
pub(crate) leaf_cache: Arc<RwLock<Option<Cache>>>,
pub(crate) last_key: Option<String>,
pub(crate) prefix: String,
}
impl<St> Stream for StateDeltaPrefixRawStream<St>
where
St: Stream<Item = Result<(String, Vec<u8>)>>,
{
type Item = Result<(String, Vec<u8>)>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
// This implementation interleaves items from the underlying stream with
// items in cache layers. To do this, it tracks the last key it
// returned, then, for each item in the underlying stream, searches for
// cached keys that lie between the last-returned key and the item's key,
// checking whether the cached key represents a deletion requiring further
// scanning. This process is illustrated as follows:
//
// ◇ skip ◇ skip ▲ yield ▲ yield ▲ yield
// │ │ │ │ │
// ░ pick ──────────────▶ ░ pick ────────▶ █ pick ────────▶ █ pick ─────────▶ █ pick
// ▲ ▲ ▲ ▲ ▲
// ▲ │ ▲ │ ▲ │ ▲ │ ▲ │
// write│ │ │ │ │ │ │ │ │ │
// layer│ │ █ │ │ █ │ │█ │ █ │ █ │
// │ │ ░ │ ░ │ ░│ │ ░ │ ░ │
// │ ░ │ ░ │ ░ │ │ ░ │ ░ │
// │ █ │ █ │ █│ │ █ │ █ │
// │ █ █ │ █ █ │ █ │ █ │ █ █ │ █ █
// │ █ │ █ │ █ │ █ │ █
// ─┼(─────]────keys─▶ ─┼──(───]────▶ ─┼────(─]────▶ ─┼─────(]────▶ ─┼──────(──]─▶
// │ ▲ █ █ │ █ █ │ █ █ │ █ █ │ █ █
// │
// │search range of key-value pairs in cache layers that could
// │affect whether to yield the next item in the underlying stream
// Optimization: ensure we have a peekable item in the underlying stream before continuing.
let mut this = self.project();
ready!(this.underlying.as_mut().poll_peek(cx));
// Now that we're ready to interleave the next underlying item with any
// cache layers, lock them all for the duration of the method, using a
// SmallVec to (hopefully) store all the guards on the stack.
let mut layer_guards = SmallVec::<[_; 8]>::new();
for layer in this.layers.iter() {
layer_guards.push(layer.read());
}
// Tacking the leaf cache onto the list is important to not miss any values.
// It's stored separately so that the contents of the
layer_guards.push(this.leaf_cache.read());
loop {
// Obtain a reference to the next key-value pair from the underlying stream.
let peeked = match ready!(this.underlying.as_mut().poll_peek(cx)) {
// If we get an underlying error, bubble it up immediately.
Some(Err(_e)) => return this.underlying.poll_next(cx),
// Otherwise, pass through the peeked value.
Some(Ok(pair)) => Some(pair),
None => None,
};
// To determine whether or not we should return the peeked value, we
// need to search the cache layers for keys that are between the last
// key we returned (exclusive, so we make forward progress on the
// stream) and the peeked key (inclusive, because we need to find out
// whether or not there was a covering deletion).
let search_range = (
this.last_key
.as_ref()
.map(Bound::Excluded)
.unwrap_or(Bound::Included(this.prefix)),
peeked
.map(|(k, _)| Bound::Included(k))
.unwrap_or(Bound::Unbounded),
);
// It'd be slightly cleaner to initialize `leftmost_pair` with the
// peeked contents, but that would taint `leftmost_pair` with a
// `peeked` borrow, and we may need to mutate the underlying stream
// later. Instead, initialize it with `None` to only search the
// cache layers, and compare at the end.
let mut leftmost_pair = None;
for layer in layer_guards.iter() {
// Find this layer's leftmost key-value pair in the search range.
let found_pair = layer
.as_ref()
.expect("layer must not have been applied")
.unwritten_changes
.range::<String, _>(search_range)
.take_while(|(k, _v)| k.starts_with(this.prefix.as_str()))
.next();
// Check whether the new pair, if any, is the new leftmost pair.
match (leftmost_pair, found_pair) {
// We want to replace the pair even when the key is equal,
// so that we always prefer a newer value over an older value.
(Some((leftmost_k, _)), Some((k, v))) if k <= leftmost_k => {
leftmost_pair = Some((k, v));
}
(None, Some((k, v))) => {
leftmost_pair = Some((k, v));
}
_ => {}
}
}
// Overwrite a Vec, attempting to reuse its existing allocation.
let overwrite_in_place = |dst: &mut Option<String>, src: &str| {
if let Some(ref mut dst) = dst {
dst.clear();
dst.push_str(src);
} else {
*dst = Some(src.to_owned());
}
};
match (leftmost_pair, peeked) {
(Some((k, v)), peeked) => {
// Since we searched for cached keys less than or equal to
// the peeked key, we know that the cached pair takes
// priority over the peeked pair.
//
// If the keys are exactly equal, we advance the underlying stream.
if peeked.map(|(kp, _)| kp) == Some(k) {
let _ = this.underlying.as_mut().poll_next(cx);
}
overwrite_in_place(this.last_key, k);
if let Some(v) = v {
// If the value is Some, we have a key-value pair to yield.
return Poll::Ready(Some(Ok((k.clone(), v.clone()))));
} else {
// If the value is None, this pair represents a deletion,
// so continue looping until we find a non-deleted pair.
continue;
}
}
(None, Some(_)) => {
// There's no cache hit before the peeked pair, so we want
// to extract and return it from the underlying stream.
let Poll::Ready(Some(Ok((k, v)))) = this.underlying.as_mut().poll_next(cx)
else {
unreachable!("peeked stream must yield peeked item");
};
overwrite_in_place(this.last_key, &k);
return Poll::Ready(Some(Ok((k, v))));
}
(None, None) => {
// Terminate the stream, no more items are available.
return Poll::Ready(None);
}
}
}
}
}
// This implementation is almost exactly the same as the one above, but with
// minor tweaks to work with string keys and to read different fields from the cache.
// Update them together.
#[pin_project]
pub struct StateDeltaPrefixKeysStream<St>
where
St: Stream<Item = Result<String>>,
{
#[pin]
pub(crate) underlying: Peekable<St>,
pub(crate) layers: Vec<Arc<RwLock<Option<Cache>>>>,
pub(crate) leaf_cache: Arc<RwLock<Option<Cache>>>,
pub(crate) last_key: Option<String>,
pub(crate) prefix: String,
}
impl<St> Stream for StateDeltaPrefixKeysStream<St>
where
St: Stream<Item = Result<String>>,
{
type Item = Result<String>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
// This implementation interleaves items from the underlying stream with
// items in cache layers. To do this, it tracks the last key it
// returned, then, for each item in the underlying stream, searches for
// cached keys that lie between the last-returned key and the item's key,
// checking whether the cached key represents a deletion requiring further
// scanning. This process is illustrated as follows:
//
// ◇ skip ◇ skip ▲ yield ▲ yield ▲ yield
// │ │ │ │ │
// ░ pick ──────────────▶ ░ pick ────────▶ █ pick ────────▶ █ pick ─────────▶ █ pick
// ▲ ▲ ▲ ▲ ▲
// ▲ │ ▲ │ ▲ │ ▲ │ ▲ │
// write│ │ │ │ │ │ │ │ │ │
// layer│ │ █ │ │ █ │ │█ │ █ │ █ │
// │ │ ░ │ ░ │ ░│ │ ░ │ ░ │
// │ ░ │ ░ │ ░ │ │ ░ │ ░ │
// │ █ │ █ │ █│ │ █ │ █ │
// │ █ █ │ █ █ │ █ │ █ │ █ █ │ █ █
// │ █ │ █ │ █ │ █ │ █
// ─┼(─────]────keys─▶ ─┼──(───]────▶ ─┼────(─]────▶ ─┼─────(]────▶ ─┼──────(──]─▶
// │ ▲ █ █ │ █ █ │ █ █ │ █ █ │ █ █
// │
// │search range of key-value pairs in cache layers that could
// │affect whether to yield the next item in the underlying stream
// Optimization: ensure we have a peekable item in the underlying stream before continuing.
let mut this = self.project();
ready!(this.underlying.as_mut().poll_peek(cx));
// Now that we're ready to interleave the next underlying item with any
// cache layers, lock them all for the duration of the method, using a
// SmallVec to (hopefully) store all the guards on the stack.
let mut layer_guards = SmallVec::<[_; 8]>::new();
for layer in this.layers.iter() {
layer_guards.push(layer.read());
}
// Tacking the leaf cache onto the list is important to not miss any values.
// It's stored separately so that the contents of the
layer_guards.push(this.leaf_cache.read());
loop {
// Obtain a reference to the next key-value pair from the underlying stream.
let peeked = match ready!(this.underlying.as_mut().poll_peek(cx)) {
// If we get an underlying error, bubble it up immediately.
Some(Err(_e)) => return this.underlying.poll_next(cx),
// Otherwise, pass through the peeked value.
Some(Ok(pair)) => Some(pair),
None => None,
};
// To determine whether or not we should return the peeked value, we
// need to search the cache layers for keys that are between the last
// key we returned (exclusive, so we make forward progress on the
// stream) and the peeked key (inclusive, because we need to find out
// whether or not there was a covering deletion).
let search_range = (
this.last_key
.as_ref()
.map(Bound::Excluded)
.unwrap_or(Bound::Included(this.prefix)),
peeked.map(Bound::Included).unwrap_or(Bound::Unbounded),
);
// It'd be slightly cleaner to initialize `leftmost_pair` with the
// peeked contents, but that would taint `leftmost_pair` with a
// `peeked` borrow, and we may need to mutate the underlying stream
// later. Instead, initialize it with `None` to only search the
// cache layers, and compare at the end.
let mut leftmost_pair = None;
for layer in layer_guards.iter() {
// Find this layer's leftmost key-value pair in the search range.
let found_pair = layer
.as_ref()
.expect("layer must not have been applied")
.unwritten_changes
.range::<String, _>(search_range)
.take_while(|(k, _v)| k.starts_with(this.prefix.as_str()))
.next();
// Check whether the new pair, if any, is the new leftmost pair.
match (leftmost_pair, found_pair) {
// We want to replace the pair even when the key is equal,
// so that we always prefer a newer value over an older value.
(Some((leftmost_k, _)), Some((k, v))) if k <= leftmost_k => {
leftmost_pair = Some((k, v));
}
(None, Some((k, v))) => {
leftmost_pair = Some((k, v));
}
_ => {}
}
}
// Overwrite a Vec, attempting to reuse its existing allocation.
let overwrite_in_place = |dst: &mut Option<String>, src: &str| {
if let Some(ref mut dst) = dst {
dst.clear();
dst.push_str(src);
} else {
*dst = Some(src.to_owned());
}
};
match (leftmost_pair, peeked) {
(Some((k, v)), peeked) => {
// Since we searched for cached keys less than or equal to
// the peeked key, we know that the cached pair takes
// priority over the peeked pair.
//
// If the keys are exactly equal, we advance the underlying stream.
if peeked == Some(k) {
let _ = this.underlying.as_mut().poll_next(cx);
}
overwrite_in_place(this.last_key, k);
if v.is_some() {
// If the value is Some, we have a key-value pair to yield.
return Poll::Ready(Some(Ok(k.clone())));
} else {
// If the value is None, this pair represents a deletion,
// so continue looping until we find a non-deleted pair.
continue;
}
}
(None, Some(_)) => {
// There's no cache hit before the peeked pair, so we want
// to extract and return it from the underlying stream.
let Poll::Ready(Some(Ok(k))) = this.underlying.as_mut().poll_next(cx) else {
unreachable!("peeked stream must yield peeked item");
};
overwrite_in_place(this.last_key, &k);
return Poll::Ready(Some(Ok(k)));
}
(None, None) => {
// Terminate the stream, no more items are available.
return Poll::Ready(None);
}
}
}
}
}
#[pin_project]
/// A stream of key-value pairs that interleaves a nonverifiable storage and caching layers.
// This implementation differs from [`StateDeltaNonconsensusPrefixRawStream`] sin how
// it specifies the search space for the cache.
pub struct StateDeltaNonconsensusRangeRawStream<St>
where
St: Stream<Item = Result<(Vec<u8>, Vec<u8>)>>,
{
#[pin]
pub(crate) underlying: Peekable<St>,
pub(crate) layers: Vec<Arc<RwLock<Option<Cache>>>>,
pub(crate) leaf_cache: Arc<RwLock<Option<Cache>>>,
pub(crate) last_key: Option<Vec<u8>>,
pub(crate) prefix: Option<Vec<u8>>,
pub(crate) range: (Option<Vec<u8>>, Option<Vec<u8>>),
}
impl<St> Stream for StateDeltaNonconsensusRangeRawStream<St>
where
St: Stream<Item = Result<(Vec<u8>, Vec<u8>)>>,
{
type Item = Result<(Vec<u8>, Vec<u8>)>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
// This implementation interleaves items from the underlying stream with
// items in cache layers. To do this, it tracks the last key it
// returned, then, for each item in the underlying stream, searches for
// cached keys that lie between the last-returned key and the item's key,
// checking whether the cached key represents a deletion requiring further
// scanning. This process is illustrated as follows:
//
// ◇ skip ◇ skip ▲ yield ▲ yield ▲ yield
// │ │ │ │ │
// ░ pick ──────────────▶ ░ pick ────────▶ █ pick ────────▶ █ pick ─────────▶ █ pick
// ▲ ▲ ▲ ▲ ▲
// ▲ │ ▲ │ ▲ │ ▲ │ ▲ │
// write│ │ │ │ │ │ │ │ │ │
// layer│ │ █ │ │ █ │ │█ │ █ │ █ │
// │ │ ░ │ ░ │ ░│ │ ░ │ ░ │
// │ ░ │ ░ │ ░ │ │ ░ │ ░ │
// │ █ │ █ │ █│ │ █ │ █ │
// │ █ █ │ █ █ │ █ │ █ │ █ █ │ █ █
// │ █ │ █ │ █ │ █ │ █
// ─┼(─────]────keys─▶ ─┼──(───]────▶ ─┼────(─]────▶ ─┼─────(]────▶ ─┼──────(──]─▶
// │ ▲ █ █ │ █ █ │ █ █ │ █ █ │ █ █
// │
// │search range of key-value pairs in cache layers that could
// │affect whether to yield the next item in the underlying stream
// Optimization: ensure we have a peekable item in the underlying stream before continuing.
let mut this = self.project();
ready!(this.underlying.as_mut().poll_peek(cx));
// Now that we're ready to interleave the next underlying item with any
// cache layers, lock them all for the duration of the method, using a
// SmallVec to (hopefully) store all the guards on the stack.
let mut layer_guards = SmallVec::<[_; 8]>::new();
for layer in this.layers.iter() {
layer_guards.push(layer.read());
}
// Tacking the leaf cache onto the list is important to not miss any values.
// It's stored separately so that the contents of the
layer_guards.push(this.leaf_cache.read());
let (binding_prefix, binding_start, binding_end) = (Vec::new(), Vec::new(), Vec::new());
let prefix = this.prefix.as_ref().unwrap_or(&binding_prefix);
let start = this.range.0.as_ref().unwrap_or(&binding_start);
let end = this.range.1.as_ref().unwrap_or(&binding_end);
let mut prefix_start = Vec::with_capacity(prefix.len() + start.len());
let mut prefix_end = Vec::with_capacity(prefix.len() + end.len());
prefix_start.extend(prefix);
prefix_start.extend(start);
prefix_end.extend(prefix);
prefix_end.extend(end);
loop {
// Obtain a reference to the next key-value pair from the underlying stream.
let peeked = match ready!(this.underlying.as_mut().poll_peek(cx)) {
// If we get an underlying error, bubble it up immediately.
Some(Err(_e)) => return this.underlying.poll_next(cx),
// Otherwise, pass through the peeked value.
Some(Ok(pair)) => Some(pair),
None => None,
};
// We want to decide which key to return next, so we have to inspect the cache layers.
// To do this, we have to define a search space so that we cover updates and new insertions
// that could affect the next key to return.
let lower_bound = match this.last_key.as_ref() {
Some(k) => Bound::Excluded(k),
None => Bound::Included(prefix_start.as_ref()),
};
let upper_bound = match peeked {
Some((k, _v)) => Bound::Included(k),
None => this
.range
.1
.as_ref()
.map_or(Bound::Unbounded, |_| Bound::Excluded(prefix_end.as_ref())),
};
let search_range = (lower_bound, upper_bound);
tracing::debug!(
"searching cache layers for key-value pairs in range {:?}",
search_range
);
// It'd be slightly cleaner to initialize `leftmost_pair` with the
// peeked contents, but that would taint `leftmost_pair` with a
// `peeked` borrow, and we may need to mutate the underlying stream
// later. Instead, initialize it with `None` to only search the
// cache layers, and compare at the end.
let mut leftmost_pair = None;
for layer in layer_guards.iter() {
// Find this layer's leftmost key-value pair in the search range.
let found_pair = layer
.as_ref()
.expect("layer must not have been applied")
.nonverifiable_changes
.range::<Vec<u8>, _>(search_range)
.take_while(|(k, v)| {
tracing::debug!(?v, ?k, "found key-value pair in cache layer");
match peeked {
Some((peeked_k, _)) => {
k.starts_with(prefix.as_slice()) && k <= &peeked_k
}
None => k.starts_with(prefix.as_slice()),
}
})
.next();
// Check whether the new pair, if any, is the new leftmost pair.
match (leftmost_pair, found_pair) {
// We want to replace the pair even when the key is equal,
// so that we always prefer a newer value over an older value.
(Some((leftmost_k, _)), Some((k, v))) if k <= leftmost_k => {
leftmost_pair = Some((k, v));
}
(None, Some((k, v))) => {
leftmost_pair = Some((k, v));
}
_ => {}
}
}
// Overwrite a Vec, attempting to reuse its existing allocation.
let overwrite_in_place = |dst: &mut Option<Vec<u8>>, src: &[u8]| {
if let Some(ref mut dst) = dst {
dst.clear();
dst.extend_from_slice(src);
} else {
*dst = Some(src.to_vec());
}
};
match (leftmost_pair, peeked) {
(Some((k, v)), peeked) => {
// Since we searched for cached keys less than or equal to
// the peeked key, we know that the cached pair takes
// priority over the peeked pair.
//
// If the keys are exactly equal, we advance the underlying stream.
if peeked.map(|(kp, _)| kp) == Some(k) {
let _ = this.underlying.as_mut().poll_next(cx);
}
overwrite_in_place(this.last_key, k);
if let Some(v) = v {
// If the value is Some, we have a key-value pair to yield.
return Poll::Ready(Some(Ok((k.clone(), v.clone()))));
} else {
// If the value is None, this pair represents a deletion,
// so continue looping until we find a non-deleted pair.
continue;
}
}
(None, Some(_)) => {
// There's no cache hit before the peeked pair, so we want
// to extract and return it from the underlying stream.
let Poll::Ready(Some(Ok((k, v)))) = this.underlying.as_mut().poll_next(cx)
else {
unreachable!("peeked stream must yield peeked item");
};
overwrite_in_place(this.last_key, &k);
return Poll::Ready(Some(Ok((k, v))));
}
(None, None) => {
// Terminate the stream, no more items are available.
return Poll::Ready(None);
}
}
}
}
}