pageserver: add a fringe tracker for the vectored search

The fringe of a vectored search is the set of layers intersect
by the ranges in the search at any given point. The keyspace
intersecting each layer is tracked and updated as the search will
dictate. The double indexing will allow the search to continue from
the highest Lsn.

pageserver/src/tenant/storage_layer.rs

@@ -20,7 +20,9 @@ use pageserver_api::keyspace::{KeySpace, KeySpaceRandomAccum};
 use pageserver_api::models::{
     LayerAccessKind, LayerResidenceEvent, LayerResidenceEventReason, LayerResidenceStatus,
 };
-use std::cmp::Reverse;
+use std::cmp::{Ordering, Reverse};
+use std::collections::hash_map::Entry;
+use std::collections::{BinaryHeap, HashMap};
 use std::ops::Range;
 use std::sync::Mutex;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
@@ -210,6 +212,94 @@ pub(crate) enum ReadableLayerDesc {
     },
 }
 
+/// Wraper for 'ReadableLayerDesc' sorted by Lsn
+#[derive(Debug)]
+struct ReadableLayerDescOrdered(ReadableLayerDesc);
+
+/// Data structure which maintains a fringe of layers for the
+/// read path. The fringe is the set of layers which intersects
+/// the current keyspace that the search is descending on.
+/// Each layer tracks the keyspace that intersects it.
+///
+/// The fringe must appear sorted by Lsn. Hence, it uses
+/// a two layer indexing scheme.
+#[derive(Debug)]
+pub(crate) struct LayerFringe {
+    layers_by_lsn: BinaryHeap<ReadableLayerDescOrdered>,
+    layers: HashMap<ReadableLayerDesc, KeySpace>,
+}
+
+impl LayerFringe {
+    pub(crate) fn new() -> Self {
+        LayerFringe {
+            layers_by_lsn: BinaryHeap::new(),
+            layers: HashMap::new(),
+        }
+    }
+
+    pub(crate) fn next_layer(&mut self) -> Option<(ReadableLayerDesc, KeySpace)> {
+        let handle = match self.layers_by_lsn.pop() {
+            Some(h) => h,
+            None => return None,
+        };
+
+        let removed = self.layers.remove_entry(&handle.0);
+        match removed {
+            Some((layer, keyspace)) => Some((layer, keyspace)),
+            None => panic!(),
+        }
+    }
+
+    pub(crate) fn update(&mut self, layer: ReadableLayerDesc, keyspace: KeySpace) {
+        let entry = self.layers.entry(layer.clone());
+        match entry {
+            Entry::Occupied(mut entry) => {
+                entry.get_mut().merge(&keyspace);
+            }
+            Entry::Vacant(entry) => {
+                self.layers_by_lsn
+                    .push(ReadableLayerDescOrdered(entry.key().clone()));
+                entry.insert(keyspace);
+            }
+        }
+    }
+}
+
+impl Default for LayerFringe {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Ord for ReadableLayerDescOrdered {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let ord = self.0.get_lsn_ceil().cmp(&other.0.get_lsn_ceil());
+        if ord == std::cmp::Ordering::Equal {
+            self.0
+                .get_lsn_floor()
+                .cmp(&other.0.get_lsn_floor())
+                .reverse()
+        } else {
+            ord
+        }
+    }
+}
+
+impl PartialOrd for ReadableLayerDescOrdered {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl PartialEq for ReadableLayerDescOrdered {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.get_lsn_floor() == other.0.get_lsn_floor()
+            && self.0.get_lsn_ceil() == other.0.get_lsn_ceil()
+    }
+}
+
+impl Eq for ReadableLayerDescOrdered {}
+
 impl ReadableLayerDesc {
     pub(crate) fn get_lsn_floor(&self) -> Lsn {
         match self {