pageserver/layer_map: add an in memory layer handle type

`InMemoryLayerHandle` can be thought of as the counterpart
of `PersistentLayerDesc`. A future commit will exploit this
similarity and shove both in an enum.

pageserver/src/tenant/layer_map.rs

@@ -340,6 +340,35 @@ where
     }
 }
 
+#[derive(PartialEq, Eq, Hash, Debug, Clone)]
+pub enum InMemoryLayerHandle {
+    Open {
+        lsn_floor: Lsn,
+        end_lsn: Lsn,
+    },
+    Frozen {
+        idx: usize,
+        lsn_floor: Lsn,
+        end_lsn: Lsn,
+    },
+}
+
+impl InMemoryLayerHandle {
+    pub fn get_lsn_floor(&self) -> Lsn {
+        match self {
+            InMemoryLayerHandle::Open { lsn_floor, .. } => *lsn_floor,
+            InMemoryLayerHandle::Frozen { lsn_floor, .. } => *lsn_floor,
+        }
+    }
+
+    pub fn get_end_lsn(&self) -> Lsn {
+        match self {
+            InMemoryLayerHandle::Open { end_lsn, .. } => *end_lsn,
+            InMemoryLayerHandle::Frozen { end_lsn, .. } => *end_lsn,
+        }
+    }
+}
+
 impl LayerMap {
     ///
     /// Find the latest layer (by lsn.end) that covers the given
@@ -534,6 +563,43 @@ impl LayerMap {
         self.historic.iter()
     }
 
+    /// Get a handle for the first in memory layer that matches the provided predicate.
+    /// The handle should be used with [`Self::get_in_memory_layer`] to retrieve the actual layer.
+    ///
+    /// Note: [`Self::find_in_memory_layer`] and [`Self::get_in_memory_layer`] should be called during
+    /// the same exclusive region established by holding the layer manager lock.
+    pub fn find_in_memory_layer<Pred>(&self, mut pred: Pred) -> Option<InMemoryLayerHandle>
+    where
+        Pred: FnMut(&Arc<InMemoryLayer>) -> bool,
+    {
+        if let Some(open) = &self.open_layer {
+            if pred(open) {
+                return Some(InMemoryLayerHandle::Open {
+                    lsn_floor: open.get_lsn_range().start,
+                    end_lsn: open.get_lsn_range().end,
+                });
+            }
+        }
+
+        let pos = self.frozen_layers.iter().rev().position(pred);
+        pos.map(|rev_idx| {
+            let idx = self.frozen_layers.len() - 1 - rev_idx;
+            InMemoryLayerHandle::Frozen {
+                idx,
+                lsn_floor: self.frozen_layers[idx].get_lsn_range().start,
+                end_lsn: self.frozen_layers[idx].get_lsn_range().end,
+            }
+        })
+    }
+
+    /// Get the layer pointed to by the provided handle.
+    pub fn get_in_memory_layer(&self, handle: &InMemoryLayerHandle) -> Option<Arc<InMemoryLayer>> {
+        match handle {
+            InMemoryLayerHandle::Open { .. } => self.open_layer.clone(),
+            InMemoryLayerHandle::Frozen { idx, .. } => self.frozen_layers.get(*idx).cloned(),
+        }
+    }
+
     ///
     /// Divide the whole given range of keys into sub-ranges based on the latest
     /// image layer that covers each range at the specified lsn (inclusive).