semantic layer map operations (#4618)

## Problem

ref #4373

## Summary of changes

A step towards immutable layer map. I decided to finish the refactor
with this new approach and apply
#4455 on this patch later.

In this PR, we moved all modifications of the layer map to one place
with semantic operations like `initialize_local_layers`,
`finish_compact_l0`, `finish_gc_timeline`, etc, which is now part
of `LayerManager`. This makes it easier to build new features upon
this PR:

* For immutable storage state refactor, we can simply replace the layer
map with `ArcSwap<LayerMap>` and remove the `layers` lock. Moving
towards it requires us to put all layer map changes in a single place as
in #4455.
* For manifest, we can write to manifest in each of the semantic
functions.

---------

Signed-off-by: Alex Chi Z <[email protected]>
Co-authored-by: Christian Schwarz <[email protected]>

Author: skyzh

pageserver/src/tenant.rs

@@ -429,7 +429,7 @@ impl Tenant {
                     .layers
                     .read()
                     .await
-                    .0
+                    .layer_map()
                     .iter_historic_layers()
                     .next()
                     .is_some(),

pageserver/src/tenant/layer_map.rs

@@ -659,7 +659,7 @@ mod tests {
 
         use crate::tenant::{
             storage_layer::{AsLayerDesc, PersistentLayerDesc},
-            timeline::LayerFileManager,
+            timeline::layer_manager::LayerFileManager,
         };
 
         use super::*;

pageserver/src/tenant/storage_layer.rs

@@ -41,7 +41,7 @@ pub use inmemory_layer::InMemoryLayer;
 pub use layer_desc::{PersistentLayerDesc, PersistentLayerKey};
 pub use remote_layer::RemoteLayer;
 
-use super::layer_map::BatchedUpdates;
+use super::timeline::layer_manager::LayerManager;
 
 pub fn range_overlaps<T>(a: &Range<T>, b: &Range<T>) -> bool
 where
@@ -170,7 +170,7 @@ impl LayerAccessStats {
     ///
     /// See [`record_residence_event`] for why you need to do this while holding the layer map lock.
     pub(crate) fn for_loading_layer(
-        layer_map_lock_held_witness: &BatchedUpdates<'_>,
+        layer_map_lock_held_witness: &LayerManager,
         status: LayerResidenceStatus,
     ) -> Self {
         let new = LayerAccessStats(Mutex::new(LayerAccessStatsLocked::default()));
@@ -189,7 +189,7 @@ impl LayerAccessStats {
     /// See [`record_residence_event`] for why you need to do this while holding the layer map lock.
     pub(crate) fn clone_for_residence_change(
         &self,
-        layer_map_lock_held_witness: &BatchedUpdates<'_>,
+        layer_map_lock_held_witness: &LayerManager,
         new_status: LayerResidenceStatus,
     ) -> LayerAccessStats {
         let clone = {
@@ -221,7 +221,7 @@ impl LayerAccessStats {
     ///
     pub(crate) fn record_residence_event(
         &self,
-        _layer_map_lock_held_witness: &BatchedUpdates<'_>,
+        _layer_map_lock_held_witness: &LayerManager,
         status: LayerResidenceStatus,
         reason: LayerResidenceEventReason,
     ) {

pageserver/src/tenant/storage_layer/remote_layer.rs

@@ -4,9 +4,9 @@
 use crate::config::PageServerConf;
 use crate::context::RequestContext;
 use crate::repository::Key;
-use crate::tenant::layer_map::BatchedUpdates;
 use crate::tenant::remote_timeline_client::index::LayerFileMetadata;
 use crate::tenant::storage_layer::{Layer, ValueReconstructResult, ValueReconstructState};
+use crate::tenant::timeline::layer_manager::LayerManager;
 use anyhow::{bail, Result};
 use pageserver_api::models::HistoricLayerInfo;
 use std::ops::Range;
@@ -224,7 +224,7 @@ impl RemoteLayer {
     /// Create a Layer struct representing this layer, after it has been downloaded.
     pub fn create_downloaded_layer(
         &self,
-        layer_map_lock_held_witness: &BatchedUpdates<'_>,
+        layer_map_lock_held_witness: &LayerManager,
         conf: &'static PageServerConf,
         file_size: u64,
     ) -> Arc<dyn PersistentLayer> {