Add API docs for all overridden local overlay methods

These docs explain the implementation relative to the local store
originals. The original declaration of virtual methods can still be
consulted for proper interface-level documentation.

src/libstore/local-overlay-store.hh

@@ -68,7 +68,10 @@ struct LocalOverlayStoreConfig : virtual LocalStoreConfig
 };
 
 /**
- * Variation of local store using overlayfs for the store dir.
+ * Variation of local store using OverlayFS for the store directory.
+ *
+ * Documentation on overridden methods states how they differ from their
+ * `LocalStore` counterparts.
  */
 class LocalOverlayStore : public virtual LocalOverlayStoreConfig, public virtual LocalStore
 {
@@ -99,30 +102,77 @@ public:
     }
 
 private:
-    // Overridden methods…
-
+    /**
+     * First copy up any lower store realisation with the same key, so we
+     * merge rather than mask it.
+     */
     void registerDrvOutput(const Realisation & info) override;
 
+    /**
+     * Check lower store if upper DB does not have.
+     */
     void queryPathInfoUncached(const StorePath & path,
         Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override;
 
+    /**
+     * Check lower store if upper DB does not have.
+     *
+     * In addition, copy up metadata for lower store objects (and their
+     * closure). (I.e. Optimistically cache in the upper DB.)
+     */
     bool isValidPathUncached(const StorePath & path) override;
 
+    /**
+     * Check the lower store and upper DB.
+     */
     void queryReferrers(const StorePath & path, StorePathSet & referrers) override;
 
+    /**
+     * Check the lower store and upper DB.
+     */
     StorePathSet queryValidDerivers(const StorePath & path) override;
 
+    /**
+     * Check lower store if upper DB does not have.
+     */
     std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
 
+    /**
+     * First copy up any lower store realisation with the same key, so we
+     * merge rather than mask it.
+     */
     void registerValidPaths(const ValidPathInfos & infos) override;
 
+    /**
+     * Check lower store if upper DB does not have.
+     */
     void queryRealisationUncached(const DrvOutput&,
         Callback<std::shared_ptr<const Realisation>> callback) noexcept override;
 
+    /**
+     * Call `remountIfNecessary` after collecting garbage normally.
+     */
     void collectGarbage(const GCOptions & options, GCResults & results) override;
 
+    /**
+     * Check which layers the store object exists in to try to avoid
+     * needing to remount.
+     */
     void deleteStorePath(const Path & path, uint64_t & bytesFreed) override;
 
+    /**
+     * Deduplicate by removing store objects from the upper layer that
+     * are now in the lower layer.
+     *
+     * This implementation will not cause duplications, but addition of
+     * new store objects to the lower layer can instill induce them
+     * (there is no way to prevent that). This cleans up those
+     * duplications.
+     *
+     * @note We do not yet optomise the upper layer in the normal way
+     * (hardlink) yet. We would like to, but it requires more
+     * refactoring of existing code to support this sustainably.
+     */
     void optimiseStore() override;
 
     /**
@@ -147,8 +197,16 @@ private:
      */
     void queryGCReferrers(const StorePath & path, StorePathSet & referrers) override;
 
+    /**
+     * Call the `remountHook` if we have done something such that the
+     * OverlayFS needed to be remounted. See that hook's user-facing
+     * documentation for further details.
+     */
     void remountIfNecessary();
 
+    /**
+     * State for `remountIfNecessary`
+     */
     std::atomic_bool _remountRequired = false;
 };