blockstorage: Add prune locks to BlockManager

This change also introduces an aditional buffer of 10 blocks (PRUNE_LOCK_BUFFER) that will not be pruned before the best block.

Co-authored-by: Luke Dashjr <luke-jr+git@utopios.org>

src/node/blockstorage.cpp

@@ -21,6 +21,8 @@
 #include <util/system.h>
 #include <validation.h>
 
+#include <unordered_map>
+
 namespace node {
 std::atomic_bool fImporting(false);
 std::atomic_bool fReindex(false);
@@ -230,6 +232,11 @@ void BlockManager::FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPr
            nLastBlockWeCanPrune, count);
 }
 
+void BlockManager::UpdatePruneLock(const std::string& name, const PruneLockInfo& lock_info) {
+    AssertLockHeld(::cs_main);
+    m_prune_locks[name] = lock_info;
+}
+
 CBlockIndex* BlockManager::InsertBlockIndex(const uint256& hash)
 {
     AssertLockHeld(cs_main);

src/node/blockstorage.h

@@ -13,6 +13,7 @@
 
 #include <atomic>
 #include <cstdint>
+#include <unordered_map>
 #include <vector>
 
 extern RecursiveMutex cs_main;
@@ -65,6 +66,10 @@ struct CBlockIndexHeightOnlyComparator {
     bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const;
 };
 
+struct PruneLockInfo {
+    int height_first{std::numeric_limits<int>::max()}; //! Height of earliest block that should be kept and not pruned
+};
+
 /**
  * Maintains a tree of blocks (stored in `m_block_index`) which is consulted
  * to determine where the most-work tip is.
@@ -118,6 +123,14 @@ private:
     /** Dirty block file entries. */
     std::set<int> m_dirty_fileinfo;
 
+    /**
+     * Map from external index name to oldest block that must not be pruned.
+     *
+     * @note Internally, only blocks at height (height_first - PRUNE_LOCK_BUFFER - 1) and
+     * below will be pruned, but callers should avoid assuming any particular buffer size.
+     */
+    std::unordered_map<std::string, PruneLockInfo> m_prune_locks GUARDED_BY(::cs_main);
+
 public:
     BlockMap m_block_index GUARDED_BY(cs_main);
 
@@ -175,6 +188,9 @@ public:
     //! Check whether the block associated with this index entry is pruned or not.
     bool IsBlockPruned(const CBlockIndex* pblockindex) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Create or update a prune lock identified by its name
+    void UpdatePruneLock(const std::string& name, const PruneLockInfo& lock_info) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     ~BlockManager()
     {
         Unload();