index: Allow atomic commits of index state to be extended.

src/index/base.cpp

@@ -41,9 +41,9 @@ bool BaseIndex::DB::ReadBestBlock(CBlockLocator& locator) const
     return success;
 }
 
-bool BaseIndex::DB::WriteBestBlock(const CBlockLocator& locator)
+void BaseIndex::DB::WriteBestBlock(CDBBatch& batch, const CBlockLocator& locator)
 {
-    return Write(DB_BEST_BLOCK, locator);
+    batch.Write(DB_BEST_BLOCK, locator);
 }
 
 BaseIndex::~BaseIndex()
@@ -95,7 +95,11 @@ void BaseIndex::ThreadSync()
         int64_t last_locator_write_time = 0;
         while (true) {
             if (m_interrupt) {
-                WriteBestBlock(pindex);
+                m_best_block_index = pindex;
+                // No need to handle errors in Commit. If it fails, the error will be already be
+                // logged. The best way to recover is to continue, as index cannot be corrupted by
+                // a missed commit to disk for an advanced index state.
+                Commit();
                 return;
             }
 
@@ -103,9 +107,10 @@ void BaseIndex::ThreadSync()
                 LOCK(cs_main);
                 const CBlockIndex* pindex_next = NextSyncBlock(pindex);
                 if (!pindex_next) {
-                    WriteBestBlock(pindex);
                     m_best_block_index = pindex;
                     m_synced = true;
+                    // No need to handle errors in Commit. See rationale above.
+                    Commit();
                     break;
                 }
                 pindex = pindex_next;
@@ -119,8 +124,10 @@ void BaseIndex::ThreadSync()
             }
 
             if (last_locator_write_time + SYNC_LOCATOR_WRITE_INTERVAL < current_time) {
-                WriteBestBlock(pindex);
+                m_best_block_index = pindex;
                 last_locator_write_time = current_time;
+                // No need to handle errors in Commit. See rationale above.
+                Commit();
             }
 
             CBlock block;
@@ -144,15 +151,22 @@ void BaseIndex::ThreadSync()
     }
 }
 
-bool BaseIndex::WriteBestBlock(const CBlockIndex* block_index)
+bool BaseIndex::Commit()
 {
-    LOCK(cs_main);
-    if (!GetDB().WriteBestBlock(chainActive.GetLocator(block_index))) {
-        return error("%s: Failed to write locator to disk", __func__);
+    CDBBatch batch(GetDB());
+    if (!CommitInternal(batch) || !GetDB().WriteBatch(batch)) {
+        return error("%s: Failed to commit latest %s state", __func__, GetName());
     }
     return true;
 }
 
+bool BaseIndex::CommitInternal(CDBBatch& batch)
+{
+    LOCK(cs_main);
+    GetDB().WriteBestBlock(batch, chainActive.GetLocator(m_best_block_index));
+    return true;
+}
+
 void BaseIndex::BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* pindex,
                                const std::vector<CTransactionRef>& txn_conflicted)
 {
@@ -224,9 +238,10 @@ void BaseIndex::ChainStateFlushed(const CBlockLocator& locator)
         return;
     }
 
-    if (!GetDB().WriteBestBlock(locator)) {
-        error("%s: Failed to write locator to disk", __func__);
-    }
+    // No need to handle errors in Commit. If it fails, the error will be already be logged. The
+    // best way to recover is to continue, as index cannot be corrupted by a missed commit to disk
+    // for an advanced index state.
+    Commit();
 }
 
 bool BaseIndex::BlockUntilSyncedToCurrentChain()

src/index/base.h

@@ -32,7 +32,7 @@ protected:
         bool ReadBestBlock(CBlockLocator& locator) const;
 
         /// Write block locator of the chain that the txindex is in sync with.
-        bool WriteBestBlock(const CBlockLocator& locator);
+        void WriteBestBlock(CDBBatch& batch, const CBlockLocator& locator);
     };
 
 private:
@@ -54,8 +54,15 @@ private:
     /// over and the sync thread exits.
     void ThreadSync();
 
-    /// Write the current chain block locator to the DB.
-    bool WriteBestBlock(const CBlockIndex* block_index);
+    /// Write the current index state (eg. chain block locator and subclass-specific items) to disk.
+    ///
+    /// Recommendations for error handling:
+    /// If called on a successor of the previous committed best block in the index, the index can
+    /// continue processing without risk of corruption, though the index state will need to catch up
+    /// from further behind on reboot. If the new state is not a successor of the previous state (due
+    /// to a chain reorganization), the index must halt until Commit succeeds or else it could end up
+    /// getting corrupted.
+    bool Commit();
 
 protected:
     void BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* pindex,
@@ -69,6 +76,10 @@ protected:
     /// Write update index entries for a newly connected block.
     virtual bool WriteBlock(const CBlock& block, const CBlockIndex* pindex) { return true; }
 
+    /// Virtual method called internally by Commit that can be overridden to atomically
+    /// commit more index state.
+    virtual bool CommitInternal(CDBBatch& batch);
+
     virtual DB& GetDB() const = 0;
 
     /// Get the name of the index for display in logs.