Require WriteUndoDataForBlock() to hold mutex cs_main

Mutex cs_main is already held by the caller of WriteUndoDataForBlock().
This change is needed to require CBlockIndex::GetUndoPos() to hold
cs_main and CBlockIndex::nStatus to be guarded by cs_main in the
following commits without adding 2 unnecessary cs_main locks to
WriteUndoDataForBlock().

Author: jonatack

src/node/blockstorage.cpp

@@ -712,6 +712,7 @@ static bool WriteBlockToDisk(const CBlock& block, FlatFilePos& pos, const CMessa
 
 bool BlockManager::WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)
 {
+    AssertLockHeld(::cs_main);
     // Write undo information to disk
     if (pindex->GetUndoPos().IsNull()) {
         FlatFilePos _pos;

src/node/blockstorage.h

@@ -7,12 +7,15 @@
 
 #include <fs.h>
 #include <protocol.h> // For CMessageHeader::MessageStartChars
+#include <sync.h>
 #include <txdb.h>
 
 #include <atomic>
 #include <cstdint>
 #include <vector>
 
+extern RecursiveMutex cs_main;
+
 class ArgsManager;
 class BlockValidationState;
 class CBlock;
@@ -146,7 +149,8 @@ class BlockManager
     /** Get block file info entry for one block file */
     CBlockFileInfo* GetBlockFileInfo(size_t n);
 
-    bool WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams);
+    bool WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     FlatFilePos SaveBlockToDisk(const CBlock& block, int nHeight, CChain& active_chain, const CChainParams& chainparams, const FlatFilePos* dbp);