blockstorage: Make m_block_index own CBlockIndex's

Instead of having CBlockIndex's live on the heap, which requires manual
memory management, have them be owned by m_block_index. This means that
they will live and die with BlockManager.

A change to BlockManager::LookupBlockIndex:
- Previously, it was a const member function returning a non-const CBlockIndex*
- Now, there's are const and non-const versions of
  BlockManager::LookupBlockIndex returning a CBlockIndex with the same
  const-ness as the member function:
    (e.g. const CBlockIndex* LookupBlockIndex(...) const)

See next commit for some weirdness that this eliminates.

The range based for-loops are modernize (using auto + destructuring) in
a future commit.

2 files changed, 34 insertions, 23 deletions

diff --git a/src/node/blockstorage.cpp b/src/node/blockstorage.cpp
index 8a99130fd0..a13c1a9956 100644
--- a/src/node/blockstorage.cpp
+++ b/src/node/blockstorage.cpp
@@ -32,11 +32,18 @@ static FILE* OpenUndoFile(const FlatFilePos& pos, bool fReadOnly = false);
 static FlatFileSeq BlockFileSeq();
 static FlatFileSeq UndoFileSeq();
 
-CBlockIndex* BlockManager::LookupBlockIndex(const uint256& hash) const
+CBlockIndex* BlockManager::LookupBlockIndex(const uint256& hash)
+{
+    AssertLockHeld(cs_main);
+    BlockMap::iterator it = m_block_index.find(hash);
+    return it == m_block_index.end() ? nullptr : &it->second;
+}
+
+const CBlockIndex* BlockManager::LookupBlockIndex(const uint256& hash) const
 {
     AssertLockHeld(cs_main);
     BlockMap::const_iterator it = m_block_index.find(hash);
-    return it == m_block_index.end() ? nullptr : it->second;
+    return it == m_block_index.end() ? nullptr : &it->second;
 }
 
 CBlockIndex* BlockManager::AddToBlockIndex(const CBlockHeader& block)
@@ -47,20 +54,22 @@ CBlockIndex* BlockManager::AddToBlockIndex(const CBlockHeader& block)
     uint256 hash = block.GetHash();
     BlockMap::iterator it = m_block_index.find(hash);
     if (it != m_block_index.end()) {
-        return it->second;
+        return &it->second;
     }
 
     // Construct new block index object
-    CBlockIndex* pindexNew = new CBlockIndex(block);
+    CBlockIndex new_index{block};
     // We assign the sequence id to blocks only when the full data is available,
     // to avoid miners withholding blocks but broadcasting headers, to get a
     // competitive advantage.
-    pindexNew->nSequenceId = 0;
-    BlockMap::iterator mi = m_block_index.insert(std::make_pair(hash, pindexNew)).first;
+    new_index.nSequenceId = 0;
+    BlockMap::iterator mi = m_block_index.insert(std::make_pair(hash, std::move(new_index))).first;
+
+    CBlockIndex* pindexNew = &(*mi).second;
     pindexNew->phashBlock = &((*mi).first);
     BlockMap::iterator miPrev = m_block_index.find(block.hashPrevBlock);
     if (miPrev != m_block_index.end()) {
-        pindexNew->pprev = (*miPrev).second;
+        pindexNew->pprev = &(*miPrev).second;
         pindexNew->nHeight = pindexNew->pprev->nHeight + 1;
         pindexNew->BuildSkip();
     }
@@ -80,8 +89,8 @@ void BlockManager::PruneOneBlockFile(const int fileNumber)
     AssertLockHeld(cs_main);
     LOCK(cs_LastBlockFile);
 
-    for (const auto& entry : m_block_index) {
-        CBlockIndex* pindex = entry.second;
+    for (auto& entry : m_block_index) {
+        CBlockIndex* pindex = &entry.second;
         if (pindex->nFile == fileNumber) {
             pindex->nStatus &= ~BLOCK_HAVE_DATA;
             pindex->nStatus &= ~BLOCK_HAVE_UNDO;
@@ -202,12 +211,13 @@ CBlockIndex* BlockManager::InsertBlockIndex(const uint256& hash)
     // Return existing
     BlockMap::iterator mi = m_block_index.find(hash);
     if (mi != m_block_index.end()) {
-        return (*mi).second;
+        return &(*mi).second;
     }
 
     // Create new
-    CBlockIndex* pindexNew = new CBlockIndex();
-    mi = m_block_index.insert(std::make_pair(hash, pindexNew)).first;
+    CBlockIndex new_index{};
+    mi = m_block_index.insert(std::make_pair(hash, std::move(new_index))).first;
+    CBlockIndex* pindexNew = &(*mi).second;
     pindexNew->phashBlock = &((*mi).first);
 
     return pindexNew;
@@ -224,8 +234,8 @@ bool BlockManager::LoadBlockIndex(
     // Calculate nChainWork
     std::vector<std::pair<int, CBlockIndex*>> vSortedByHeight;
     vSortedByHeight.reserve(m_block_index.size());
-    for (const std::pair<const uint256, CBlockIndex*>& item : m_block_index) {
-        CBlockIndex* pindex = item.second;
+    for (std::pair<const uint256, CBlockIndex>& item : m_block_index) {
+        CBlockIndex* pindex = &item.second;
         vSortedByHeight.push_back(std::make_pair(pindex->nHeight, pindex));
     }
     sort(vSortedByHeight.begin(), vSortedByHeight.end());
@@ -327,10 +337,6 @@ void BlockManager::Unload()
 {
     m_blocks_unlinked.clear();
 
-    for (const BlockMap::value_type& entry : m_block_index) {
-        delete entry.second;
-    }
-
     m_block_index.clear();
 
     m_blockfile_info.clear();
@@ -386,8 +392,8 @@ bool BlockManager::LoadBlockIndexDB(ChainstateManager& chainman)
     // Check presence of blk files
     LogPrintf("Checking all blk files are present...\n");
     std::set<int> setBlkDataFiles;
-    for (const std::pair<const uint256, CBlockIndex*>& item : m_block_index) {
-        CBlockIndex* pindex = item.second;
+    for (const std::pair<const uint256, CBlockIndex>& item : m_block_index) {
+        const CBlockIndex* pindex = &item.second;
         if (pindex->nStatus & BLOCK_HAVE_DATA) {
             setBlkDataFiles.insert(pindex->nFile);
         }
diff --git a/src/node/blockstorage.h b/src/node/blockstorage.h
index 42e46797d2..be1ed83064 100644
--- a/src/node/blockstorage.h
+++ b/src/node/blockstorage.h
@@ -5,6 +5,7 @@
 #ifndef BITCOIN_NODE_BLOCKSTORAGE_H
 #define BITCOIN_NODE_BLOCKSTORAGE_H
 
+#include <chain.h>
 #include <fs.h>
 #include <protocol.h> // For CMessageHeader::MessageStartChars
 #include <sync.h>
@@ -20,7 +21,6 @@ class ArgsManager;
 class BlockValidationState;
 class CBlock;
 class CBlockFileInfo;
-class CBlockIndex;
 class CBlockUndo;
 class CChain;
 class CChainParams;
@@ -52,7 +52,11 @@ extern bool fPruneMode;
 /** Number of MiB of block files that we're trying to stay below. */
 extern uint64_t nPruneTarget;
 
-typedef std::unordered_map<uint256, CBlockIndex*, BlockHasher> BlockMap;
+// Because validation code takes pointers to the map's CBlockIndex objects, if
+// we ever switch to another associative container, we need to either use a
+// container that has stable addressing (true of all std associative
+// containers), or make the key a `std::unique_ptr<CBlockIndex>`
+typedef std::unordered_map<uint256, CBlockIndex, BlockHasher> BlockMap;
 
 struct CBlockIndexWorkComparator {
     bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const;
@@ -144,7 +148,8 @@ public:
     //! Mark one block file as pruned (modify associated database entries)
     void PruneOneBlockFile(const int fileNumber) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-    CBlockIndex* LookupBlockIndex(const uint256& hash) const EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    CBlockIndex* LookupBlockIndex(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    const CBlockIndex* LookupBlockIndex(const uint256& hash) const EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
     /** Get block file info entry for one block file */
     CBlockFileInfo* GetBlockFileInfo(size_t n);