refactor: return reference instead of pointer

The return value of BlockManager::GetFirstBlock must always be non-null. This
can be inferred by the implementation, which has an assertion that the return
value is not null. A raw pointer should only be returned if the result may be
null. In this case a reference is more appropriate.

src/node/blockstorage.cpp

@@ -588,7 +588,7 @@ bool BlockManager::IsBlockPruned(const CBlockIndex& block) const
     return m_have_pruned && !(block.nStatus & BLOCK_HAVE_DATA) && (block.nTx > 0);
 }
 
-const CBlockIndex* BlockManager::GetFirstBlock(const CBlockIndex& upper_block, uint32_t status_mask, const CBlockIndex* lower_block) const
+const CBlockIndex& BlockManager::GetFirstBlock(const CBlockIndex& upper_block, uint32_t status_mask, const CBlockIndex* lower_block) const
 {
     AssertLockHeld(::cs_main);
     const CBlockIndex* last_block = &upper_block;
@@ -596,7 +596,7 @@ const CBlockIndex* BlockManager::GetFirstBlock(const CBlockIndex& upper_block, u
     while (last_block->pprev && ((last_block->pprev->nStatus & status_mask) == status_mask)) {
         if (lower_block) {
             // Return if we reached the lower_block
-            if (last_block == lower_block) return lower_block;
+            if (last_block == lower_block) return *lower_block;
             // if range was surpassed, means that 'lower_block' is not part of the 'upper_block' chain
             // and so far this is not allowed.
             assert(last_block->nHeight >= lower_block->nHeight);
@@ -604,13 +604,13 @@ const CBlockIndex* BlockManager::GetFirstBlock(const CBlockIndex& upper_block, u
         last_block = last_block->pprev;
     }
     assert(last_block != nullptr);
-    return last_block;
+    return *last_block;
 }
 
 bool BlockManager::CheckBlockDataAvailability(const CBlockIndex& upper_block, const CBlockIndex& lower_block)
 {
     if (!(upper_block.nStatus & BLOCK_HAVE_DATA)) return false;
-    return GetFirstBlock(upper_block, BLOCK_HAVE_DATA, &lower_block) == &lower_block;
+    return &GetFirstBlock(upper_block, BLOCK_HAVE_DATA, &lower_block) == &lower_block;
 }
 
 // If we're using -prune with -reindex, then delete block files that will be ignored by the

src/node/blockstorage.h

@@ -422,11 +422,11 @@ public:
      * @param lower_block The earliest possible block to return. If null, the
      *                    search can extend to the genesis block.
      *
-     * @return A non-null pointer to the earliest block between `upper_block`
+     * @return A reference to the earliest block between `upper_block`
      *         and `lower_block`, inclusive, such that every block between the
      *         returned block and `upper_block` has `status_mask` flags set.
      */
-    const CBlockIndex* GetFirstBlock(
+    const CBlockIndex& GetFirstBlock(
         const CBlockIndex& upper_block LIFETIMEBOUND,
         uint32_t status_mask,
         const CBlockIndex* lower_block LIFETIMEBOUND = nullptr

src/rpc/blockchain.cpp

@@ -870,7 +870,7 @@ std::optional<int> GetPruneHeight(const BlockManager& blockman, const CChain& ch
     // If the chain tip is pruned, everything is pruned.
     if (!((chain_tip->nStatus & BLOCK_HAVE_MASK) == BLOCK_HAVE_MASK)) return chain_tip->nHeight;
 
-    const auto& first_unpruned{*CHECK_NONFATAL(blockman.GetFirstBlock(*chain_tip, /*status_mask=*/BLOCK_HAVE_MASK, first_block))};
+    const auto& first_unpruned{blockman.GetFirstBlock(*chain_tip, /*status_mask=*/BLOCK_HAVE_MASK, first_block)};
     if (&first_unpruned == first_block) {
         // All blocks between first_block and chain_tip have data, so nothing is pruned.
         return std::nullopt;
@@ -3116,8 +3116,8 @@ static RPCHelpMan dumptxoutset()
         if (node.chainman->m_blockman.IsPruneMode()) {
             LOCK(node.chainman->GetMutex());
             const CBlockIndex* current_tip{node.chainman->ActiveChain().Tip()};
-            const CBlockIndex* first_block{node.chainman->m_blockman.GetFirstBlock(*current_tip, /*status_mask=*/BLOCK_HAVE_MASK)};
+            const CBlockIndex& first_block{node.chainman->m_blockman.GetFirstBlock(*current_tip, /*status_mask=*/BLOCK_HAVE_MASK)};
-            if (first_block->nHeight > target_index->nHeight) {
+            if (first_block.nHeight > target_index->nHeight) {
                 throw JSONRPCError(RPC_MISC_ERROR, "Could not roll back to requested height since necessary block data is already pruned.");
             }
         }

src/test/blockmanager_tests.cpp

@@ -119,7 +119,7 @@ BOOST_FIXTURE_TEST_CASE(blockmanager_block_data_availability, TestChain100Setup)
     };
 
     // 1) Return genesis block when all blocks are available
-    BOOST_CHECK_EQUAL(blockman.GetFirstBlock(tip, BLOCK_HAVE_DATA), chainman->ActiveChain()[0]);
+    BOOST_CHECK_EQUAL(&blockman.GetFirstBlock(tip, BLOCK_HAVE_DATA), chainman->ActiveChain()[0]);
     BOOST_CHECK(blockman.CheckBlockDataAvailability(tip, *chainman->ActiveChain()[0]));
 
     // 2) Check lower_block when all blocks are available
@@ -133,7 +133,7 @@ BOOST_FIXTURE_TEST_CASE(blockmanager_block_data_availability, TestChain100Setup)
     func_prune_blocks(last_pruned_block);
 
     // 3) The last block not pruned is in-between upper-block and the genesis block
-    BOOST_CHECK_EQUAL(blockman.GetFirstBlock(tip, BLOCK_HAVE_DATA), first_available_block);
+    BOOST_CHECK_EQUAL(&blockman.GetFirstBlock(tip, BLOCK_HAVE_DATA), first_available_block);
     BOOST_CHECK(blockman.CheckBlockDataAvailability(tip, *first_available_block));
     BOOST_CHECK(!blockman.CheckBlockDataAvailability(tip, *last_pruned_block));
 }