Make nSequenceId init value constants

Make it easier to follow what the values come without having to go
over the comments, plus easier to maintain

src/chain.h

@@ -35,6 +35,9 @@ static constexpr int64_t MAX_FUTURE_BLOCK_TIME = 2 * 60 * 60;
  * MAX_FUTURE_BLOCK_TIME.
  */
 static constexpr int64_t TIMESTAMP_WINDOW = MAX_FUTURE_BLOCK_TIME;
+ //! Init values for CBlockIndex nSequenceId when loaded from disk
+static constexpr int32_t SEQ_ID_BEST_CHAIN_FROM_DISK = 0;
+static constexpr int32_t SEQ_ID_INIT_FROM_DISK = 1;
 
 /**
  * Maximum gap between node time and block time used
@@ -191,9 +194,9 @@ public:
     uint32_t nNonce{0};
 
     //! (memory only) Sequential id assigned to distinguish order in which blocks are received.
-    //! Initialized to 1 when loading blocks from disk, except for blocks belonging to the best chain
+    //! Initialized to SEQ_ID_INIT_FROM_DISK{1} when loading blocks from disk, except for blocks
-    //! which overwrite it to 0.
+    //! belonging to the best chain which overwrite it to SEQ_ID_BEST_CHAIN_FROM_DISK{0}.
-    int32_t nSequenceId{1};
+    int32_t nSequenceId{SEQ_ID_INIT_FROM_DISK};
 
     //! (memory only) Maximum nTime in the chain up to and including this block.
     unsigned int nTimeMax{0};

src/node/blockstorage.cpp

@@ -215,7 +215,7 @@ CBlockIndex* BlockManager::AddToBlockIndex(const CBlockHeader& block, CBlockInde
     // 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 = 1;
+    pindexNew->nSequenceId = SEQ_ID_INIT_FROM_DISK;
 
     pindexNew->phashBlock = &((*mi).first);
     BlockMap::iterator miPrev = m_block_index.find(block.hashPrevBlock);

src/validation.cpp

@@ -4724,7 +4724,7 @@ bool Chainstate::LoadChainTip()
     // to maintain a consistent best tip over reboots
     auto target = tip;
     while (target) {
-        target->nSequenceId = 0;
+        target->nSequenceId = SEQ_ID_BEST_CHAIN_FROM_DISK;
         target = target->pprev;
     }
 
@@ -5379,7 +5379,9 @@ void ChainstateManager::CheckBlockIndex()
                 }
             }
         }
-        if (!pindex->HaveNumChainTxs()) assert(pindex->nSequenceId <= 1); // nSequenceId can't be set higher than 1 for blocks that aren't linked (negative is used for preciousblock, 0 for active chain)
+        // nSequenceId can't be set higher than SEQ_ID_INIT_FROM_DISK{1} for blocks that aren't linked
+        // (negative is used for preciousblock, SEQ_ID_BEST_CHAIN_FROM_DISK{0} for active chain when loaded from disk)
+        if (!pindex->HaveNumChainTxs()) assert(pindex->nSequenceId <= SEQ_ID_INIT_FROM_DISK);
         // VALID_TRANSACTIONS is equivalent to nTx > 0 for all nodes (whether or not pruning has occurred).
         // HAVE_DATA is only equivalent to nTx > 0 (or VALID_TRANSACTIONS) if no pruning has occurred.
         if (!m_blockman.m_have_pruned) {

src/validation.h

@@ -1021,9 +1021,10 @@ public:
      * Every received block is assigned a unique and increasing identifier, so we
      * know which one to give priority in case of a fork.
      */
-    /** Blocks loaded from disk are assigned id 1 (0 if they belong to the best
+    /** Blocks loaded from disk are assigned id SEQ_ID_INIT_FROM_DISK{1}
-     * chain loaded from disk), so start the counter at 2. **/
+     * (SEQ_ID_BEST_CHAIN_FROM_DISK{0} if they belong to the best chain loaded from disk),
-    int32_t nBlockSequenceId GUARDED_BY(::cs_main) = 2;
+     * so start the counter after that. **/
+    int32_t nBlockSequenceId GUARDED_BY(::cs_main) = SEQ_ID_INIT_FROM_DISK + 1;
     /** Decreasing counter (used by subsequent preciousblock calls). */
     int32_t nBlockReverseSequenceId = -1;
     /** chainwork for the last block that preciousblock has been applied to. */
@@ -1034,7 +1035,7 @@ public:
     void ResetBlockSequenceCounters() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
         AssertLockHeld(::cs_main);
-        nBlockSequenceId = 2;
+        nBlockSequenceId = SEQ_ID_INIT_FROM_DISK + 1;
         nBlockReverseSequenceId = -1;
     }