validation, test: Improve and document nChainTx check for testability

Co-authored-by: Ryan Ofsky <[email protected]>

Author: fjahr

src/kernel/chainparams.cpp

@@ -495,7 +495,7 @@ class CRegTestParams : public CChainParams
             {
                 .height = 110,
                 .hash_serialized = AssumeutxoHash{uint256S("0x1ebbf5850204c0bdb15bf030f47c7fe91d45c44c712697e4509ba67adb01c618")},
-                .nChainTx = 110,
+                .nChainTx = 111,
                 .blockhash = uint256S("0x696e92821f65549c7ee134edceeeeaaa4105647a3c4fd9f298c0aec0ab50425c")
             },
             {

src/test/validation_tests.cpp

@@ -138,11 +138,11 @@ BOOST_AUTO_TEST_CASE(test_assumeutxo)
 
     const auto out110 = *params->AssumeutxoForHeight(110);
     BOOST_CHECK_EQUAL(out110.hash_serialized.ToString(), "1ebbf5850204c0bdb15bf030f47c7fe91d45c44c712697e4509ba67adb01c618");
-    BOOST_CHECK_EQUAL(out110.nChainTx, 110U);
+    BOOST_CHECK_EQUAL(out110.nChainTx, 111U);
 
     const auto out110_2 = *params->AssumeutxoForBlockhash(uint256S("0x696e92821f65549c7ee134edceeeeaaa4105647a3c4fd9f298c0aec0ab50425c"));
     BOOST_CHECK_EQUAL(out110_2.hash_serialized.ToString(), "1ebbf5850204c0bdb15bf030f47c7fe91d45c44c712697e4509ba67adb01c618");
-    BOOST_CHECK_EQUAL(out110_2.nChainTx, 110U);
+    BOOST_CHECK_EQUAL(out110_2.nChainTx, 111U);
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/validation.cpp

@@ -4844,10 +4844,14 @@ void ChainstateManager::CheckBlockIndex()
     CBlockIndex* pindexFirstAssumeValid = nullptr; // Oldest ancestor of pindex which has BLOCK_ASSUMED_VALID
     while (pindex != nullptr) {
         nNodes++;
-        if (pindex->pprev && pindex->nTx > 0) {
-            // nChainTx should increase monotonically
-            assert(pindex->pprev->nChainTx <= pindex->nChainTx);
-        }
+        // Make sure nChainTx sum is correctly computed.
+        unsigned int prev_chain_tx = pindex->pprev ? pindex->pprev->nChainTx : 0;
+        assert((pindex->nChainTx == pindex->nTx + prev_chain_tx)
+               // For testing, allow transaction counts to be completely unset.
+               || (pindex->nChainTx == 0 && pindex->nTx == 0)
+               // For testing, allow this nChainTx to be unset if previous is also unset.
+               || (pindex->nChainTx == 0 && prev_chain_tx == 0 && pindex->pprev));
+
         if (pindexFirstAssumeValid == nullptr && pindex->nStatus & BLOCK_ASSUMED_VALID) pindexFirstAssumeValid = pindex;
         if (pindexFirstInvalid == nullptr && pindex->nStatus & BLOCK_FAILED_VALID) pindexFirstInvalid = pindex;
         if (pindexFirstMissing == nullptr && !(pindex->nStatus & BLOCK_HAVE_DATA)) {