validation: fix ActivateSnapshot to use hardcoded nChainTx

This fixes an oversight from the move of nChainTx from the user-supplied
snapshot metadata into the hardcoded assumeutxo chainparams.

Since the nChainTx is now unused in the metadata, it should be removed
in a future commit.

src/node/utxo_snapshot.h

@@ -24,6 +24,8 @@ class SnapshotMetadata
 
     //! Necessary to "fake" the base nChainTx so that we can estimate progress during
     //! initial block download for the assumeutxo chainstate.
+    //!
+    //! TODO: this is unused and should be removed.
     unsigned int m_nchaintx = 0;
 
     SnapshotMetadata() { }

src/test/validation_chainstatemanager_tests.cpp

@@ -233,6 +233,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_activate_snapshot, TestChain100Setup)
 
     // Mine 10 more blocks, putting at us height 110 where a valid assumeutxo value can
     // be found.
+    constexpr int snapshot_height = 110;
     mineBlocks(10);
     initial_size += 10;
     initial_total_coins += 10;
@@ -265,14 +266,31 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_activate_snapshot, TestChain100Setup)
             metadata.m_base_blockhash = uint256::ONE;
     }));
 
-    BOOST_REQUIRE(CreateAndActivateUTXOSnapshot(m_node, m_path_root));
+    constexpr int bad_nchaintx = 9999;
+
+    BOOST_REQUIRE(CreateAndActivateUTXOSnapshot(
+        m_node, m_path_root, [](CAutoFile& auto_infile, SnapshotMetadata& metadata) {
+            // Provide an nChainTx that differs from the hardcoded one.
+            //
+            // Ultimately this malleation check should be removed when we remove
+            // the now-unnecessary nChainTx from the user-specified snapshot metadata.
+            metadata.m_nchaintx = bad_nchaintx;
+    }));
 
     // Ensure our active chain is the snapshot chainstate.
     BOOST_CHECK(!chainman.ActiveChainstate().m_from_snapshot_blockhash.IsNull());
     BOOST_CHECK_EQUAL(
         chainman.ActiveChainstate().m_from_snapshot_blockhash,
         *chainman.SnapshotBlockhash());
 
+    const AssumeutxoData& au_data = *ExpectedAssumeutxo(snapshot_height, ::Params());
+    const CBlockIndex* tip = chainman.ActiveTip();
+
+    // Ensure that, despite a bad nChainTx value being in the snapshot, activation
+    // uses the hardcoded value from chainparams.
+    BOOST_CHECK_EQUAL(tip->nChainTx, au_data.nChainTx);
+    BOOST_CHECK(tip->nChainTx != bad_nchaintx);
+
     // To be checked against later when we try loading a subsequent snapshot.
     uint256 loaded_snapshot_blockhash{*chainman.SnapshotBlockhash()};
 

src/validation.cpp

@@ -5346,7 +5346,7 @@ bool ChainstateManager::PopulateAndValidateSnapshot(
     }
 
     assert(index);
-    index->nChainTx = metadata.m_nchaintx;
+    index->nChainTx = au_data.nChainTx;
     snapshot_chainstate.setBlockIndexCandidates.insert(snapshot_start_block);
 
     LogPrintf("[snapshot] validated snapshot (%.2f MB)\n",