[refactor] Replace global int nScriptCheckThreads with bool

The global nScriptCheckThreads int is confusing and is only needed for
its int-ness in AppInitMain. Move all `-par` parsing logic there and
replace the int nScriptCheckThreads with a bool
g_parallel_script_checks.

Also tidy up logic and improve comments.

src/init.cpp

@@ -1061,15 +1061,6 @@ bool AppInitParameterInteraction()
         incrementalRelayFee = CFeeRate(n);
     }
 
-    // -par=0 means autodetect, but nScriptCheckThreads==0 means no concurrency
-    nScriptCheckThreads = gArgs.GetArg("-par", DEFAULT_SCRIPTCHECK_THREADS);
-    if (nScriptCheckThreads <= 0)
-        nScriptCheckThreads += GetNumCores();
-    if (nScriptCheckThreads <= 1)
-        nScriptCheckThreads = 0;
-    else if (nScriptCheckThreads > MAX_SCRIPTCHECK_THREADS)
-        nScriptCheckThreads = MAX_SCRIPTCHECK_THREADS;
-
     // block pruning; get the amount of disk space (in MiB) to allot for block & undo files
     int64_t nPruneArg = gArgs.GetArg("-prune", 0);
     if (nPruneArg < 0) {
@@ -1256,11 +1247,26 @@ bool AppInitMain(NodeContext& node)
     InitSignatureCache();
     InitScriptExecutionCache();
 
-    LogPrintf("Script verification uses %d additional threads\n", std::max(nScriptCheckThreads - 1, 0));
-    if (nScriptCheckThreads) {
-        for (int i=0; i<nScriptCheckThreads-1; i++)
+    int script_threads = gArgs.GetArg("-par", DEFAULT_SCRIPTCHECK_THREADS);
+    if (script_threads <= 0) {
+        // -par=0 means autodetect (number of cores - 1 script threads)
+        // -par=-n means "leave n cores free" (number of cores - n - 1 script threads)
+        script_threads += GetNumCores();
+    }
+
+    // Subtract 1 because the main thread counts towards the par threads
+    script_threads = std::max(script_threads - 1, 0);
+
+    // Number of script-checking threads <= MAX_SCRIPTCHECK_THREADS
+    script_threads = std::min(script_threads, MAX_SCRIPTCHECK_THREADS);
+
+    LogPrintf("Script verification uses %d additional threads\n", script_threads);
+    if (script_threads >= 1) {
+        g_parallel_script_checks = true;
+        for (int i = 0; i < script_threads; ++i) {
             threadGroup.create_thread([i]() { return ThreadScriptCheck(i); });
         }
+    }
 
     // Start the lightweight task scheduler thread
     CScheduler::Function serviceLoop = std::bind(&CScheduler::serviceQueue, &scheduler);

src/test/setup_common.cpp

@@ -102,9 +102,12 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
         throw std::runtime_error(strprintf("ActivateBestChain failed. (%s)", FormatStateMessage(state)));
     }
 
-    nScriptCheckThreads = 3;
-    for (int i = 0; i < nScriptCheckThreads - 1; i++)
+    // Start script-checking threads. Set g_parallel_script_checks to true so they are used.
+    constexpr int script_check_threads = 2;
+    for (int i = 0; i < script_check_threads; ++i) {
         threadGroup.create_thread([i]() { return ThreadScriptCheck(i); });
+    }
+    g_parallel_script_checks = true;
 
     m_node.banman = MakeUnique<BanMan>(GetDataDir() / "banlist.dat", nullptr, DEFAULT_MISBEHAVING_BANTIME);
     m_node.connman = MakeUnique<CConnman>(0x1337, 0x1337); // Deterministic randomness for tests.

src/validation.cpp

@@ -107,7 +107,7 @@ CBlockIndex *pindexBestHeader = nullptr;
 Mutex g_best_block_mutex;
 std::condition_variable g_best_block_cv;
 uint256 g_best_block;
-int nScriptCheckThreads = 0;
+bool g_parallel_script_checks{false};
 std::atomic_bool fImporting(false);
 std::atomic_bool fReindex(false);
 bool fHavePruned = false;
@@ -2069,7 +2069,7 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
 
     CBlockUndo blockundo;
 
-    CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : nullptr);
+    CCheckQueueControl<CScriptCheck> control(fScriptChecks && g_parallel_script_checks ? &scriptcheckqueue : nullptr);
 
     std::vector<int> prevheights;
     CAmount nFees = 0;
@@ -2130,7 +2130,7 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
             std::vector<CScriptCheck> vChecks;
             bool fCacheResults = fJustCheck; /* Don't cache results if we're actually connecting blocks (still consult the cache, though) */
             TxValidationState tx_state;
-            if (fScriptChecks && !CheckInputs(tx, tx_state, view, flags, fCacheResults, fCacheResults, txdata[i], nScriptCheckThreads ? &vChecks : nullptr)) {
+            if (fScriptChecks && !CheckInputs(tx, tx_state, view, flags, fCacheResults, fCacheResults, txdata[i], g_parallel_script_checks ? &vChecks : nullptr)) {
                 // Any transaction validation failure in ConnectBlock is a block consensus failure
                 state.Invalid(BlockValidationResult::BLOCK_CONSENSUS,
                               tx_state.GetRejectReason(), tx_state.GetDebugMessage());

src/validation.h

@@ -76,8 +76,8 @@ static const unsigned int BLOCKFILE_CHUNK_SIZE = 0x1000000; // 16 MiB
 /** The pre-allocation chunk size for rev?????.dat files (since 0.8) */
 static const unsigned int UNDOFILE_CHUNK_SIZE = 0x100000; // 1 MiB
 
-/** Maximum number of script-checking threads allowed */
-static const int MAX_SCRIPTCHECK_THREADS = 16;
+/** Maximum number of dedicated script-checking threads allowed */
+static const int MAX_SCRIPTCHECK_THREADS = 15;
 /** -par default (number of script-checking threads, 0 = auto) */
 static const int DEFAULT_SCRIPTCHECK_THREADS = 0;
 /** Number of blocks that can be requested at any given time from a single peer. */
@@ -146,7 +146,10 @@ extern std::condition_variable g_best_block_cv;
 extern uint256 g_best_block;
 extern std::atomic_bool fImporting;
 extern std::atomic_bool fReindex;
-extern int nScriptCheckThreads;
+/** Whether there are dedicated script-checking threads running.
+ * False indicates all script checking is done on the main threadMessageHandler thread.
+ */
+extern bool g_parallel_script_checks;
 extern bool fRequireStandard;
 extern bool fCheckBlockIndex;
 extern bool fCheckpointsEnabled;