[backport#17342][refactor] Remove global int nScriptCheckThreads

Summary:
The global nScriptCheckThreads int is confusing and is only needed for
its int-ness in AppInitMain. Move all `-par` parsing logic there and
~~replace~~ remove the int nScriptCheckThreads ~~with a bool g_parallel_script_checks.~~ (see D521)

Also tidy up logic and improve comments.

bitcoin/bitcoin@5506ecf

---

Concludes backport of Core [[bitcoin/bitcoin#17342 | PR17342]]

Depends on D7274

Test Plan:
  ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7275

Author: jnewbery

src/init.cpp

@@ -1821,17 +1821,6 @@ bool AppInitParameterInteraction(Config &config) {
                       std::ceil(nMempoolSizeMin / 1000000.0)));
     }
 
-    // -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;
-    }
-
     // Configure excessive block size.
     const uint64_t nProposedExcessiveBlockSize =
         gArgs.GetArg("-excessiveblocksize", DEFAULT_MAX_BLOCK_SIZE);
@@ -2104,10 +2093,24 @@ bool AppInitMain(Config &config, RPCServer &rpcServer,
     InitSignatureCache();
     InitScriptExecutionCache();
 
-    LogPrintf("Using %u threads for script verification\n",
-              nScriptCheckThreads);
-    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) {
+        for (int i = 0; i < script_threads; ++i) {
             threadGroup.create_thread([i]() { return ThreadScriptCheck(i); });
         }
     }

src/test/util/setup_common.cpp

@@ -124,8 +124,8 @@ TestingSetup::TestingSetup(const std::string &chainName)
                                                FormatStateMessage(state)));
         }
     }
-    nScriptCheckThreads = 3;
-    for (int i = 0; i < nScriptCheckThreads - 1; i++) {
+    constexpr int script_check_threads = 2;
+    for (int i = 0; i < script_check_threads; ++i) {
         threadGroup.create_thread([i]() { return ThreadScriptCheck(i); });
     }
 

src/validation.cpp

@@ -87,7 +87,6 @@ CBlockIndex *pindexBestHeader = nullptr;
 Mutex g_best_block_mutex;
 std::condition_variable g_best_block_cv;
 uint256 g_best_block;
-int nScriptCheckThreads = 0;
 std::atomic_bool fImporting(false);
 std::atomic_bool fReindex(false);
 bool fHavePruned = false;

src/validation.h

@@ -78,8 +78,8 @@ 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;
 /**
@@ -176,7 +176,6 @@ 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;
 extern bool fRequireStandard;
 extern bool fCheckBlockIndex;
 extern bool fCheckpointsEnabled;