remove superfluous use of runtime.SetFinalizer on SQLiteRows

The commit removes the use of runtime.SetFinalizer to finalize
SQLiteRows since only serves to close the associated SQLiteStmt which
already has a registered finalizer.

It also fixes a race and potential panic in SQLiteRows.Close around the
SQLiteRows.s field (*SQLiteStmt) which is accessed without a mutex being
held, but modified with it held (null'd out). Further the mutex we are
holding is that of the SQLiteStmt so a subsequent call to Close will
cause a panic sine it'll attempt to dereference a nil field. The fix
here is to add a mutex for closing to SQLiteRows.

Since we now also set the s field to nil when closing this commit
removes the "closed" field (since checking if s is nil is the same) and
also changes the type of "nc" (number of columns) to an int32 so that we
can pack the nc and cls fields, and add the close mutex without making
the struct any bigger.

```
goos: darwin
goarch: arm64
pkg: github.com/charlievieth/go-sqlite3
cpu: Apple M4 Pro
                                          │   x1.txt    │               x4.txt                │
                                          │   sec/op    │   sec/op     vs base                │
Suite/BenchmarkExec/Params-14               719.2n ± 2%   716.9n ± 1%        ~ (p=0.897 n=10)
Suite/BenchmarkExec/NoParams-14             506.5n ± 3%   500.1n ± 0%   -1.25% (p=0.002 n=10)
Suite/BenchmarkExecContext/Params-14        1.584µ ± 0%   1.567µ ± 1%   -1.07% (p=0.007 n=10)
Suite/BenchmarkExecContext/NoParams-14      1.524µ ± 1%   1.524µ ± 1%        ~ (p=0.539 n=10)
Suite/BenchmarkExecStep-14                  443.9µ ± 3%   441.4µ ± 0%   -0.55% (p=0.011 n=10)
Suite/BenchmarkExecContextStep-14           447.8µ ± 1%   442.9µ ± 0%   -1.10% (p=0.000 n=10)
Suite/BenchmarkExecTx-14                    1.643µ ± 1%   1.640µ ± 0%        ~ (p=0.642 n=10)
Suite/BenchmarkQuery-14                     1.968µ ± 3%   1.821µ ± 1%   -7.52% (p=0.000 n=10)
Suite/BenchmarkQuerySimple-14               1.207µ ± 2%   1.040µ ± 1%  -13.84% (p=0.000 n=10)
Suite/BenchmarkQueryContext/Background-14   2.400µ ± 1%   2.320µ ± 0%   -3.31% (p=0.000 n=10)
Suite/BenchmarkQueryContext/WithCancel-14   8.847µ ± 5%   8.512µ ± 4%   -3.79% (p=0.007 n=10)
Suite/BenchmarkParams-14                    2.131µ ± 2%   1.967µ ± 1%   -7.70% (p=0.000 n=10)
Suite/BenchmarkStmt-14                      1.444µ ± 1%   1.359µ ± 1%   -5.89% (p=0.000 n=10)
Suite/BenchmarkRows-14                      61.57µ ± 1%   60.24µ ± 1%   -2.16% (p=0.000 n=10)
Suite/BenchmarkStmtRows-14                  60.15µ ± 1%   59.08µ ± 1%   -1.78% (p=0.000 n=10)
Suite/BenchmarkQueryParallel-14             960.9n ± 1%   420.8n ± 2%  -56.21% (p=0.000 n=10)
geomean                                     4.795µ        4.430µ        -7.62%
```

sqlite3.go

@@ -381,7 +381,7 @@ type SQLiteStmt struct {
 	s      *C.sqlite3_stmt
 	t      string
 	closed bool
-	cls    bool
+	cls    bool // True if the statement was created by SQLiteConn.Query
 }
 
 // SQLiteResult implements sql.Result.
@@ -393,12 +393,12 @@ type SQLiteResult struct {
 // SQLiteRows implements driver.Rows.
 type SQLiteRows struct {
 	s        *SQLiteStmt
-	nc       int
+	nc       int32 // Number of columns
+	cls      bool  // True if we need to close the parent statement in Close
 	cols     []string
 	decltype []string
-	cls      bool
-	closed   bool
 	ctx      context.Context // no better alternative to pass context into Next() method
+	closemu  sync.Mutex
 }
 
 type functionInfo struct {
@@ -2008,14 +2008,12 @@ func (s *SQLiteStmt) query(ctx context.Context, args []driver.NamedValue) (drive
 
 	rows := &SQLiteRows{
 		s:        s,
-		nc:       int(C.sqlite3_column_count(s.s)),
+		nc:       int32(C.sqlite3_column_count(s.s)),
+		cls:      s.cls,
 		cols:     nil,
 		decltype: nil,
-		cls:      s.cls,
-		closed:   false,
 		ctx:      ctx,
 	}
-	runtime.SetFinalizer(rows, (*SQLiteRows).Close)
 
 	return rows, nil
 }
@@ -2112,24 +2110,28 @@ func (s *SQLiteStmt) Readonly() bool {
 
 // Close the rows.
 func (rc *SQLiteRows) Close() error {
-	rc.s.mu.Lock()
+	rc.closemu.Lock()
-	if rc.s.closed || rc.closed {
+	defer rc.closemu.Unlock()
-		rc.s.mu.Unlock()
+	s := rc.s
+	if s == nil {
+		return nil
+	}
+	rc.s = nil // remove reference to SQLiteStmt
+	s.mu.Lock()
+	if s.closed {
+		s.mu.Unlock()
 		return nil
 	}
-	rc.closed = true
 	if rc.cls {
-		rc.s.mu.Unlock()
+		s.mu.Unlock()
-		return rc.s.Close()
+		return s.Close()
 	}
-	rv := C.sqlite3_reset(rc.s.s)
+	rv := C.sqlite3_reset(s.s)
 	if rv != C.SQLITE_OK {
-		rc.s.mu.Unlock()
+		s.mu.Unlock()
-		return rc.s.c.lastError()
+		return s.c.lastError()
 	}
-	rc.s.mu.Unlock()
+	s.mu.Unlock()
-	rc.s = nil
-	runtime.SetFinalizer(rc, nil)
 	return nil
 }
 
@@ -2137,9 +2139,9 @@ func (rc *SQLiteRows) Close() error {
 func (rc *SQLiteRows) Columns() []string {
 	rc.s.mu.Lock()
 	defer rc.s.mu.Unlock()
-	if rc.s.s != nil && rc.nc != len(rc.cols) {
+	if rc.s.s != nil && int(rc.nc) != len(rc.cols) {
 		rc.cols = make([]string, rc.nc)
-		for i := 0; i < rc.nc; i++ {
+		for i := 0; i < int(rc.nc); i++ {
 			rc.cols[i] = C.GoString(C.sqlite3_column_name(rc.s.s, C.int(i)))
 		}
 	}
@@ -2149,7 +2151,7 @@ func (rc *SQLiteRows) Columns() []string {
 func (rc *SQLiteRows) declTypes() []string {
 	if rc.s.s != nil && rc.decltype == nil {
 		rc.decltype = make([]string, rc.nc)
-		for i := 0; i < rc.nc; i++ {
+		for i := 0; i < int(rc.nc); i++ {
 			rc.decltype[i] = strings.ToLower(C.GoString(C.sqlite3_column_decltype(rc.s.s, C.int(i))))
 		}
 	}

sqlite3_test.go

@@ -2111,6 +2111,7 @@ var benchmarks = []testing.InternalBenchmark{
 	{Name: "BenchmarkStmt", F: benchmarkStmt},
 	{Name: "BenchmarkRows", F: benchmarkRows},
 	{Name: "BenchmarkStmtRows", F: benchmarkStmtRows},
+	{Name: "BenchmarkQueryParallel", F: benchmarkQueryParallel},
 }
 
 func (db *TestDB) mustExec(sql string, args ...any) sql.Result {
@@ -2568,3 +2569,20 @@ func benchmarkStmtRows(b *testing.B) {
 		}
 	}
 }
+
+func benchmarkQueryParallel(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		db, err := sql.Open("sqlite3", ":memory:")
+		if err != nil {
+			panic(err)
+		}
+		db.SetMaxOpenConns(runtime.NumCPU())
+		defer db.Close()
+		var i int64
+		for pb.Next() {
+			if err := db.QueryRow("SELECT 1, 2, 3, 4").Scan(&i, &i, &i, &i); err != nil {
+				panic(err)
+			}
+		}
+	})
+}