Fix exponential memory allocation in SQLiteConn.Exec

This commit fixes SQLiteConn.Exec to not duplicate the provided SQL
statement and to pass the Go string to libsqlite3 instead of creating
a C string copy and passing that. The issue with the previous
implementation was that it would create a new C string copy of the
original Go string each time it stepped through a SQL statement. This
led to exponential memory usage when Exec'ing an argument that contained
multiple SQL statements (which is common when initializing a database
from a SQL dump).

This commit is a slimmed down version of PR mattn#1133:
  mattn#1133

```
goos: darwin
goarch: arm64
pkg: github.com/mattn/go-sqlite3
cpu: Apple M1 Max
                           │    b.txt     │                n.txt                │
                           │    sec/op    │   sec/op     vs base                │
Suite/BenchmarkExec-10        1.278µ ± 1%   1.022µ ± 2%  -19.99% (p=0.000 n=10)
Suite/BenchmarkExecStep-10   1762.9µ ± 0%   900.9µ ± 1%  -48.90% (p=0.000 n=10)
geomean                       47.47µ        30.35µ       -36.06%

                           │     b.txt      │                n.txt                 │
                           │      B/op      │     B/op      vs base                │
Suite/BenchmarkExec-10           128.0 ± 0%     120.0 ± 0%   -6.25% (p=0.000 n=10)
Suite/BenchmarkExecStep-10   5279.94Ki ± 0%   31.34Ki ± 0%  -99.41% (p=0.000 n=10)
geomean                        25.69Ki        1.916Ki       -92.54%

                           │    b.txt    │                n.txt                │
                           │  allocs/op  │  allocs/op   vs base                │
Suite/BenchmarkExec-10        7.000 ± 0%    6.000 ± 0%  -14.29% (p=0.000 n=10)
Suite/BenchmarkExecStep-10   7.000k ± 0%   3.003k ± 0%  -57.10% (p=0.000 n=10)
geomean                       221.4         134.2       -39.36%
```

sqlite3.go

@@ -137,6 +137,22 @@ _sqlite3_prepare_v2_internal(sqlite3 *db, const char *zSql, int nBytes, sqlite3_
 }
 #endif
 
+static int _sqlite3_prepare_v2(sqlite3 *db, const char *zSql, int nBytes, sqlite3_stmt **ppStmt, int *oBytes) {
+	const char *tail = NULL;
+	int rv = _sqlite3_prepare_v2_internal(db, zSql, nBytes, ppStmt, &tail);
+	if (rv != SQLITE_OK) {
+		return rv;
+	}
+	if (tail == NULL) {
+		return rv; // NB: this should not happen
+	}
+	// Set oBytes to the number of bytes consumed instead of using the **pzTail
+	// out param since that requires storing a Go pointer in a C pointer, which
+	// is not allowed by CGO and will cause runtime.cgoCheckPointer to fail.
+	*oBytes = tail - zSql;
+	return rv;
+}
+
 void _sqlite3_result_text(sqlite3_context* ctx, const char* s) {
   sqlite3_result_text(ctx, s, -1, &free);
 }
@@ -858,51 +874,61 @@ func (c *SQLiteConn) Exec(query string, args []driver.Value) (driver.Result, err
 }
 
 func (c *SQLiteConn) exec(ctx context.Context, query string, args []driver.NamedValue) (driver.Result, error) {
-	start := 0
+	var (
+		stmtArgs []driver.NamedValue
+		start    int
+		s        SQLiteStmt // escapes to the heap so reuse it
+		sz       C.int      // number of query bytes consumed: escapes to the heap
+	)
+	query = strings.TrimSpace(query)
 	for {
-		s, err := c.prepare(ctx, query)
-		if err != nil {
-			return nil, err
+		s = SQLiteStmt{c: c} // reset
+		sz = 0
+		rv := C._sqlite3_prepare_v2(c.db, (*C.char)(unsafe.Pointer(stringData(query))),
+			C.int(len(query)), &s.s, &sz)
+		if rv != C.SQLITE_OK {
+			return nil, c.lastError()
 		}
+		query = strings.TrimSpace(query[sz:])
+
 		var res driver.Result
-		if s.(*SQLiteStmt).s != nil {
-			stmtArgs := make([]driver.NamedValue, 0, len(args))
+		if s.s != nil {
 			na := s.NumInput()
 			if len(args)-start < na {
-				s.Close()
+				s.finalize()
 				return nil, fmt.Errorf("not enough args to execute query: want %d got %d", na, len(args))
 			}
 			// consume the number of arguments used in the current
 			// statement and append all named arguments not
 			// contained therein
-			if len(args[start:start+na]) > 0 {
-				stmtArgs = append(stmtArgs, args[start:start+na]...)
-				for i := range args {
-					if (i < start || i >= na) && args[i].Name != "" {
-						stmtArgs = append(stmtArgs, args[i])
-					}
-				}
-				for i := range stmtArgs {
-					stmtArgs[i].Ordinal = i + 1
+			if stmtArgs == nil {
+				stmtArgs = make([]driver.NamedValue, 0, na)
+			}
+			stmtArgs = append(stmtArgs[:0], args[start:start+na]...)
+			for i := range args {
+				if (i < start || i >= na) && args[i].Name != "" {
+					stmtArgs = append(stmtArgs, args[i])
 				}
 			}
-			res, err = s.(*SQLiteStmt).exec(ctx, stmtArgs)
+			for i := range stmtArgs {
+				stmtArgs[i].Ordinal = i + 1
+			}
+			var err error
+			res, err = s.exec(ctx, stmtArgs)
 			if err != nil && err != driver.ErrSkip {
-				s.Close()
+				s.finalize()
 				return nil, err
 			}
 			start += na
 		}
-		tail := s.(*SQLiteStmt).t
-		s.Close()
-		if tail == "" {
+		s.finalize()
+		if len(query) == 0 {
 			if res == nil {
 				// https://github.com/mattn/go-sqlite3/issues/963
 				res = &SQLiteResult{0, 0}
 			}
 			return res, nil
 		}
-		query = tail
 	}
 }
 
@@ -1914,6 +1940,13 @@ func (s *SQLiteStmt) Close() error {
 	return nil
 }
 
+func (s *SQLiteStmt) finalize() {
+	if s.s != nil {
+		C.sqlite3_finalize(s.s)
+		s.s = nil
+	}
+}
+
 // NumInput return a number of parameters.
 func (s *SQLiteStmt) NumInput() int {
 	return int(C.sqlite3_bind_parameter_count(s.s))

sqlite3_test.go

@@ -2106,6 +2106,7 @@ var tests = []testing.InternalTest{
 
 var benchmarks = []testing.InternalBenchmark{
 	{Name: "BenchmarkExec", F: benchmarkExec},
+	{Name: "BenchmarkExecStep", F: benchmarkExecStep},
 	{Name: "BenchmarkQuery", F: benchmarkQuery},
 	{Name: "BenchmarkParams", F: benchmarkParams},
 	{Name: "BenchmarkStmt", F: benchmarkStmt},
@@ -2465,6 +2466,16 @@ func benchmarkExec(b *testing.B) {
 	}
 }
 
+var largeSelectStmt = strings.Repeat("select 1;\n", 1_000)
+
+func benchmarkExecStep(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		if _, err := db.Exec(largeSelectStmt); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
 // benchmarkQuery is benchmark for query
 func benchmarkQuery(b *testing.B) {
 	for i := 0; i < b.N; i++ {

unsafe_go120.go

@@ -0,0 +1,17 @@
+//go:build !go1.21
+// +build !go1.21
+
+package sqlite3
+
+import "unsafe"
+
+// stringData is a safe version of unsafe.StringData that handles empty strings.
+func stringData(s string) *byte {
+	if len(s) != 0 {
+		b := *(*[]byte)(unsafe.Pointer(&s))
+		return &b[0]
+	}
+	// The return value of unsafe.StringData
+	// is unspecified if the string is empty.
+	return &placeHolder[0]
+}

unsafe_go121.go

@@ -0,0 +1,23 @@
+//go:build go1.21
+// +build go1.21
+
+// The unsafe.StringData function was made available in Go 1.20 but it
+// was not until Go 1.21 that Go was changed to interpret the Go version
+// in go.mod (1.19 as of writing this) as the minimum version required
+// instead of the exact version.
+//
+// See: https://github.com/golang/go/issues/59033
+
+package sqlite3
+
+import "unsafe"
+
+// stringData is a safe version of unsafe.StringData that handles empty strings.
+func stringData(s string) *byte {
+	if len(s) != 0 {
+		return unsafe.StringData(s)
+	}
+	// The return value of unsafe.StringData
+	// is unspecified if the string is empty.
+	return &placeHolder[0]
+}