Fix exponential memory allocation in Exec and improve performance

This commit changes SQLiteConn.Exec to use the raw Go query string
instead of repeatedly converting it to a C string (which it would do for
every statement in the provided query). This yields a ~20% performance
improvement for a query containing one statement and a significantly
larger improvement when the query contains multiple statements as is
common when importing a SQL dump (our benchmark shows a 5x improvement
for handling 1k SQL statements).

Additionally, this commit improves the performance of Exec by 2x or more
and makes number and size of allocations constant when there are no bind
parameters (the performance improvement scales with the number of SQL
statements in the query). This is achieved by having the entire query
processed in C code thus requiring only one CGO call.

The speedup for Exec'ing single statement queries means that wrapping
simple statements in a transaction is now twice as fast.

This commit also improves the test coverage of Exec, which previously
failed to test that Exec could process multiple statements like INSERT.
It also adds some Exec specific benchmarks that highlight both the
improvements here and the overhead of using a cancellable Context.

This commit is a slimmed down and improved version of PR #1133:
  #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/Params-10             1.434µ ± 1%   1.186µ ± 0%  -17.27% (p=0.000 n=10)
Suite/BenchmarkExec/NoParams-10          1267.5n ± 0%   759.2n ± 1%  -40.10% (p=0.000 n=10)
Suite/BenchmarkExecContext/Params-10      2.886µ ± 0%   2.517µ ± 0%  -12.80% (p=0.000 n=10)
Suite/BenchmarkExecContext/NoParams-10    2.605µ ± 1%   1.829µ ± 1%  -29.81% (p=0.000 n=10)
Suite/BenchmarkExecStep-10               1852.6µ ± 1%   582.3µ ± 0%  -68.57% (p=0.000 n=10)
Suite/BenchmarkExecContextStep-10        3053.3µ ± 3%   582.0µ ± 0%  -80.94% (p=0.000 n=10)
Suite/BenchmarkExecTx-10                  4.126µ ± 2%   2.200µ ± 1%  -46.67% (p=0.000 n=10)
geomean                                   16.40µ        8.455µ       -48.44%

                                       │      b.txt      │                n.txt                │
                                       │      B/op       │    B/op     vs base                 │
Suite/BenchmarkExec/Params-10                 248.0 ± 0%   240.0 ± 0%    -3.23% (p=0.000 n=10)
Suite/BenchmarkExec/NoParams-10              128.00 ± 0%   64.00 ± 0%   -50.00% (p=0.000 n=10)
Suite/BenchmarkExecContext/Params-10          408.0 ± 0%   400.0 ± 0%    -1.96% (p=0.000 n=10)
Suite/BenchmarkExecContext/NoParams-10        288.0 ± 0%   208.0 ± 0%   -27.78% (p=0.000 n=10)
Suite/BenchmarkExecStep-10               5406674.50 ± 0%   64.00 ± 0%  -100.00% (p=0.000 n=10)
Suite/BenchmarkExecContextStep-10         5566758.5 ± 0%   208.0 ± 0%  -100.00% (p=0.000 n=10)
Suite/BenchmarkExecTx-10                      712.0 ± 0%   520.0 ± 0%   -26.97% (p=0.000 n=10)
geomean                                     4.899Ki        189.7        -96.22%

                                       │     b.txt     │               n.txt                │
                                       │   allocs/op   │ allocs/op   vs base                │
Suite/BenchmarkExec/Params-10              10.000 ± 0%   9.000 ± 0%  -10.00% (p=0.000 n=10)
Suite/BenchmarkExec/NoParams-10             7.000 ± 0%   4.000 ± 0%  -42.86% (p=0.000 n=10)
Suite/BenchmarkExecContext/Params-10        12.00 ± 0%   11.00 ± 0%   -8.33% (p=0.000 n=10)
Suite/BenchmarkExecContext/NoParams-10      9.000 ± 0%   6.000 ± 0%  -33.33% (p=0.000 n=10)
Suite/BenchmarkExecStep-10               7000.000 ± 0%   4.000 ± 0%  -99.94% (p=0.000 n=10)
Suite/BenchmarkExecContextStep-10        9001.000 ± 0%   6.000 ± 0%  -99.93% (p=0.000 n=10)
Suite/BenchmarkExecTx-10                    27.00 ± 0%   18.00 ± 0%  -33.33% (p=0.000 n=10)
geomean                                     74.60        7.224       -90.32%
```

sqlite3.go

@@ -137,6 +137,57 @@ _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;
+}
+
+// _sqlite3_exec_no_args executes all of the statements in zSql. None of the
+// statements are allowed to have positional arguments.
+int _sqlite3_exec_no_args(sqlite3 *db, const char *zSql, int nBytes, int64_t *rowid, int64_t *changes) {
+	while (*zSql && nBytes > 0) {
+		sqlite3_stmt *stmt;
+		const char *tail;
+		int rv = sqlite3_prepare_v2(db, zSql, nBytes, &stmt, &tail);
+		if (rv != SQLITE_OK) {
+			return rv;
+		}
+
+		// Process statement
+		do {
+			rv = _sqlite3_step_internal(stmt);
+		} while (rv == SQLITE_ROW);
+
+		// Only record the number of changes made by the last statement.
+		*changes = sqlite3_changes64(db);
+		*rowid = sqlite3_last_insert_rowid(db);
+
+		if (rv != SQLITE_OK && rv != SQLITE_DONE) {
+			sqlite3_finalize(stmt);
+			return rv;
+		}
+		rv = sqlite3_finalize(stmt);
+		if (rv != SQLITE_OK) {
+			return rv;
+		}
+
+		nBytes -= tail - zSql;
+		zSql = tail;
+	}
+	return SQLITE_OK;
+}
+
 void _sqlite3_result_text(sqlite3_context* ctx, const char* s) {
   sqlite3_result_text(ctx, s, -1, &free);
 }
@@ -858,54 +909,119 @@ 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
+	// Trim the query. This is mostly important for getting rid
+	// of any trailing space.
+	query = strings.TrimSpace(query)
+	if len(args) > 0 {
+		return c.execArgs(ctx, query, args)
+	}
+	return c.execNoArgs(ctx, query)
+}
+
+func (c *SQLiteConn) execArgs(ctx context.Context, query string, args []driver.NamedValue) (driver.Result, error) {
+	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
+	)
 	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
+			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
 	}
 }
 
+// execNoArgsSync processes every SQL statement in query. All processing occurs
+// in C code, which reduces the overhead of CGO calls.
+func (c *SQLiteConn) execNoArgsSync(query string) (_ driver.Result, err error) {
+	var rowid, changes C.int64_t
+	rv := C._sqlite3_exec_no_args(c.db, (*C.char)(unsafe.Pointer(stringData(query))),
+		C.int(len(query)), &rowid, &changes)
+	if rv != C.SQLITE_OK {
+		err = c.lastError()
+	}
+	return &SQLiteResult{id: int64(rowid), changes: int64(changes)}, err
+}
+
+func (c *SQLiteConn) execNoArgs(ctx context.Context, query string) (driver.Result, error) {
+	done := ctx.Done()
+	if done == nil {
+		return c.execNoArgsSync(query)
+	}
+
+	// Fast check if the Context is cancelled
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	ch := make(chan struct{})
+	defer close(ch)
+	go func() {
+		select {
+		case <-done:
+			C.sqlite3_interrupt(c.db)
+			// Wait until signaled. We need to ensure that this goroutine
+			// will not call interrupt after this method returns, which is
+			// why we can't check if only done is closed when waiting below.
+			<-ch
+		case <-ch:
+		}
+	}()
+
+	res, err := c.execNoArgsSync(query)
+
+	// Stop the goroutine and make sure we're at a point where
+	// sqlite3_interrupt cannot be called again.
+	ch <- struct{}{}
+
+	if isInterruptErr(err) {
+		err = ctx.Err()
+	}
+	return res, err
+}
+
 // Query implements Queryer.
 func (c *SQLiteConn) Query(query string, args []driver.Value) (driver.Rows, error) {
 	list := make([]driver.NamedValue, len(args))
@@ -1914,6 +2030,13 @@ 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))

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]
+}