Add Go API for virtual tables

See https://www.sqlite.org/vtab.html for more details.

This work was started from
https://github.com/gwenn/gosqlite/blob/master/vtab.{c,go} and adds:

- Porting the API to go-sqlite3 APIs.
- Support for >= Go 1.6 without requiring the `cgocheck` flag to be changed.
- Filling out the unfinished callback functions for the `Vtable` struct.
- A simple `Context` API layer for ease of use when adding modules.

Tests are included.

vtable_test.go

@@ -0,0 +1,132 @@
+// Copyright (C) 2014 Yasuhiro Matsumoto <mattn.jp@gmail.com>.
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package sqlite3
+
+import (
+	"database/sql"
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type testModule struct {
+	t        *testing.T
+	intarray []int
+}
+
+type testVTab struct {
+	intarray []int
+}
+
+type testVTabCursor struct {
+	vTab  *testVTab
+	index int
+}
+
+func (m testModule) Create(c *SQLiteConn, args []string) (VTab, error) {
+	assert.True(m.t, len(args) == 6, "six arguments expected")
+	assert.Equal(m.t, "test", args[0], "module name")
+	assert.Equal(m.t, "main", args[1], "db name")
+	assert.Equal(m.t, "vtab", args[2], "table name")
+	assert.Equal(m.t, "'1'", args[3], "first arg")
+	assert.Equal(m.t, "2", args[4], "second arg")
+	assert.Equal(m.t, "three", args[5], "third arg")
+	err := c.DeclareVTab("CREATE TABLE x(test TEXT)")
+	if err != nil {
+		return nil, err
+	}
+	return &testVTab{m.intarray}, nil
+}
+
+func (m testModule) Connect(c *SQLiteConn, args []string) (VTab, error) {
+	return m.Create(c, args)
+}
+
+func (m testModule) DestroyModule() {}
+
+func (v *testVTab) BestIndex(cst []InfoConstraint, ob []InfoOrderBy) (*IndexResult, error) {
+	used := make([]bool, 0, len(cst))
+	for range cst {
+		used = append(used, false)
+	}
+	return &IndexResult{
+		Used:           used,
+		IdxNum:         0,
+		IdxStr:         "test-index",
+		AlreadyOrdered: true,
+		EstimatedCost:  100,
+		EstimatedRows:  200,
+	}, nil
+}
+
+func (v *testVTab) Disconnect() error {
+	return nil
+}
+
+func (v *testVTab) Destroy() error {
+	return nil
+}
+
+func (v *testVTab) Open() (VTabCursor, error) {
+	return &testVTabCursor{v, 0}, nil
+}
+
+func (vc *testVTabCursor) Close() error {
+	return nil
+}
+
+func (vc *testVTabCursor) Filter(idxNum int, idxStr string, vals []interface{}) error {
+	vc.index = 0
+	return nil
+}
+
+func (vc *testVTabCursor) Next() error {
+	vc.index++
+	return nil
+}
+
+func (vc *testVTabCursor) EOF() bool {
+	return vc.index >= len(vc.vTab.intarray)
+}
+
+func (vc *testVTabCursor) Column(c *Context, col int) error {
+	if col != 0 {
+		return fmt.Errorf("column index out of bounds: %d", col)
+	}
+	c.ResultInt(vc.vTab.intarray[vc.index])
+	return nil
+}
+
+func (vc *testVTabCursor) Rowid() (int64, error) {
+	return int64(vc.index), nil
+}
+
+func TestCreateModule(t *testing.T) {
+	tempFilename := TempFilename(t)
+	defer os.Remove(tempFilename)
+	intarray := []int{1, 2, 3}
+	sql.Register("sqlite3_TestCreateModule", &SQLiteDriver{
+		ConnectHook: func(conn *SQLiteConn) error {
+			return conn.CreateModule("test", testModule{t, intarray})
+		},
+	})
+	db, err := sql.Open("sqlite3_TestCreateModule", tempFilename)
+	assert.Nil(t, err, "could not open db")
+	_, err = db.Exec("CREATE VIRTUAL TABLE vtab USING test('1', 2, three)")
+	assert.Nil(t, err, "could not create vtable")
+
+	var i, value int
+	rows, err := db.Query("SELECT rowid, * FROM vtab WHERE test = '3'")
+	assert.Nil(t, err, "couldn't select from virtual table")
+	for rows.Next() {
+		rows.Scan(&i, &value)
+		assert.Equal(t, intarray[i], value)
+	}
+	_, err = db.Exec("DROP TABLE vtab")
+	assert.Nil(t, err, "couldn't drop virtual table")
+}