Fix "cannot start a transaction within a transaction" issue

[why]
If db.BeginTx(ctx, nil) context is cancelled too fast, "BEGIN" statement can be
completed inside DB, but we still try to cancel it with sqlite3_interrupt.
In such case we get context.Cancelled or context.DeadlineExceeded from exec(),
but operation really completed. Connection returned into pool, and returns "cannot
start a transaction within a transaction" error for next db.BeginTx() call.

[how]
If we get context cancelled on "BEGIN" statement, call "ROLLBACK" to clean-up
connection state. Don't return cancellation error from exec() if operation completed
without sqlite3_interrupt.

[testing]
Added unit-test which reproduces issue.

sqlite3.go

@@ -864,6 +864,14 @@ func (c *SQLiteConn) Begin() (driver.Tx, error) {
 
 func (c *SQLiteConn) begin(ctx context.Context) (driver.Tx, error) {
 	if _, err := c.exec(ctx, c.txlock, nil); err != nil {
+		select {
+		case <-ctx.Done():
+			// context cancellation can be handled between c.txlock completes inside DB and we notice its result.
+			// in such case we may leave transaction opened, but exec() returns context cancellation error.
+			// Rollback will make sure we don't leave connection in that state. It is no-op otherwise.
+			_, _ = c.exec(context.Background(), "ROLLBACK", nil)
+		default:
+		}
 		return nil, err
 	}
 	return &SQLiteTx{c}, nil
@@ -1895,13 +1903,14 @@ func (s *SQLiteStmt) exec(ctx context.Context, args []namedValue) (driver.Result
 		return rv.r, rv.err
 	case <-ctx.Done():
 		select {
-		case <-resultCh: // no need to interrupt
+		case rv := <-resultCh: // no need to interrupt, operation completed in db
+			return rv.r, rv.err
 		default:
 			// this is still racy and can be no-op if executed between sqlite3_* calls in execSync.
 			C.sqlite3_interrupt(s.c.db)
-			<-resultCh // ensure goroutine completed
+			<-resultCh // wait for goroutine completed
+			return nil, ctx.Err()
 		}
-		return nil, ctx.Err()
 	}
 }
 
@@ -1992,7 +2001,7 @@ func (rc *SQLiteRows) Next(dest []driver.Value) error {
 		resultCh <- rc.nextSyncLocked(dest)
 	}()
 	select {
-	case err := <- resultCh:
+	case err := <-resultCh:
 		return err
 	case <-rc.ctx.Done():
 		select {

sqlite3_go18_test.go

@@ -136,6 +136,44 @@ func TestShortTimeout(t *testing.T) {
 	}
 }
 
+func TestBeginTxCancel(t *testing.T) {
+	srcTempFilename := TempFilename(t)
+	defer os.Remove(srcTempFilename)
+
+	db, err := sql.Open("sqlite3", srcTempFilename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	initDatabase(t, db, 100)
+
+	wg := sync.WaitGroup{}
+	// create many go-routines to expose racy issue
+	for i := 0; i < 10000; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+
+			ctx, cancel := context.WithCancel(context.Background())
+			go cancel() // make it cancel concurrently with exec("BEGIN");
+			tx, err := db.BeginTx(ctx, nil)
+			switch err {
+			case nil:
+				switch err := tx.Rollback(); err {
+				case nil, sql.ErrTxDone:
+				default:
+					t.Error(err)
+				}
+			case context.Canceled:
+			default:
+				// must not fail with "cannot start a transaction within a transaction"
+				t.Error(err)
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 func TestQueryRowContextCancel(t *testing.T) {
 	srcTempFilename := TempFilename(t)
 	defer os.Remove(srcTempFilename)