Fix "cannot start a transaction within a transaction" issue (#764) (#765)

* 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]
Handle status code returned from cancelled operation.

[testing]
Added unit-test which reproduces issue.

* Reduce TestQueryRowContextCancelParallel concurrency

[why]
Tests times out in travis-ci when run with -race option.

1 files changed, 74 insertions, 0 deletions

diff --git a/sqlite3_go113_test.go b/sqlite3_go113_test.go
new file mode 100644
index 0000000..74036f8
--- /dev/null
+++ b/sqlite3_go113_test.go
@@ -0,0 +1,74 @@
+// Copyright (C) 2019 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.
+
+// +build go1.13,cgo
+
+package sqlite3
+
+import (
+	"context"
+	"database/sql"
+	"database/sql/driver"
+	"os"
+	"testing"
+)
+
+func TestBeginTxCancel(t *testing.T) {
+	srcTempFilename := TempFilename(t)
+	defer os.Remove(srcTempFilename)
+
+	db, err := sql.Open("sqlite3", srcTempFilename)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	db.SetMaxOpenConns(10)
+	db.SetMaxIdleConns(5)
+
+	defer db.Close()
+	initDatabase(t, db, 100)
+
+	// create several go-routines to expose racy issue
+	for i := 0; i < 1000; i++ {
+		func() {
+			ctx, cancel := context.WithCancel(context.Background())
+			conn, err := db.Conn(ctx)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer func() {
+				if err := conn.Close(); err != nil {
+					t.Error(err)
+				}
+			}()
+
+			err = conn.Raw(func(driverConn interface{}) error {
+				d, ok := driverConn.(driver.ConnBeginTx)
+				if !ok {
+					t.Fatal("unexpected: wrong type")
+				}
+
+				go cancel() // make it cancel concurrently with exec("BEGIN");
+				tx, err := d.BeginTx(ctx, driver.TxOptions{})
+				switch err {
+				case nil:
+					switch err := tx.Rollback(); err {
+					case nil, sql.ErrTxDone:
+					default:
+						return err
+					}
+				case context.Canceled:
+				default:
+					// must not fail with "cannot start a transaction within a transaction"
+					return err
+				}
+				return nil
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+		}()
+	}
+}