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.

sqlite3.go

@@ -1918,6 +1918,14 @@ func (s *SQLiteStmt) Exec(args []driver.Value) (driver.Result, error) {
 	return s.exec(context.Background(), list)
 }
 
+func isInterruptErr(err error) bool {
+	sqliteErr, ok := err.(Error)
+	if ok {
+		return sqliteErr.Code == ErrInterrupt
+	}
+	return false
+}
+
 // exec executes a query that doesn't return rows. Attempts to honor context timeout.
 func (s *SQLiteStmt) exec(ctx context.Context, args []namedValue) (driver.Result, error) {
 	if ctx.Done() == nil {
@@ -1933,19 +1941,22 @@ func (s *SQLiteStmt) exec(ctx context.Context, args []namedValue) (driver.Result
 		r, err := s.execSync(args)
 		resultCh <- result{r, err}
 	}()
+	var rv result
 	select {
-	case rv := <-resultCh:
-		return rv.r, rv.err
+	case rv = <-resultCh:
 	case <-ctx.Done():
 		select {
-		case <-resultCh: // no need to interrupt
+		case rv = <-resultCh: // no need to interrupt, operation completed in db
 		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
+			rv = <-resultCh // wait for goroutine completed
+			if isInterruptErr(rv.err) {
+				return nil, ctx.Err()
+			}
 		}
-		return nil, ctx.Err()
 	}
+	return rv.r, rv.err
 }
 
 func (s *SQLiteStmt) execSync(args []namedValue) (driver.Result, error) {

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)
+			}
+		}()
+	}
+}

sqlite3_go18_test.go

@@ -136,6 +136,44 @@ func TestShortTimeout(t *testing.T) {
 	}
 }
 
+func TestExecContextCancel(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()
+
+	ts := time.Now()
+	initDatabase(t, db, 1000)
+	spent := time.Since(ts)
+	if spent < 100*time.Millisecond {
+		t.Skip("test will be too racy, as ExecContext below will be too fast.")
+	}
+
+	// expected to be extremely slow query
+	q := `
+INSERT INTO test_table (key1, key_id, key2, key3, key4, key5, key6, data)
+SELECT t1.key1 || t2.key1, t1.key_id || t2.key_id, t1.key2 || t2.key2, t1.key3 || t2.key3, t1.key4 || t2.key4, t1.key5 || t2.key5, t1.key6 || t2.key6, t1.data || t2.data
+FROM test_table t1 LEFT OUTER JOIN test_table t2`
+	// expect query above take ~ same time as setup above
+	ctx, cancel := context.WithTimeout(context.Background(), spent/2)
+	defer cancel()
+	ts = time.Now()
+	r, err := db.ExecContext(ctx, q)
+	// racy check
+	if r != nil {
+		n, err := r.RowsAffected()
+		t.Log(n, err, time.Since(ts))
+	}
+	if err != context.DeadlineExceeded {
+		t.Fatal(err, ctx.Err())
+	}
+}
+
 func TestQueryRowContextCancel(t *testing.T) {
 	srcTempFilename := TempFilename(t)
 	defer os.Remove(srcTempFilename)
@@ -191,7 +229,7 @@ func TestQueryRowContextCancelParallel(t *testing.T) {
 	testCtx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	for i := 0; i < 50; i++ {
+	for i := 0; i < 10; i++ {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()