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_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()