Allow detect whether it's in a database transaction for a context.Context (#21756)

Fix #19513

This PR introduce a new db method `InTransaction(context.Context)`,
and also builtin check on `db.TxContext` and `db.WithTx`.
There is also a new method `db.AutoTx` has been introduced but could be used by other PRs.

`WithTx` will always open a new transaction, if a transaction exist in context, return an error.
`AutoTx` will try to open a new transaction if no transaction exist in context.
That means it will always enter a transaction if there is no error.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: 6543 <6543@obermui.de>

models/db/context_test.go

@@ -0,0 +1,33 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package db_test
+
+import (
+	"context"
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInTransaction(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	assert.False(t, db.InTransaction(db.DefaultContext))
+	assert.NoError(t, db.WithTx(db.DefaultContext, func(ctx context.Context) error {
+		assert.True(t, db.InTransaction(ctx))
+		return nil
+	}))
+
+	ctx, committer, err := db.TxContext(db.DefaultContext)
+	assert.NoError(t, err)
+	defer committer.Close()
+	assert.True(t, db.InTransaction(ctx))
+	assert.Error(t, db.WithTx(ctx, func(ctx context.Context) error {
+		assert.True(t, db.InTransaction(ctx))
+		return nil
+	}))
+}