Add aggregator; improve itest workflow

* Factor JSON evaluation and  construction base URLs out of of checker.
* Move json path matching to util.
* Add csaf_aggregator (as additional command)
* Improve itest workflow to checkout the branch where it is running on.

resolve #105
resolve  #72

Co-authored-by: tschmidtb51 <65305130+tschmidtb51@users.noreply.github.com>
Co-authored-by: Bernhard Reiter <bernhard@intevation.de>
Co-authored-by: Fadi Abbud <fadi.abbud@intevation.de>

cmd/csaf_aggregator/lazytransaction.go

@@ -0,0 +1,86 @@
+// This file is Free Software under the MIT License
+// without warranty, see README.md and LICENSES/MIT.txt for details.
+//
+// SPDX-License-Identifier: MIT
+//
+// SPDX-FileCopyrightText: 2022 German Federal Office for Information Security (BSI) <https://www.bsi.bund.de>
+// Software-Engineering: 2022 Intevation GmbH <https://intevation.de>
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/csaf-poc/csaf_distribution/util"
+)
+
+type lazyTransaction struct {
+	src    string
+	dstDir string
+	dst    string
+}
+
+func newLazyTransaction(src, dstDir string) *lazyTransaction {
+	return &lazyTransaction{
+		src:    src,
+		dstDir: dstDir,
+	}
+}
+
+func (lt *lazyTransaction) Src() string {
+	return lt.src
+}
+
+func (lt *lazyTransaction) Dst() (string, error) {
+	if lt.dst != "" {
+		return lt.dst, nil
+	}
+
+	srcBase := filepath.Base(lt.src)
+
+	folder := filepath.Join(lt.dstDir, srcBase)
+
+	dst, err := util.MakeUniqDir(folder)
+	if err != nil {
+		return "", err
+	}
+
+	// Copy old content into new.
+	if err := util.DeepCopy(lt.dst, lt.src); err != nil {
+		os.RemoveAll(dst)
+		return "", err
+	}
+	lt.dst = dst
+
+	return dst, nil
+}
+
+func (lt *lazyTransaction) rollback() error {
+	if lt.dst == "" {
+		return nil
+	}
+	err := os.RemoveAll(lt.dst)
+	lt.dst = ""
+	return err
+}
+
+func (lt *lazyTransaction) commit() error {
+	if lt.dst == "" {
+		return nil
+	}
+	defer func() { lt.dst = "" }()
+
+	// Switch directories.
+	symlink := filepath.Join(lt.dstDir, filepath.Base(lt.src))
+	if err := os.Symlink(lt.dstDir, symlink); err != nil {
+		os.RemoveAll(lt.dstDir)
+		return err
+	}
+	if err := os.Rename(symlink, lt.src); err != nil {
+		os.RemoveAll(lt.dstDir)
+		return err
+	}
+
+	return os.RemoveAll(lt.src)
+}