added an unsafe rebase snapshot

Unsafe rebase a qcow2 snapshot to a none exist the raw backing file.
Signed-off-by: NHaotong Chen <hachen@redhat.com>

qemu/tests/cfg/unsafe_rebase_to_none_existing_backing_file.cfg

+- unsafe_rebase_to_none_existing_backing_file:
+    only raw
+    virt_test_type = qemu
+    type = unsafe_rebase_to_none_existing_backing_file
+    kill_vm = yes
+    start_vm = no
+    force_create_image = no
+    image_chain = "image1 sn"
+    image_format_image1 = "raw"
+    image_name_sn = "images/sn"
+    rebase_mode_sn = unsafe
+    none_existing_image_name = a-none-existing-image.raw
+    # set size to "", so during snapshot creation
+    # the cmdline will not have specified size option
+    image_size_sn = ""
+    image_format_sn = qcow2
+    variants:
+        - compat_0.10:
+            image_extra_params = "compat=0.10"
+        - compat_default:

qemu/tests/unsafe_rebase_to_none_existing_backing_file.py

+import logging
+import json
+
+from avocado.utils import process
+from virttest import data_dir
+from virttest.qemu_storage import QemuImg
+
+from qemu.tests.qemu_disk_img import QemuImgTest
+from qemu.tests.qemu_disk_img import generate_base_snapshot_pair
+
+
+def run(test, params, env):
+    """
+    Unsafe rebase a qcow2 snapshot to a none existing the raw backing file.
+
+    1. create a qcow2 snapshot based on a raw image with compat mode 0.10/1.1
+    2. rebase the snapshot to a none exist the raw backing file
+    3. check the snapshot
+
+    :param test: Qemu test object
+    :param params: Dictionary with the test parameters
+    :param env: Dictionary with test environment
+    """
+    def _get_img_obj_and_params(tag):
+        """Get an QemuImg object and its params based on the tag."""
+        img_param = params.object_params(tag)
+        img = QemuImg(img_param, data_dir.get_data_dir(), tag)
+        return img, img_param
+
+    def _get_compat_version():
+        """Get snapshot compat version."""
+        if params.get("image_extra_params") is None:
+            # default compat version for now is 1.1
+            return "1.1"
+        return params.get("image_extra_params").split("=")[1]
+
+    def _verify_qemu_img_info(output, b_fmt, b_name):
+        """Verify qemu-img info output for this case."""
+        logging.info("Verify snapshot's backing file information.")
+        res = json.loads(output)
+        if (res["backing-filename-format"] != b_fmt or
+                res["backing-filename"] != b_name):
+            test.fail("Backing file information is not correct,"
+                      " got %s." % b_name)
+        compat = res["format-specific"]["data"]["compat"]
+        expected = _get_compat_version()
+        if (compat != expected):
+            test.fail("Snapshot's compat mode is not correct,"
+                      " got %s, expected %s." % (compat, expected))
+
+    def _verify_unsafe_rebase(img):
+        """Verify qemu-img check output for this case."""
+        logging.info("Verify snapshot's unsafe check information.")
+        res = process.run("%s check %s" % (img.image_cmd, img.image_filename),
+                          ignore_status=True)
+        expected = ["Could not open backing file", img.base_image_filename,
+                    "No such file or directory"]
+        for msg in expected:
+            if msg not in res.stderr_text:
+                test.fail("The %s should not exist." % img.base_image_filename)
+
+    gen = generate_base_snapshot_pair(params["image_chain"])
+    base, snapshot = next(gen)
+    base_img, _ = _get_img_obj_and_params(base)
+    sn_img, sn_img_params = _get_img_obj_and_params(snapshot)
+
+    logging.info("Create a snapshot %s based on %s." % (snapshot, base))
+    # workaround to assign system disk's image_name to image_name_image1
+    params["image_name_image1"] = params["image_name"]
+    QemuImgTest(test, params, env, snapshot).create_snapshot()
+    _verify_qemu_img_info(sn_img.info(output="json"),
+                          base_img.image_format, base_img.image_filename)
+
+    sn_img.base_image_filename = params["none_existing_image_name"]
+    sn_img.base_format = sn_img.base_image_filename.split('.')[-1]
+    sn_img.rebase(sn_img_params)
+    _verify_qemu_img_info(sn_img.info(output="json"),
+                          sn_img.base_format, sn_img.base_image_filename)
+
+    _verify_unsafe_rebase(sn_img)