New subject: Change in vdsm[master]: utils: atomic file write

27 Jul 2016

Petr Horáček has uploaded a new change for review.
Change subject: utils: atomic write
......................................................................
utils: atomic write
Atomic writes are needed for safe file editation. This function will
be used in following network patch 'acquire external interfaces'.
Change-Id: Icbb5a2d3ac439a334db2c9075376f219c356762c
Bug-Url: https://bugzilla.redhat.com/1195208
Signed-off-by: Petr Horáček phoracek@redhat.com
---
M lib/vdsm/utils.py
M tests/utilsTests.py
2 files changed, 73 insertions(+), 3 deletions(-)
git pull ssh://gerrit.ovirt.org:29418/vdsm refs/changes/82/61482/1

diff --git a/lib/vdsm/utils.py b/lib/vdsm/utils.py
index 0beac90..10888bb 100644
--- a/lib/vdsm/utils.py
+++ b/lib/vdsm/utils.py
@@ -834,8 +834,7 @@
     suffix, e.g. dummy_ilXaYiSn7. The name is bound to IFNAMSIZ of 16-1 chars.
     """
     suffix_len = max_length - len(prefix)
-    suffix = ''.join(random.choice(string.ascii_letters + string.digits)
-                     for _ in range(suffix_len))
+    suffix = _random_alnum_string(suffix_len)
     return prefix + suffix
@@ -929,3 +928,43 @@
     elif len(keys) == 0:
         return dict
     return rget(dict.get(keys[0]), keys[1:], default)
+
+
+@contextmanager
+def atomic_write(path, flag):
+    """Atomically write into a file.
+
+    Usage:
+
+        with atomic_write('foo.txt', 'w') as f:
+            f.write('shrubery')
+            # there are no changes on foo.txt yet
+        # now it is changed
+
+    Temporary copy of the original is created in the same folder == same file
+    system.
+    """
+    tmp_path = _generate_tmp_path(path)
+    if os.path.exists(path):
+        shutil.copyfile(path, tmp_path)
+    try:
+        with open(tmp_path, flag) as f:
+            yield f
+    except:
+        raise
+    else:
+        shutil.copyfile(tmp_path, path)
+    finally:
+        os.remove(tmp_path)
+
+
+def _generate_tmp_path(path):
+    id = _random_alnum_string(8)
+    tmp_path = '{}.{}.tmp'.format(path, id)
+    return tmp_path
+
+
+def _random_alnum_string(length):
+    return ''.join(
+        random.choice(string.ascii_letters + string.digits)
+        for _ in range(length))
diff --git a/tests/utilsTests.py b/tests/utilsTests.py
index 975cb0b..1fc4f95 100644
--- a/tests/utilsTests.py
+++ b/tests/utilsTests.py
@@ -47,7 +47,7 @@
 from monkeypatch import MonkeyPatch, MonkeyPatchScope
 from vmTestsData import VM_STATUS_DUMP
 from monkeypatch import Patch
-from testlib import forked, online_cpus
+from testlib import forked, online_cpus, namedTemporaryDir
 from testlib import permutations, expandPermutations
 from testlib import VdsmTestCase as TestCaseBase
 from testValidation import brokentest
@@ -1059,3 +1059,34 @@
         proc.start()
         self._noIntrWatchFd(myPipe, isEpoll=False, mask=select.POLLIN)
         proc.join()
+
+
+class AtomicWriteTest(TestCaseBase):
+
+    def test_create_a_new_file(self):
+        TEST_TEXT = 'shrubery'
+
+        with namedTemporaryDir() as tmp_dir:
+            test_file_path = os.path.join(tmp_dir, 'foo.txt')
+            with utils.atomic_write(test_file_path, 'w') as f:
+                f.write(TEST_TEXT)
+                self.assertFalse(os.path.exists(test_file_path))
+            self._assert_file_contains(test_file_path, TEST_TEXT)
+
+    def test_edit_file(self):
+        TEST_TEXT_1 = 'foo'
+        TEST_TEXT_2 = 'bar'
+
+        with namedTemporaryDir() as tmp_dir:
+            test_file_path = os.path.join(tmp_dir, 'foo.txt')
+            with open(test_file_path, 'w') as f:
+                f.write(TEST_TEXT_1)
+            with utils.atomic_write(test_file_path, 'w') as f:
+                f.write(TEST_TEXT_2)
+                self._assert_file_contains(test_file_path, TEST_TEXT_1)
+            self._assert_file_contains(test_file_path, TEST_TEXT_2)
+
+    def _assert_file_contains(self, path, expected_content):
+        with open(path) as f:
+            content = f.read()
+            self.assertEqual(content, expected_content)
-- 
To view, visit https://gerrit.ovirt.org/61482
To unsubscribe, visit https://gerrit.ovirt.org/settings

Gerrit-MessageType: newchange
Gerrit-Change-Id: Icbb5a2d3ac439a334db2c9075376f219c356762c
Gerrit-PatchSet: 1
Gerrit-Project: vdsm
Gerrit-Branch: master
Gerrit-Owner: Petr Horáček phoracek@redhat.com

    

Change in vdsm[master]: utils: atomic write