[
https://issues.apache.org/jira/browse/LOG4NET-376?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13821734#comment-13821734
]
Stuart Lange commented on LOG4NET-376:
--------------------------------------
Hello again. Sorry for letting this slide for a while.
We've noticed that with the log4net 1.2.12 service release, this bug has
actually gotten considerably worse. We have run into scenarios where the "to
the second" component of the timestamp gets "stuck" and never updates again for
the duration of the application. Unfortunately, I have been unable to produce
a unit test that reliably reproduces the issue (the unit test above sometimes
reproduces it), but my best guess is that this is related to the addition of
the Hashtable in AbsoluteTimeDateFormatter in LOG4NET-323. The Hashtable is
cleared and read from outside the lock, which can lead to potential race
conditions since the Hashtable is not thread-safe.
I have created my own implementation of IDateFormatter that behaves identically
to Iso8601DateFormatter, but avoids the bugs noted in this issue. My
performance testing shows that it performs nearly identically to
Iso8601DateFormatter in typical scenarios, and actually performs substantially
better in multi-threaded scenarios. I chose to make the cache state
ThreadStatic as that performs marginally better than the other options (static,
instance) in my performance testing, but the other options also perform well
and behave equivalently. Code is below. If you would like to see my
performance testing code, I can send you that as well, but it uses some of my
own custom components that would require unpacking to post in copy-paste
friendly form.
public class StandardDateFormatter : IDateFormatter
{
// Using ThreadStatic is a micro-optimization. Making it static or
instance state also works.
// ThreadStatic performs marginally better in scenarios where the same
instance of the formatter
// is being hit from multiple threads that are using different
timestamps.
// Performance is roughly equivalent in single-threaded scenarios.
[ThreadStatic]
private static Tuple<long, string> _lastTicksToTheSecond;
public void FormatDate(DateTime dateToFormat, TextWriter writer)
{
var ticksToTheSecond = dateToFormat.Ticks - dateToFormat.Ticks %
TimeSpan.TicksPerSecond;
var lastToTheSecond = _lastTicksToTheSecond;
string toTheSecondString;
if (lastToTheSecond != null && lastToTheSecond.Item1 ==
ticksToTheSecond)
{
toTheSecondString = lastToTheSecond.Item2;
}
else
{
toTheSecondString = dateToFormat.ToString("yyyy-MM-dd
HH:mm:ss");
_lastTicksToTheSecond = Tuple.Create(ticksToTheSecond,
toTheSecondString);
}
writer.Write(toTheSecondString);
writer.Write(',');
var millisecond = dateToFormat.Millisecond;
if (millisecond < 100)
writer.Write('0');
if (millisecond < 10)
writer.Write('0');
writer.Write(millisecond);
}
}
> Race condition in AbsoluteTimeDateFormatter
> -------------------------------------------
>
> Key: LOG4NET-376
> URL: https://issues.apache.org/jira/browse/LOG4NET-376
> Project: Log4net
> Issue Type: Bug
> Affects Versions: 1.2.11
> Reporter: Stuart Lange
> Assignee: Dominik Psenner
> Fix For: 1.2.12
>
>
> AbsoluteTimeDateFormatter's caching of the "to the second" timestamp string
> is not thread-safe. It is possible for one thread to clear the check (that
> this timestamp matches the currently cached "to the second" timestamp), but
> then end up using an incorrect "to the second" timestamp string if another
> thread has changed it in the meantime.
> In our organization, we see this bug fairly regularly because we have a mix
> of "real time" loggers that immediately write out log lines and "batching"
> loggers that defer logging to a background task that runs every second. We
> therefore regularly see log lines where the timestamp is off by a second or
> two.
> The following unit tests demonstrates the bug:
> [TestFixture]
> [Explicit]
> public class Log4netTimestampBug
> {
> /// <summary>
> /// This test demonstrates a bug with the log4net default time
> formatter (Iso8601DateFormatter)
> /// where the logged timestamp can be seconds off from the actual
> input timestamp
> /// The bug is caused to a race condition in the base class
> AbsoluteTimeDateFormatter
> /// because this class caches the timestamp string to the second but
> it is possible for
> /// the timestamp as written by a different thread to "sneak in" and
> be used by another
> /// thread erroneously (the checking and usage of this string is not
> done under a lock, only
> /// its modification)
> /// </summary>
> [Test]
> public void Test()
> {
> var now = DateTime.Now;
> var times = Enumerable.Range(1, 1000000).Select(i =>
> now.AddMilliseconds(i)).ToList();
> var sb1 = new StringBuilder();
> var sb2 = new StringBuilder();
> var task1 = Task.Run(() => WriteAllTheTimes(times, new
> StringWriter(sb1)));
> var task2 = Task.Delay(50).ContinueWith(t =>
> WriteAllTheTimes(times, new StringWriter(sb2)));
> Task.WaitAll(task1, task2);
> var task1Strings = GetTimeStrings(sb1);
> var task2Strings = GetTimeStrings(sb2);
> var diffs = Enumerable.Range(0, times.Count).Where(i =>
> task1Strings[i] != task2Strings[i]).ToList();
> Console.WriteLine("found {0} instances where the formatted
> timestamps are not the same", diffs.Count);
> Console.WriteLine();
> var diffToLookAt = diffs.FirstOrDefault(i => i - 10 > 0 && i + 10
> < times.Count);
> if (diffToLookAt != 0)
> {
> Console.WriteLine("Example Diff:");
> Console.WriteLine();
> Console.WriteLine("Index Original Timestamp Task 1
> Format Task 2 Format");
> for (int i = diffToLookAt - 10; i < diffToLookAt + 10; i++)
> {
> Console.WriteLine("{0,-7} {1} {2} {3} {4}", i,
> times[i].ToString("yyyy-MM-dd HH:mm:ss,fff"),
> task1Strings[i], task2Strings[i], i ==
> diffToLookAt ? "**** DIFF HERE ****" : "");
> }
> }
> CollectionAssert.AreEqual(task1Strings, task2Strings);
> }
> private static List<string> GetTimeStrings(StringBuilder sb1)
> {
> return sb1.ToString().Split(new[] {'\r', '\n'},
> StringSplitOptions.RemoveEmptyEntries).ToList();
> }
> private static void WriteAllTheTimes(IEnumerable<DateTime> times,
> TextWriter writer)
> {
> var formatter = new Iso8601DateFormatter();
> foreach (var t in times)
> {
> formatter.FormatDate(t, writer);
> writer.WriteLine();
> }
> }
> }
--
This message was sent by Atlassian JIRA
(v6.1#6144)
