[logback-dev] [JIRA] Issue Comment Edited: (LBCLASSIC-253) MDC.clear() in child thread improperly affects parent.

[Thomas Becker (JIRA)]

    [ http://jira.qos.ch/browse/LBCLASSIC-253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=11969#action_11969 ] 

Thomas Becker edited comment on LBCLASSIC-253 at 3/1/11 4:34 PM:
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FYI the issue here seems to be that the LogbackMDCAdapter is clearing the HashMap before calling copyOnInheritThreadLocal.remove().  The HashMap instance itself is shared with the parent thread since T childValue(T parentValue) is not overridden when the thread local is created.  As far as I can tell, the clear() is unnecessary (and undesirable).

      was (Author: tbecker):
    FYI the issue here seems to be that the LogbackMDCAdapter is clearing the HashMap before calling copyOnInheritThreadLocal.remove().  The HashMap instance itself is shared with the parent thread since T childValue(T parentValue) is not overridden when the thread local is created.
  
> MDC.clear() in child thread improperly affects parent.
> ------------------------------------------------------
>
>                 Key: LBCLASSIC-253
>                 URL: http://jira.qos.ch/browse/LBCLASSIC-253
>             Project: logback-classic
>          Issue Type: Bug
>          Components: Other
>    Affects Versions: 0.9.28
>         Environment: JDK 1.6.0_21
>            Reporter: Thomas Becker
>            Assignee: Logback dev list
>
> I want to upgrade logback to 0.9.28 but one of my unit tests caught this bug in the LogbackMDCAdapter.  Basically, if a child thread clears the MDC, the parent thread's copy is cleared as well.  This simple unit test illustrates the problem.
> import org.junit.Test;
> import org.slf4j.MDC;
> /**
>  * Demonstrates a bug in LogbackMDCAdapter.  The clear() executed by the child thread should not affect the parent.
>  */
> public class MDCTest {
>     @Test
>     public void testBug() throws InterruptedException {
>         MDC.put("foo", "bar");
>         assertEquals("bar", MDC.get("foo"));
>         
>         Thread clearer = new Thread() {
>             @Override
>             public void run() {
>                 MDC.clear();
>                 assertNull(MDC.get("foo"));
>             }
>         };
>         clearer.start();
>         clearer.join();
>         //Fails
>         assertEquals("bar", MDC.get("foo"));
>     }
> }

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