[I] Removing all keys from `ThreadContextMap` does not clear the `ThreadLocal` (logging-log4j2)

["ppkarwasz (via GitHub)" <gi...@apache.org>]

ppkarwasz opened a new issue, #2523:
URL: https://github.com/apache/logging-log4j2/issues/2523

   The implementations of [`ThreadContextMap#remove`](https://logging.apache.org/log4j/2.x/javadoc/log4j-api/org/apache/logging/log4j/spi/ThreadContextMap#remove(java.lang.String)) and [`CleanableThreadContextMap#removeAll`](https://logging.apache.org/log4j/2.x/javadoc/log4j-api/org/apache/logging/log4j/spi/CleanableThreadContextMap#removeAll(java.lang.Iterable)) in the `DefaultThreadContextMap` and `CopyOnWriteSortedArrayThreadContextMap` implementations do not clear the underlying `ThreadLocal`, after all keys have been removed from the context map.
   
   This issue prevents `CopyOnWriteSortedArrayThreadContextMap` to be safely used in a web application, if code like this is used:
   
   ```java
   ThreadContext.put("key", "value");
   try {
       ...
   } finally {
       ThreadContext.remove("key");
   }
   ```
   
   **Remark**: this issue does not affect the `GarbageFreeSortedArrayThreadContextMap` implementation, since leaving a non-null value of `ThreadContext` is part of its garbage-free design.


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