[LOG4J2-1538] Dynamic removal of filter may cause NPE

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.6.2
• Fix Version/s: 2.7
• Component/s: Core
• Flags: Important
• Assignee: Unassigned
• Reporter: Igor Karpov
• Created: 22/Aug/16 09:03
• Updated: 23/Aug/16 00:25
• Resolved: 23/Aug/16 00:25

Description

In my application, I need to add and remove a filter dynamically.
The problem is, that it occasionally causes NPE because of the subtle bug in
AppenderControl.isFilteredByAppenderControl:

@PerformanceSensitive
    private boolean isFilteredByAppenderControl(LogEvent event) {
        return this.getFilter() != null && Result.DENY == this.getFilter().filter(event);
    }

here, this.getFilter() is called twice.
getFilter() is defined in superclass, AbstractFilterable:

private volatile Filter filter;

so, it's volatile, and in my case, when I dynamically remove the filter it becomes null, and in certain cases it happens right between the two calls to getFilter().
The first call to getFilter() returns non-null, so the secand call is performed, which returns null and it causes NPE.

The easy fix for this is not to call getFilter() twice:

@PerformanceSensitive
    private boolean isFilteredByAppenderControl(LogEvent event) {
        Filter filter = this.getFilter(); // do not inline, filter is volatile
        return filter != null && Result.DENY == filter.filter(event);
    }