Fix: prevent ThrowableStackTraceRenderer from throwing NPEs #3934
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The `ThrowableStackTraceRenderer` class can throw a `NullPointerException` if the suppressed exceptions associated with the `Throwable` it is rendering are being concurrently mutated. This happens because `ThrowableStackTraceRenderer` invokes `Throwable#getSuppressed()` twice: once in `ThrowableStackTraceRenderer.Context.Metadata#populateMetadata()`, and a second time in `ThrowableStackTraceRenderer#renderThrowable()`, ahead of invoking `ThrowableStackTraceRenderer#renderSuppressed()`. If a racing thread manages to add a new suppressed exception to the being-logged exception between these two invocations, then the `Map<Throwable, Context.Metadata>` constructed by `populateMetadata()` will contain no mapping for the newly-added suppression, and as a result the dereference performed on line 168 explodes. Note: the unit test I am adding here requires the ability to mock `Throwable#getSuppressed()`. Since this method is `final`, I had to add a dependency on `mockito-inline`, which then required that I fix up some unrelated unit tests that had been relying on the lack of support for mocking `final` methods. This fixes apache#3929
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vy
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Sep 26, 2025
...st/src/test/java/org/apache/logging/log4j/core/appender/db/AbstractDatabaseAppenderTest.java
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...est/src/test/java/org/apache/logging/log4j/core/appender/db/AbstractDatabaseManagerTest.java
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.../java/org/apache/logging/log4j/core/appender/rolling/RandomRollingAppenderOnStartupTest.java
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log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/ThrowableStackTraceRenderer.java
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See conversation: apache#3934
Instead of modifying these tests, I added a new unit test specifically for |
vy
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Oct 3, 2025
vy
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Oct 3, 2025
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The
ThrowableStackTraceRendererclass can throw aNullPointerExceptionif the suppressed exceptions associated with theThrowableit is rendering are being concurrently mutated. This happens becauseThrowableStackTraceRendererinvokesThrowable#getSuppressed()twice: once inThrowableStackTraceRenderer.Context.Metadata#populateMetadata(), and a second time inThrowableStackTraceRenderer#renderThrowable(), ahead of invokingThrowableStackTraceRenderer#renderSuppressed(). If a racing thread manages to add a new suppressed exception to the being-logged exception between these two invocations, then theMap<Throwable, Context.Metadata>constructed bypopulateMetadata()will contain no mapping for the newly-added suppression, and as a result the dereference performed on line 168 explodes.Note: the unit test I am adding here requires the ability to mock
Throwable#getSuppressed(). Since this method isfinal, I had to add a dependency onmockito-inline, which then required that I fix up some unrelated unit tests that had been relying on the lack of support for mockingfinalmethods.This fixes #3929