Zschimmer commented on issue #3929:
URL:
https://github.com/apache/logging-log4j2/issues/3929#issuecomment-4249816946
The NullPointerException reproducible without using threads. It appears when
calling addSuppressed some microseconds after the exception has been logged.
Then the line is not written to the log file.
This inhibits us to use the TLS vulnerability fix #4002.
Example in Scala, Log4jNullPointerExceptionTest.sc:
```//> using dep org.apache.logging.log4j:log4j-api:2.25.4
//> using dep org.apache.logging.log4j:log4j-core:2.25.4
//> using dep org.apache.logging.log4j:log4j-slf4j2-impl:2.25.4
//> using dep com.lmax:disruptor:3.4.4
//> using dep org.slf4j:slf4j-api:2.0.17
import java.lang.System.setProperty
import org.slf4j.LoggerFactory
// Log4j 2.25 logs NullPointerException to stderr without aborting the test
setProperty("log4j2.configurationFile", "./log4j2.xml")
setProperty("log4j2.contextSelector",
"org.apache.logging.log4j.core.async.AsyncLoggerContextSelector")
val logger = LoggerFactory.getLogger("Log4jNullPointerExceptionTest")
val suppressed = new Exception("Suppressed")
(1 to 10_000).foreach: i =>
val exception = new Exception("Log4jNullPointerExceptionTest")
val nanoseconds = i * 10 % 1_000_000
logger.debug(s"${nanoseconds / 1000.0}µs", exception)
Thread.sleep(0, nanoseconds)
// Adding a suppressed exception while the AsyncLogger is still logging
the exception may
// cause a NullPointerException (visible in stderr output)
exception.addSuppressed(suppressed)
```
log4j2.xml:
```
<?xml version="1.0" encoding="UTF-8"?>
<configuration status="WARN">
<appenders>
<randomAccessFile name="file" fileName="test.log" append="false"
immediateFlush="false">
<patternLayout
pattern="%d{yyyy-MM-dd HH:mm:ss.SSS} %level %logger - %message%n"
charset="UTF-8"/>
</randomAccessFile>
</appenders>
<loggers>
<root level="trace">
<appenderRef level="trace" ref="file"/>
</root>
</loggers>
</configuration>
```
Run with
`scala Log4jNullPointerExceptionTest.sc`
On my computer, three times a NullPointerException is logged to stderr after
8µs.
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