Broken logger initialization in 2.24.1

[kelunik]

Description

LoggerContext / LoggerRegistry don't play well together in using weak references, which can result in a null logger if the GC kicks in at a bad timing.

Configuration

Version: 2.24.1

Operating system: Linux / macOS

JDK: 21.0.4

Logs

java.lang.NullPointerException

Cannot invoke "org.apache.logging.log4j.spi.ExtendedLogger.logIfEnabled(String, org.apache.logging.log4j.Level, org.apache.logging.log4j.Marker, String)" because "this.logger" is null

at org.apache.logging.slf4j.Log4jLogger.warn(Log4jLogger.java:233)
at <internal company code>

Reproduction

• LoggerContext keeps a local reference to newLogger, but this isn't safe to keep a reference (see below).
• LoggerRegistry.putIfAbsent takes the instance and creates a WeakReference from it.
• Unfortunate GC timing might result in the weak reference being cleared and the following getLogger call returning null

Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. For example, a Java compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner.

https://docs.oracle.com/javase/specs/jls/se21/html/jls-12.html#jls-12.6.1

Fix

#2936 should fix this, but is currently unreleased.

Reference.reachabilityFence would also fix this, but is JDK 9+ only.

See also: https://shipilev.net/jvm/anatomy-quarks/8-local-var-reachability/

[kelunik]

Full reproducer: https://gist.github.com/kelunik/be51e5e2fdf746bbc86383b922bbb05e

[ppkarwasz]

@kelunik,

Thank you for the report.

The LoggerRegistry implementation was changed in 2.25.0-SNAPSHOT. Can you check if that fixes your issue?
Our snapshots are located in the https://repository.apache.org/snapshots/ Maven repository (see also our download page).