feat(singlejar): Add Log4j plugins cache combiner

[stevebarrau]

In singlejar, add support for combining Log4j2 plugins cache file.

Log4j2 plugins are Java annotations collected by a compiler plugin into .dat files for Log4j2 runtime to find them fast. With correct dependency on a java_plugin, Bazel already runs the java plugin compiler correctly. The behavior is correct on bazel run JAR, but not on _deploy.jar fat jars.

The silent clobbering of files in general, and the example of these Log4j2 .dat files in particular, is discussed in #7330.

• File structure of Log4j2Plugins.dat files
• the test data was generated by a python script, happy to commit it as well
• How to merge Log4j2Plugins.dat?

I tested this fix in my project using Bazel 7.0.2, compiling //src:java_tools_prebuilt.zip, and overriding @remote_java_tools_darwin_arm64 and @remote_java_tools_linux.

[stevebarrau]

@sgowroji what question do I need to answer for "awaiting-user-response" to get removed?

@softprops @jwilliams-ocient from #7330; does this help the issue for you?

[sgowroji]

@stevebarrau Could you please take a look at the failing checks?

[stevebarrau]

@sgowroji PTAL.

[hvadehra]

cc @cushon

[stevebarrau]

Ping @cushon

[meteorcloudy]

@cushon Can you please take a look?

[cushon]

@hvadehra @meteorcloudy for Blaze we don't need to support log4j, so supporting this isn't a priority for me. I also don't want to stand in the way of progress if you want to support this in Bazel. I left a note in #7330 about the idea of trying to make this kind of thing more pluggable, but there may not be great alternatives to doing it directly in singlejar. If that's something you want to pursue, perhaps the new 'combiner' here could be factored into a separate file and only wired up for Bazel.

[cushon]

Instead of extracting this helper, would it be possible to share implementation with Concatenator with composition, similar to what ManifestCombiner does?

[stevebarrau]

Composed in e416b51

[cushon]

Would it be possible to move Log4J2PluginDatCombiner into a separate file?

[stevebarrau]

Extracted in 8e901f0

[stevebarrau]

@cushon I would prefer to author this in Java to reuse the log4j merger logic from existing implementation. Ideally I agree with you if we could provide a set of custom combiners as configuration instead of having to add complexity/features in singlejar this would be very nice.

Where could we start to pry open singlejar and allow external combiners to be configured?

In the short term, if I address the comments, is this something that could live in singlejar until we rollout a configurability mechanism?

[stevebarrau]

@shs96c if this ends up being authorable in Java, would https://github.com/bazel-contrib/rules_jvm be a good home for this log4j jar combiner extension?

[shs96c]

contrib_rules_jvm is too high a level. You might want this in rules_jvm_external so maven artifacts we create are correct, but the only way for this to work with a deploy jar is to have this in rules_Java or wherever singlejar lives.

[stevebarrau]

@cushon PTAL.