Avoid to copy Logstash's classes into the uber jar

[Andrea Selva]

Logstash information:

Please include the following information:

1. Logstash version (e.g. bin/logstash --version) 8.4.0
2. Logstash installation source (e.g. built from source, with a package manager: DEB/RPM, expanded from tar or zip archive, docker)
3. How is Logstash being run (e.g. as a service/service manager: systemd, upstart, etc. Via command line, docker/kubernetes)
4. How was the Logstash Plugin installed

JVM (e.g. java -version):

If the affected version of Logstash is 7.9 (or earlier), or if it is NOT using the bundled JDK or using the 'no-jdk' version in 7.10 (or higher), please provide the following information:

1. JVM version (java -version) (bundled)
2. JVM installation source (e.g. from the Operating System's package manager, from source, etc).
3. Value of the JAVA_HOME environment variable if set.

OS version (uname -a if on a Unix-like system):

Description of the problem including expected versus actual behavior:
The uber jar (and the gem) created contains also the Logstash classes. This could create weird problems when loaded inside a runtime Logstash, because the classloader could load a class from this jar and not from the runtime Logstash creating weird linkage errors.

Steps to reproduce:

1. Build the jar with ./gradlew jar, eventually fixing the build.gradle with:

diff --git a/build.gradle b/build.gradle
index 5f54ad3..4a85c25 100644
--- a/build.gradle
+++ b/build.gradle
@@ -49,7 +49,7 @@ dependencies {
    compileOnly 'org.apache.logging.log4j:log4j-api:2.17.0' // provided by Logstash
    compileOnly 'org.apache.logging.log4j:log4j-core:2.17.0' // provided by Logstash
 
-    implementation fileTree(dir: LOGSTASH_CORE_PATH, include: "**/logstash-core.jar")
+    implementation fileTree(dir: LOGSTASH_CORE_PATH, include: "**/lib/jars/logstash-core.jar")

    testImplementation 'junit:junit:4.12'
    testImplementation 'org.jruby:jruby-complete:9.2.20.1'

2. verify that the created jar contains classes from Logstash:

$ jar tf ./build/libs/logstash-input-java_input_example-1.0.2.jar
META-INF/
META-INF/MANIFEST.MF
org/
org/logstashplugins/
org/logstashplugins/JavaInputExample.class
org/logstash/
org/logstash/JavaVersionUtils.class
org/logstash/Rubyfier.class
org/logstash/Valuefier$Converter.class
org/logstash/FieldReference$IllegalSyntaxException.class
org/logstash/Timestamp.class
org/logstash/Javafier.class
org/logstash/execution/
org/logstash/execution/ConvergeResultExt$FailedActionExt.class
org/logstash/execution/QueueBatch.class
org/logstash/execution/PipelineReporterExt$SnapshotExt.class

[Andrea Selva]

A possible solution is drafted in PRs:

• #15
• elastic/logstash#13979

Avoid to use the shadow plugin to create the uber jar and just copy the dependency jars into the gem. To identify which is the plugin's entry point jar add a manifest to the jar itself and modify the plugin loading loading process on the Logstash side to read and respect this manifest.