Memory leak as a result of no cleanup for ThreadLocal in CodedOutputStream??

Discussed in https://github.com/open-telemetry/opentelemetry-java/discussions/7082

^{Originally posted by asclark109 February 7, 2025}
I am using the io.opentelemetry:opentelemetry-exporter-common:1.38.0 jar in my java web application project running on Tomcat 10. I am getting memory leaks at application shutdown (one is a io.netty.util.internal.InternalThreadLocalMap that is tracked in Netty). The other appears below.

07-Feb-2025 12:36:08.735 SEVERE [main] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [agtest-trunk] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@658c5a19]) and a value of type [io.opentelemetry.exporter.internal.marshal.CodedOutputStream.OutputStreamEncoder] (value [io.opentelemetry.exporter.internal.marshal.CodedOutputStream$OutputStreamEncoder@421e361]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.

I have looked at your class in release 1.38.0 and on main: CodedOutputStream.java

I notice that a ThreadLocal is created and updated but never cleaned up (i.e. there is no call to do THREAD_LOCAL_CODED_OUTPUT_STREAM.remove()).

https://github.com/open-telemetry/opentelemetry-java/blob/30d16eb3d393f85d99175a62ed246e369e25b906/exporters/common/src/main/java/io/opentelemetry/exporter/internal/marshal/CodedOutputStream.java#L85-L104

If someone can offer help to get around this (or patch a fix), it would be appreciated. thanks.

Only discussion page I could find on ThreadLocals https://github.com/open-telemetry/opentelemetry-java/discussions/6584

Start by reading exporters/common/src/main/java/io/opentelemetry/exporter/internal/marshal/CodedOutputStream.java around lines 85-104, then review discussions 7082 and 6584 for the ThreadLocal context. Confirm how the encoder is reused and where cleanup can occur safely; done means the exporter no longer leaves the reported ThreadLocal behind during application shutdown without breaking encoding behavior.