[JENKINS-71970] Memory leak involving BufferedBuildListener

[jglick]

JENKINS-71970, amending #309.

Reproduced by running mvn hpi:run with

diff --git pom.xml pom.xml
index c4482a2..646fe12 100644
--- pom.xml
+++ pom.xml
@@ -63,7 +63,7 @@
     </pluginRepositories>
     <properties>
         <changelist>999999-SNAPSHOT</changelist>
-        <jenkins.version>2.361.4</jenkins.version>
+        <jenkins.version>2.414.1</jenkins.version>
         <no-test-jar>false</no-test-jar>
         <useBeta>true</useBeta>
         <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>
@@ -72,8 +72,8 @@
         <dependencies>
             <dependency>
                 <groupId>io.jenkins.tools.bom</groupId>
-                <artifactId>bom-2.361.x</artifactId>
-                <version>2102.v854b_fec19c92</version>
+                <artifactId>bom-2.414.x</artifactId>
+                <version>2423.vce598171d115</version>
                 <scope>import</scope>
                 <type>pom</type>
             </dependency>

plus updating workflow-support from UC and installing mock-slave, creating a mock slave, and running

node('static') {
    int i = 0
    while (true) {
        sh "echo ${i}"
        i++
    }
}

then

watch 'jmap -histo:live …

or

watch 'jmap -histo:live … | fgrep BufferedBuildListener'

There were multiple issues. First of all, removing the listener on Channel.onClosed is not enough (and may in fact be useless) when there are lots of different listeners being created while the channel is open. The listener needed to be associated with the stream (BufferedBuildListener), not its Replacement which is intended to be transient. Closing the stream (if that ever happens) should also remove the listener. And collecting the stream (which happens routinely) should remove the listener.

With the fix, I was able to run thousands of sh steps in a row on the same open agent channel, and memory did not increase significantly (in particular the byte[] usage would go up to ~10Mb but then drop again), nor did any classes from this package stay in the top screenful.