RFR: 8187450: JNI local refs exceeds capacity warning in NetworkInterface::getAll

[Chris Hegarty]

On Sun, 14 Mar 2021 12:29:45 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> This is an adaptation of a patch originally written by Shafi Ahmad in
>> a comment on the JBS page but never submitted or merged.
>> 
>> With -Xcheck:jni, the method java.net.NetworkInterface.getAll very
>> quickly breaches the default JNI local refs threshold (32). Exactly when
>> this happens depends upon the number of network interfaces (in state "UP")
>> visible to Java. On Linux, with current Trunk, 2 network interfaces is
>> sufficient to breach:
>> 
>>     $ ./addif.sh 0
>>     1 interfaces
>>     $ sudo ip netns exec jbase $JAVA_HOME/bin/java -Xcheck:jni NITest
>>      (nothing)
>>     $ ./addif.sh 1
>>     2 interfaces
>>     $ sudo ip netns exec jbase $JAVA_HOME/bin/java -Xcheck:jni NITest
>>     WARNING: JNI local refs: 33, exceeds capacity: 32
>>             at java.net.NetworkInterface.getAll(java.base/Native Method)
>>             at java.net.NetworkInterface.getNetworkInterfaces(java.base/NetworkInterface.java:351)
>>             at NITest.main(NITest.java:3)
>> 
>> This patch improves the situation:
>> 
>>     $ ./addif.sh 3
>>     4 interfaces
>>     $ sudo ip netns exec jbase $JAVA_HOME/bin/java -Xcheck:jni NITest
>>      (nothing)
>>     $ ./addif.sh 4
>>     5 interfaces
>>     $ sudo ip netns exec jbase $JAVA_HOME/bin/java -Xcheck:jni NITest
>>     WARNING: JNI local refs: 33, exceeds capacity: 32
>>             at java.net.NetworkInterface.getAll(java.base/Native Method)
>>             at java.net.NetworkInterface.getNetworkInterfaces(java.base/NetworkInterface.java:351)
>>             at NITest.main(NITest.java:3)
>> 
>> 
>> Once the JNI local refs threshold is breached, the threshold is raised.
>> With the patch, it takes 10 network interfaces to breach the new
>> threshold 
>> 
>>     $ ./addif.sh 9
>>     10 interfaces
>>     $ sudo ip netns exec jbase $JAVA_HOME/bin/java -Xcheck:jni NITest
>>     WARNING: JNI local refs: 33, exceeds capacity: 32
>>             at java.net.NetworkInterface.getAll(java.base/Native Method)
>>             at java.net.NetworkInterface.getNetworkInterfaces(java.base/NetworkInterface.java:351)
>>             at NITest.main(NITest.java:3)
>>     WARNING: JNI local refs: 66, exceeds capacity: 65
>>             at java.net.NetworkInterface.getAll(java.base/Native Method)
>>             at java.net.NetworkInterface.getNetworkInterfaces(java.base/NetworkInterface.java:351)
>>             at NITest.main(NITest.java:3)
>> 
>> Without the patch it takes 5.
>> 
>> Helper scripts for testing on Linux. `setupnet.sh`:
>> 
>>     #!/bin/bash
>>     set -euo pipefail
>> 
>>     namespace=${namespace:-jbase}
>>     sudo ip netns add ${namespace}
>> 
>> And `addif.sh`:
>> 
>>     #!/bin/bash
>>     set -euo pipefail
>> 
>>     namespace=${namespace:-jbase}
>>     num="$1"
>> 
>>     sudo ip link add name vethhost${num} type veth peer name veth${namespace}${num}
>>     sudo ip link set veth${namespace}${num} netns ${namespace}
>>     sudo ip addr add 192.168.2.${num}/24 dev vethhost${num}
>>     sudo ip netns exec ${namespace} ip addr add 192.168.2.${num}/24 dev veth${namespace}${num}
>>     sudo ip link set vethhost${num} up
>>     sudo ip netns exec ${namespace} ip link set veth${namespace}${num} up
>> 
>>     count="$(sudo ip netns exec ${namespace} ip link show |grep UP | wc -l)"
>>     echo "${count} interfaces"
>
> I think this looks okay. I assume the deletion of the locals refs at the end of createNetworkInterface aren't really needed, but harmless. It may be useful to update test/jdk/java/net/NetworkInterface/Test.java to have it run with -Xcheck:jni for both the default and -Djava.net.preferIPv4Stack=true cases.

No need to force-push - just push. The Skara workflow will generate incremental diffs per-push and flatten all commits into one before final integration into the JDK mainline.

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PR: https://git.openjdk.java.net/jdk/pull/2963