RFR: 8289919: [test] LoadLibraryUnloadTest.java failed with "Failed to unload native library" [v2]

[Jaikiran Pai]

On Wed, 13 Jul 2022 14:06:00 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> The test `java/lang/ClassLoader/loadLibraryUnload/LoadLibraryUnloadTest.java`
>> Fails intermittently when expected output from a subprocess is not found.
>> 
>> I suspect a race between the Cleaner that is going to call JNI_OnUnload (in NativeLibraries.java:377) when the ClassLoader is no longer referenced and the test code that exits as soon as it detects that the p.Class1 is no longer referenced.
>> 
>> The proposed fix is to create a canary object referenced by the native library and released when the library is unloaded.
>> The Java side of the test provides the canary object and uses a WeakReference to wait for it to be released.
>> When released the child process exits and the driver test will find all of the output it expects.
>
> Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Cleanup waiting for library unload

test/jdk/java/lang/ClassLoader/loadLibraryUnload/libloadLibraryUnload.c line 48:

> 46:     if (ref == NULL) {
> 47:         // Only create a single GlobalRef
> 48:         ref = (*env)->NewGlobalRef(env, obj);

Hello Roger, do we not use any synchronization here because even if both winning threads end up concurrently here, the `Object` instance they will be setting as a global ref will be the same one? Ofcourse, there will then be two calls to `NewGlobalRef` for the same instance. I am unaware what the semantics of that call will be in such cases and if it is of any concern.

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PR: https://git.openjdk.org/jdk/pull/9474