RFR (S): 8238676: jni crashes on accessing it from process exit hook

[gerard ziemski]

Sorry, yes I am OK with the change.

I think there is some room to interpretation here, and I'd have done it 
slightly differently, but I'm fine with your interpretation.


cheers

On 2/28/20 11:16 PM, David Holmes wrote:
> Hi Gerard,
>
> Are you okay with this?
>
> Thanks,
> David
>
> On 26/02/2020 10:11 am, David Holmes wrote:
>> Hi Gerard,
>>
>> Thanks for taking a look at this.
>>
>> On 26/02/2020 2:45 am, gerard ziemski wrote:
>>> hi David,
>>>
>>> On 2/18/20 8:04 PM, David Holmes wrote:
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8238676
>>>> webrev: http://cr.openjdk.java.net/~dholmes/8238676/webrev/
>>>>
>>>> If an application tries to use JNI from an atexit handler, the 
>>>> attempt can't succeed (the VM has already logically terminated) but 
>>>> it should not crash. The JNI Invocation API code was missing some 
>>>> checks in places and wasn't aware of the possibility of trying to 
>>>> make calls from the VMThread.
>>>>
>>>> Testing:
>>>>  - new test added for the JNI Invocation API
>>>>  - tiers 1-3
>>>>
>>>> Thanks,
>>>> David
>>>
>>> In jni_GetEnv() we have:
>>>
>>> 4120   if (vm_created == 0) {
>>> 4121     *penv = NULL;
>>> 4122     ret = JNI_EDETACHED;
>>> 4123     return ret;
>>> 4124   }
>>>
>>> but in jni_DetachCurrentThread() we have:
>>>
>>> 4063   if (vm_created == 0) {
>>> 4064     HOTSPOT_JNI_DETACHCURRENTTHREAD_RETURN(JNI_ERR);
>>> 4065     return JNI_ERR;
>>> 4066   }
>>>
>>> should lines 4064,4065 perhaps be:
>>>
>>> 4064 HOTSPOT_JNI_DETACHCURRENTTHREAD_RETURN(JNI_EDETACHED);
>>> 4065     return JNI_EDETACHED;
>>>
>>> to be consistent?
>>
>> Well ... ideally the JNI spec would have considered the question of 
>> "what if the JavaVM is no longer live?" and had an JNI_ENOJVM errro 
>> code. But it doesn't. Nor does it allow for any JNI method to return 
>> JNI_ERR for unexpected errors situations. So we have to adhere to the 
>> specifications for each method.
>>
>> For GetEnv the spec only allows for returning JNI_EDETACHED, 
>> JNI_EVERSION or JNI_OK. As you can't be attached to a VM that doesn't 
>> exist then JNI_EDETACHED is the only possible return code - and it 
>> isn't wrong, it just doesn't tell you why you aren't attached.
>>
>> If you apply the same logic to DetachCurrentThread, and follow the 
>> spec then it states
>>
>> "Trying to detach a thread that is not attached is a no-op."
>>
>> which suggests we do nothing and report JNI_OK. But I think that 
>> would be a disservice to the programmer in the case where the JVM is 
>> no longer live. And we're allowed to return "a suitable JNI error 
>> code on failure" so that is what I chose to do. And I return JNI_ERR 
>> rather than JNI_EDETACHED for two reasons:
>>
>> 1. It is consistent with what we do for attach in the same circumstance:
>>
>> jint JNICALL jni_AttachCurrentThread(JavaVM *vm, void **penv, void 
>> *_args) {
>>    HOTSPOT_JNI_ATTACHCURRENTTHREAD_ENTRY(vm, penv, _args);
>>    if (vm_created == 0) {
>>      HOTSPOT_JNI_ATTACHCURRENTTHREAD_RETURN((uint32_t) JNI_ERR);
>>      return JNI_ERR;
>>    }
>>
>> 2. It would be confusing to return JNI_EDETACHED as that means 
>> "thread detached from the VM" yet the spec says "Trying to detach a 
>> thread that is not attached is a no-op.". So I need to convey that 
>> there is a more specific underlying error here.
>>
>>> Looks good otherwise.
>>
>> Thanks for the review.
>>
>> David
>> -----
>>
>>>
>>> cheers
>>>