RFR: JDK-8215482: check for cycles in type variables can provoke NPE

Vicente Romero vicente.romero at oracle.com
Fri Jan 11 18:05:22 UTC 2019

I have realized that cloning the type variable is not necessary as the 
Pair<JCTypeParameter, TypeVar> element in the table will keep a 
reference to the type variable


On 1/11/19 12:29 PM, Vicente Romero wrote:
> On 1/11/19 12:21 PM, Maurizio Cimadamore wrote:
>> It seems that another thing that your path is doing is storing all 
>> the type vars to check keyed by outermost class, so that only when 
>> you have finished entering the outermost symbol you actually start 
>> checking all pending typevars for cycles. I guess this delay is 
>> necessary otherwise you would hit a problem anyway when checking Bc 
>> for cyclicity?
> correct
>> Have you consider moving the attribution and cyclicity check in 
>> different type enter phases? For instance, leave attribution in 
>> HeaderPhase, but move the cycle check in MembersPhase ?
> I didn't try, but why would this be preferable? also as annotation 
> processing can nuke the type we would have to make sure that the types 
> are there before doing the cycle check which is something we can 
> guarantee now by keeping the check for cycles in the Header phase
>> Maurizio
> Vicente
>> On 11/01/2019 16:47, Vicente Romero wrote:
>>> Please review the fix for [1] at [2]. The NPE showed up in code like:
>>> class Outer<A extends Outer.Inner, B> {
>>>     class Inner<Bc extends B> {}
>>> }
>>> here attribution of type variable `A` during type enter phase will 
>>> trigger attribution of class `Inner` while type variable `B` hasn't 
>>> been attributed yet and thus its bound is still set to null. A 
>>> similar problem arose a while ago see [3]. The issue provoking the 
>>> current bug is that checks for cycles in type variables are done as 
>>> soon as the type variable is attributed but in cases like the one 
>>> above we can't do that until the type variable for the outer class 
>>> has been attributed too.
>>> My first try was to create a fixup table that maps the outer type to 
>>> the list of type variables defined by it or its members that happen 
>>> to be types too, and once the compiler finish entering the outer 
>>> class it would be safe to check for cycles in all the concerning 
>>> type variables. I had a mild success here as there were some trivial 
>>> cases that were working before that started failing. I realized that 
>>> it was because the annotation processing phase was setting all the 
>>> types to null, no bueno. So I decided to clone the type variables to 
>>> be stored in the fixup table and do the cycle check on the clones 
>>> which is what the current patch is doing.
>>> Thanks,
>>> Vicente
>>> [1] https://bugs.openjdk.java.net/browse/JDK-8215482
>>> [2] http://cr.openjdk.java.net/~vromero/8215482/
>>> [3] https://bugs.openjdk.java.net/browse/JDK-6660289

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