RFR: 8330465: Stable Values and Collections (Internal) [v5]

[Chen Liang]

On Wed, 15 May 2024 18:49:49 GMT, ExE Boss <duke at openjdk.org> wrote:

>> src/java.base/share/classes/jdk/internal/lang/stable/StableValueImpl.java line 240:
>> 
>>> 238:                 }
>>> 239:             } finally {
>>> 240:                 supplying = false;
>> 
>> Resetting a stable field is a bad idea. I recommend renaming this to `supplierCalled` or `supplied` so we never transition this false -> true
>
> Yes, according to the `@Stable` annotation’s JavaDoc, this is UB:
> https://github.com/openjdk/jdk/blob/8a4315f833f3700075d65fae6bc566011c837c07/src/java.base/share/classes/jdk/internal/vm/annotation/Stable.java#L74-L80

Fyi what usually happens is that if a stable field or similarly constant-folded field is promoted to constant, the constant promotion can happen to any of the previous valid values written.

MethodHandle optimisitically sets a trusted final field this way:
https://github.com/openjdk/jdk/blob/8a4315f833f3700075d65fae6bc566011c837c07/src/java.base/share/classes/java/lang/invoke/MethodHandle.java#L1868-L1870

Also a similar example in user code targeting older Java releases, before JDK 16's strong encapsulation so that enums could have been added by reflection:
https://github.com/MinecraftForge/MinecraftForge/issues/3885#issuecomment-355602542

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PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1602125255