[Ping] RFR: 8275731: CDS archived enums objects are recreated at runtime

[Ioi Lam]

Still looking for reviewers ....

Thanks
- Ioi

On 12/1/21 1:02 PM, Ioi Lam wrote:
> **Background:**
>
> In the Java Language, Enums can be tested for equality, so the constants in an Enum type must be unique. Javac compiles an enum declaration like this:
>
>
> public enum Day {  SUNDAY, MONDAY ... }
>
>
> to
>
>
> public class Day extends java.lang.Enum {
>      public static final SUNDAY = new Day("SUNDAY");
>      public static final MONDAY = new Day("MONDAY"); ...
> }
>
>
> With CDS archived heap objects, `Day::<clinit>` is executed twice: once during `java -Xshare:dump`, and once during normal JVM execution. If the archived heap objects references one of the Enum constants created at dump time, we will violate the uniqueness requirements of the Enum constants at runtime. See the test case in the description of [JDK-8275731](https://bugs.openjdk.java.net/browse/JDK-8275731)
>
> **Fix:**
>
> During -Xshare:dump, if we discovered that an Enum constant of type X is archived, we archive all constants of type X. At Runtime, type X will skip the normal execution of `X::<clinit>`. Instead, we run `HeapShared::initialize_enum_klass()` to retrieve all the constants of X that were saved at dump time.
>
> This is safe as we know that `X::<clinit>` has no observable side effect -- it only creates the constants of type X, as well as the synthetic value `X::$VALUES`, which cannot be observed until X is fully initialized.
>
> **Verification:**
>
> To avoid future problems, I added a new tool, CDSHeapVerifier, to look for similar problems where the archived heap objects reference a static field that may be recreated at runtime. There are some manual steps involved, but I analyzed the potential problems found by the tool are they are all safe (after the current bug is fixed). See cdsHeapVerifier.cpp for gory details. An example trace of this tool can be found at https://bugs.openjdk.java.net/secure/attachment/97242/enum_warning.txt
>
> **Testing:**
>
> Passed Oracle CI tiers 1-4. WIll run tier 5 as well.
>
> -------------
>
> Commit messages:
>   - 8275731: CDS archived enums objects are recreated at runtime
>
> Changes: https://git.openjdk.java.net/jdk/pull/6653/files
>   Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6653&range=00
>    Issue: https://bugs.openjdk.java.net/browse/JDK-8275731
>    Stats: 829 lines in 16 files changed: 787 ins; 2 del; 40 mod
>    Patch: https://git.openjdk.java.net/jdk/pull/6653.diff
>    Fetch: git fetch https://git.openjdk.java.net/jdk pull/6653/head:pull/6653
>
> PR: https://git.openjdk.java.net/jdk/pull/6653