RFR: 8257596: Clarify trusted final fields for record classes

[Harold Seigel]

On Tue, 8 Dec 2020 22:52:37 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> This is a follow-up on JDK-8255342 that removes non-specified JVM checks on classes with `RecordComponents` attributes.  That introduces a regression in `InstanceKlass::is_record` that returns true on a non-record class which has `RecordComponents` attribute present.   This causes unexpected semantics in `JVM_IsRecord` and `JVM_GetRecordComponents` and also a regression to trust final fields for non-record classes.
> 
> I propose to change `InstanceKlass::is_record` to match the JLS semantic of a record class, i.e. final direct subclass of `java.lang.Record` with the presence of `RecordComponents` attribute.  There is no change to JVM class file validation.   Also I propose clearly define:
>     - `JVM_IsRecord` returns true if the given class is a record i.e. final and direct subclass of `java.lang.Record` with `RecordComponents` attribute
>     - `JVM_GetRecordComponents` returns an `RecordComponents` array  if `RecordComponents` attribute is present; otherwise, returns NULL.  This does not check if it's a record class or not.  So it may return non-null on a non-record class if it has `RecordComponents` attribute.  So `JVM_GetRecordComponents` returns the content of the classfile.

Hi Mandy,
Could you replace the comment starting at line 1854 in jvm.cpp with:
// A class is a record if and only if it is final and a direct subclass
// of java.lang.Record and the presence of `Record` attribute;
// otherwise, it is not a record.

Also, replace the comment starting at line 1872 in jvm.cpp with:
// Returns an array containing the components of the Record attribute,
// or NULL if the attribute is not present.
//
// Note that this function returns the components of the Record
// attribute even if the class is not a Record.



Also, please change the name of the attribute in the comments added
to Class.java from RecordComponent to Record.

Thanks, Harold

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PR: https://git.openjdk.java.net/jdk/pull/1706