RFR: 8273152: Refactor CDS FileMapHeader loading code [v5]

[Yumin Qi]

On Wed, 6 Oct 2021 04:17:16 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Yumin Qi has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Added a helper class to facilitate checking archive
>
> src/jdk.hotspot.agent/share/native/libsaproc/ps_core_common.c line 369:
> 
>> 367: 
>> 368:       // check file magic
>> 369:       if (header._generic_header._magic != CDS_ARCHIVE_MAGIC) {
> 
> Use `header.magic()` instead?

header is a structure of CDSFileMapHeaderBase, unless we cast it to FileMapHeader.

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PR: https://git.openjdk.java.net/jdk/pull/5768