RFR: 8267666: Add option to jcmd GC.heap_dump to use existing file [v2]

[Anton Kozlov]

On Fri, 28 May 2021 10:49:32 GMT, Anton Kozlov <akozlov at openjdk.org> wrote:

>> Please review a small change that adds an option to GC.heap_dump to use an existing file. 
>> 
>> The option is necessary if the target file is a predefined file-like object, like a named pipe. This opens up a lot of possibilities to process a heap dump without storing it to the file system first.
>> 
>> Reviews of the CSR linked to the bug would be appreciated as well.
>
> Anton Kozlov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Extend HeapDumpTest

Agree, I would like to hear more feedback.

I still don't think we need to provide a (false) similarity with existing tools. For example:
* `cp` succeeds if the target file exists; `jcmd` bails out in this case
* `cp -f` unlinks and creates a new file (so it is just a convenient substitution to `rm -f; cp`); `jcmd -rewrite` rewrites the file -- this cannot be achieved without making changes to the Hotspot.

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PR: https://git.openjdk.java.net/jdk/pull/4183