RFR: 8256450: Add gz option to jmap to write a gzipped heap dump [v2]

[Chris Plummer]

On Wed, 18 Nov 2020 04:19:51 GMT, Lin Zang <lzang at openjdk.org> wrote:

>>> And the old jmap -dump can not accept "gz" option, it fails with error message printed, no matter what jvm version it work with.
>> 
>> I just want to clarify what I was referring to. I was not talking about trying to use gz with the old jmap command (from the command line). I was asking what happens if you use the old jmap command on a newer jvm that does support (and I assume expects) the new "compression level" argument to be passed, but it won't be. We've had this issue before. See [JDK-8219721](https://bugs.openjdk.java.net/browse/JDK-8219721). I haven't had a chance to look at the fix for JDK-8219721 to see if it also solves the issue with this change, or if something similar also needs to be done with this change.
>
> Hi @plummercj ,
> 
> Thanks for your explaination. I get your point. 
> The issue of JDK-8219721 was fixed by reverting the problematic change. And the root cause of that issue is there is fixed limitation of number of arguments that passing to jvm from attacher, I have figured out a way to avoid touching it when adding new options to jcmd-alike tools, However this PR doesn't even exceed the argument limitation, so it doesn't have chance to cause the similar issue. 
> 
> BRs,
> Lin

So I assume then that the `op->arg(2)` reference will return `NULL` when and older `jmap` is used? Is that correct?

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PR: https://git.openjdk.java.net/jdk/pull/1251