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

[Lin Zang]

On Wed, 18 Nov 2020 08:41:59 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> 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 an older `jmap` is used? Is that correct?

Hi @plummercj,
     For old jmap, if "gz" option is used in commandline, it prompt usage info indicating that option is illegal.
     if "gz" option is not used, the op->arg(2) will be an empty string "", which is set from the attacher side in the method execute() of VirtualMachineImpl.java.

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