RFR: 8320750: Allow a testcase to run with muliple -Xlog

[Leo Korinth]

On Mon, 27 Nov 2023 13:32:52 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

> Running a testcase with muliple -Xlog crashes JTREG test cases. This is because `Collector.toMap` is not given a merge strategy.
> 
> When the same argument is passed multiple times, I have added a merge strategy to use the latter value. This is similar to how it is implemented for `vm.opt.*` in JTREG.
> 
> If the flag tested is `-Xlog`, replace the value part with a dummy value "NONEMPTY_TEST_SENTINEL". This is because in the case of multiple `-Xlog` all values are used, and JTREG does not give a satisfactory way to represent them. This dummy value should make it hard to try to `@require` on specific values by mistake.
> 
> Tested with:
> 
>  @requires vm.opt.x.Xlog == "NONEMPTY_TEST_SENTINEL"
>  @requires vm.opt.x.Xlog == "NONEMPTY_TEST_SENTINELXXX"
>  @requires vm.opt.x.Xms == "3g"
> 
> and
> 
> JAVA_OPTIONS=-Xms3g -Xms4g
> JAVA_OPTIONS=-Xms4g -Xms3g
> JAVA_OPTIONS=-Xlog:gc* -Xlog:gc*
> ``` 
> 
> Running tier1

I think this feature will mostly be used to filter out un-allowed flag combinations, and I guess the user will seldom if ever be interested in the actual values (just the keys). Do you have an idea for something that is less fragile? I find it a bit ugly to special case `-Xlog` as was done by me and if you prefer we could set the value to the last `-Xlog`, but that is ugly as well. I wanted this to be similar as the built in `-XX` and from my understanding that code just uses the latest value.

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PR Comment: https://git.openjdk.org/jdk/pull/16824#issuecomment-1831939865