RFR: 8287007: [cgroups] Consistently use stringStream throughout parsing code

[Severin Gehwolf]

On Wed, 1 Jun 2022 09:18:38 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

> Please review this cleanup change in the cgroup subsystem which used to use hard-coded stack allocated
> buffers for concatenating strings in memory. We can use `stringStream` instead which doesn't have the issue
> of hard-coding maximum lengths (and related checks) and makes the code, thus, easier to read.
> 
> While at it, I've written basic `gtests` verifying current behaviour (and the same for the JDK code). From
> a functionality point of view this is a no-op (modulo the one bug in the substring case which seems to be
> there since day one). I'd prefer if we could defer any change in functionality to a separate bug as this is
> meant to only use stringStream throughout the cgroups code.
> 
> Testing:
> - [x] Container tests on Linux x86_64 cgroups v1 and cgroups v2
> - [x] Added tests, which I've verified also pass before the stringStream change
> 
> Thoughts?

@iklam Would you have some cycles to review this, please?

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PR: https://git.openjdk.java.net/jdk/pull/8969