RFR: 8287073: NPE from CgroupV2Subsystem.getInstance()

[Peter Levart]

On Wed, 25 May 2022 14:39:42 GMT, Peter Levart <plevart at openjdk.org> wrote:

>> Following the logic from the comment directly above the changed line, since it doesn't matter which controller we pick, pick any available controller instead of the one called "memory" specifically. This way we are guarded against getting `null` as `anyController`, which is being immediately passed down to `CgroupV2Subsystem.getInstance()` that is unprepared to accept `null` values. 
>> 
>> It is also worth noting that the previous checks (such as that at line 89) make sure that there exist at least one controller in the map.
>
> src/java.base/linux/classes/jdk/internal/platform/CgroupSubsystemFactory.java line 113:
> 
>> 111:             CgroupInfo anyController = infos.values().iterator().next();
>> 112:             CgroupSubsystem subsystem = CgroupV2Subsystem.getInstance(anyController);
>> 113:             return subsystem != null ? new CgroupMetrics(subsystem) : null;
> 
> Looking at implementation of CgroupV2Subsystem.getInstance(...), it seems that it always returns != null ...

`CgroupV1Subsystem.getInstance(...)` also claims that it never returns `null`, but has a code-path that actually returns `null` (when there is no active controller). Is this a possible outcome?

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PR: https://git.openjdk.java.net/jdk/pull/8803