8298730: Refactor subsystem_file_line_contents and add docs and tests

[jmtd]

Backport as a pre-requisite for 8313083. Not clean:

• adjustments for lack of log_trace
• remove hunks relating to the pid controller, which isn't in 8u
• move implementation of stringStream::reset from ostream.hpp to ostream.cpp in order to patch it. Upstream the move was done in 8225225 which we are not backporting
• dropped test_os_linux_cgroups.cpp from the patch as there are no gtests in 8u

I noticed a context difference in memory_and_swap_limit_in_bytes for Cgroups V1, which turns out to be because of 8284950. We might want to consider that for 8u as well.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8298730 needs maintainer approval

Issue

• JDK-8298730: Refactor subsystem_file_line_contents and add docs and tests (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk8u-dev.git pull/776/head:pull/776
$ git checkout pull/776

Update a local copy of the PR:
$ git checkout pull/776
$ git pull https://git.openjdk.org/jdk8u-dev.git pull/776/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 776

View PR using the GUI difftool:
$ git pr show -t 776

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk8u-dev/pull/776.diff

[bridgekeeper]

👋 Welcome back jdowland! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[jmtd]

@jerboaa FYI! I've not un-drafted this yet because it's not ready for review due to the test failures. I'm working on those (although running tier1 keeps getting OOM killed on my workstation).

[jerboaa]

I noticed a context difference in memory_and_swap_limit_in_bytes for Cgroups V1, which turns out to be because of 8284950. We might want to consider that for 8u as well.

This is a cgroup v1 feature and at this point on its way out. More and more systems these days are cgroups v2. I don't think that's worth doing in the stable maintenance only phase 8u is in.