[jdk17u-dev] RFR: 8298730: Refactor subsystem_file_line_contents and add docs and tests

[Paul Hohensee]

On Mon, 31 Mar 2025 13:05:13 GMT, Goetz Lindenmaier <goetz at openjdk.org> wrote:

> I backport this for parity with 17.0.16-oracle.
> 
> I had to resolve one larger chunk in cgroupSubsystem_linux.hpp.  Changes [8301479: Replace NULL with nullptr in os/linux](https://github.com/openjdk/jdk21u-dev/commit/ac9e046748a9bb6ee065dc473d82135ce36043b7) and [8238161: use os::fopen in HS code where possible](https://github.com/openjdk/jdk21u-dev/commit/f54ce84474c2ced340c92564814fa5c221415944) are not in 17. 
> 
> I did add the new code as-is wrt. nullptr. but kept the call to fopen as it was in 17 before this change.
> 
> To make it compile I add changes to ostream.hpp from https://bugs.openjdk.org/browse/JDK-8293251: Use stringStream::base() instead of as_string() when applicable
> See extra commit.

Marked as reviewed by phh (Reviewer).

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PR Review: https://git.openjdk.org/jdk17u-dev/pull/3411#pullrequestreview-2730262271