RFR: 8298730: Refactor subsystem_file_line_contents and add docs and tests [v2]

[Johan Sjölen]

On Wed, 14 Dec 2022 19:14:41 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi!
>> 
>> Citing the ticket directly:
>> 
>> The function subsystem_file_line_contents does the simple job of parsing lines in a file, but does so in a fairly complex way. This function can be simplified. The function also has a surprising API, as you need to know that in one case the format string needs to take 2 specifiers, instead of 1. 
>> 
>> Some more context:
>> 
>> `subsystem_file_line_contents` either parses files that look like this:
>> 
>> 
>> one_value
>> 
>> 
>> Called as: `subsystem_file_line_contents(ctrl, fname, nullptr, "%s")`
>> 
>> Or like this:
>> 
>> 
>> key1 val1
>> key2 val2
>> 
>> 
>> Called as:  `subsystem_file_line_contents(ctrl, fname, "key1", "%s %s")`
>> 
>> The API for the key/value case is changed to: `subsystem_file_line_contents(ctrl, fname, "key1", "%s")`. Note: `"%s"`, not `"%s %s"`.
>
> Johan Sjölen has updated the pull request incrementally with seven additional commits since the last revision:
> 
>  - Split into single-line and multiple-line tests
>  - Rename tests
>  - Revert
>  - Specify buffer limit in format specifier
>  - Fix log string
>  - Allow other whitespace than space, add test for \t
>  - More tests, including JULONG and JLONG.

test/hotspot/gtest/os/linux/test_cgroupSubsystem_linux.cpp line 103:

> 101:   err = subsystem_file_line_contents(&my_controller, test_file, "foo", "%s", &s);
> 102:   EXPECT_EQ(err, 0);
> 103:   EXPECT_STREQ(s, "bar") << "Incorrect!";

I added your test here, @jerboaa

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PR: https://git.openjdk.org/jdk/pull/11667