RFR: 8268361: Fix the infinite loop in next_line [v2]

[UncleNine]

On Tue, 8 Jun 2021 16:21:34 GMT, UncleNine <github.com+1964054+UncleNine at openjdk.org> wrote:

>> It is not obvious to me that the caller of next_line will handle the fact that we have hit EOF?
>
> In my case,  it happened in the container environment.
> the /proc filesystem of the container is provided by  lxcfs, but a lxcfs bug may make the /proc/stat mount point change, then  the file descriptor is different and fgetc function returns an EOF on error,  But c != '\n' is true and it leads to the  infinit loop.
> Below are our flamegraph in the production , it happends on serveral frameworks（micrometer, elasticsearch..）which use the api "sun/management/OperatingSystemImpl.getSystemCpuLoad"
> ![107140246-bb490f80-695b-11eb-9f62-0116079f4f4a](https://user-images.githubusercontent.com/1964054/121218982-d5690f00-c8b5-11eb-8c1e-e39f2f2bc6a5.jpg)

I use  top -Hp $PID command and the thread show 100% cpu usage
![top](https://user-images.githubusercontent.com/1964054/121224786-7ad2b180-c8bb-11eb-8c1b-e32b131d4327.png)
![jtack](https://user-images.githubusercontent.com/1964054/121224813-81612900-c8bb-11eb-80e1-29d15989dc17.png)

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PR: https://git.openjdk.java.net/jdk/pull/4378