RFR: 8286030: Avoid JVM crash when containers share the same /tmp dir [v2]

[Thomas Stuefe]

On Thu, 7 Jul 2022 21:58:55 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> src/hotspot/os/posix/perfMemory_posix.cpp line 781:
>> 
>>> 779:     // signal the process, then the file is assumed to
>>> 780:     // be stale and is removed because the files for such a
>>> 781:     // process should be in a different user specific directory.
>> 
>> I am not sure this is good. If two conflicting hotspots share the same PID in tmp, from two different users, this will probably be a setup error. Is the best way really to provoke SIGBUS in the other VM? Seems a bit harsh. 
>> 
>> Also terminology would be wrong. Its not stale then, since the target process probably exists, is a VM, and uses that file.
>
> We will get a permission error from the `kill(pid, 0)` call only after we have successfully grabbed the flock on the file. Note that if the file was created by a live JVM process that has the flock fix (i.e., this PR), regardless of which user owns the process, we will never come to here.
> 
> That the value of the `pid` variable is misleading. It is the NSPID of another JVM that created the file. If the current JVM process runs in a different PID namespace, it cannot reliably determine whether the file is stale or not.
> 
> In general, I don't think we can trust `pid` at all when containers are involved. But that's OK -- if you want to use Java in containers that share the /tmp directory, you must upgrade to a JVM that has the flock fix. Otherwise the behavior is undefined.
> 
> Otherwise, if you are:
> 
> - Not using containers. or
> - Using containers that don't share /tmp
> 
> The logic for handling the `kill(pid, 0)` error is not changed by this PR, so we are bug-for-bug compatible with older JVMs. If you think the behavior should be changed, may that should be done in a separate PR?
> 
> Or, if you run into problems like "my hsperf files are randomly deleted", a simple fix is to upgrade the JVM to one that has the flock fix :-)

Okay, you convinced me.

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PR: https://git.openjdk.org/jdk/pull/9406