AttachListener bsd differences

[David Holmes]

Hi Sonia,

Adding serviceability and Alan Bateman ...

On 5/03/2024 2:35 am, Sonia Zaldana Calles wrote:
> Hi folks,
> 
> 
> I’m working on JDK-8324683 [0].
> 
> 
> I’m hoping to unify the code for attachListener_<os>.cpp for posix 
> platforms.
> 
> 
> While reviewing linux against bsd, I noted a difference in 
> AttachListener::is_init_trigger().
> 
> 
> Both linux and AIX implementations, first try “<current 
> dir>/.attach<pid>” and failing that, attempt “/tmp/.attach<pid>”.  [1][2]
> 
> 
> On the other hand, bsd doesn’t attempt “/tmp/.attach<pid>” [3].

I think you meant it doesn't attempt <current dir>/.attach<pid>

> 
> I have been trying to do a bit of digging to see if this disparity was 
> intentional but I have found that this change precedes the MacOS port, 
> so I’m having trouble coming to a conclusion.

Looking at the history here:

https://hg.openjdk.org/macosx-port/macosx-port/hotspot/log/407770c31c18/src/os/bsd/vm/attachListener_bsd.cpp

The very first load of the file did check the current directory too:

https://hg.openjdk.org/macosx-port/macosx-port/hotspot/rev/be94a5de4d32

but that was removed in the very next upload (though they never fixed 
the comment preceding the code):

https://hg.openjdk.org/macosx-port/macosx-port/hotspot/rev/9f1dd0b1d28c

To know why you would have to ask the person that did it I'm afraid.

> I was wondering if perhaps anyone on this list could shed some light on 
> this matter and whether it would be acceptable to do the alternate path 
> on BSD too.

The init trigger mechanism was added by Alan under:

https://bugs.openjdk.org/browse/JDK-6272307

I get the sense that having the well-known file in the working directory 
was actually a bit of an anachronism. That may be why the macOS folk 
didn't bother with it (speculation on my part). AIX simply copied the 
Linux code.

Whether it would be okay/harmless/safe to have macOS/BSD also check 
there I really can't say. I'm not sure if we have code that would ever 
write the file to the target cwd these days? Hopefully Alan or current 
serviceability folk can say more.

Cheers,
David
-----

> 
> Thanks for your help,
> 
> Sonia
> 
> 
> 
> [0] https://bugs.openjdk.org/browse/JDK-8324683 
> <https://bugs.openjdk.org/browse/JDK-8324683>
> 
> [1] 
> https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/attachListener_linux.cpp#L521C3-L529C4 <https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/attachListener_linux.cpp#L521C3-L529C4>
> 
> [2] 
> https://github.com/openjdk/jdk/blob/master/src/hotspot/os/aix/attachListener_aix.cpp#L551C2-L559C4 <https://github.com/openjdk/jdk/blob/master/src/hotspot/os/aix/attachListener_aix.cpp#L551C2-L559C4>
> 
> [3] 
> https://github.com/openjdk/jdk/blob/master/src/hotspot/os/bsd/attachListener_bsd.cpp#L520C2-L522C4 <https://github.com/openjdk/jdk/blob/master/src/hotspot/os/bsd/attachListener_bsd.cpp#L520C2-L522C4>
> 
> -- 
> Sonia Zaldaña Calles
> Software Engineer, OpenJDK
> Red Hat