File locking supported on all platforms?

[Jonas Konrad]

I asked around and apparently fcntl locking can be idempotent in the 
same process at least for some fs, so locking twice in the same jvm may 
not error even on an fs that does support locks.

- Jonas

On 7/11/20 12:13 PM, Stefan Reich wrote:
> Hmm that's what I feared. So to be really sure I should do an actual 
> test. Maybe I can get away with doing two locks on the same file within 
> one JVM for testing the feature? I should try that.
> 
> Thanks
> 
> On Sat, 11 Jul 2020 at 12:10, Jonas Konrad <me at yawk.at 
> <mailto:me at yawk.at>> wrote:
> 
>     I think it should just fail silently / without error, that's what fcntl
>     does at least. I'm not sure if there's a good way to determine whether
>     locking will actually work.
> 
>     - Jonas
> 
>     On 7/11/20 11:47 AM, Stefan Reich wrote:
>      > Hi Jonas,
>      >
>      > what would happen if I try to lock a file on these platforms? Can I
>      > reliably detect this case? That would already help a lot.
>      >
>      > The use case is ensuring a database's consistency which I now do
>     over a
>      > socket protocol, but file locks would be faster and more reliable.
>      >
>      > Stefan
>      >
>      > On Sat, 11 Jul 2020 at 11:45, Jonas Konrad <me at yawk.at
>     <mailto:me at yawk.at>
>      > <mailto:me at yawk.at <mailto:me at yawk.at>>> wrote:
>      >
>      >     Hey,
>      >
>      >     Some file systems on Linux, eg old NFS, do not support
>     locking at all.
>      >
>      >     - Jonas
>      >
>      >     On 7/11/20 11:36 AM, Stefan Reich wrote:
>      >      > Hi,
>      >      >
>      >      > a quick question regarding file locks in Java. I seem to
>     remember
>      >      > reading somewhere that this feature isn't guaranteed to
>     work on all
>      >      > platforms. However, I can now find no mention of that e.g.
>     here
>      >      >
>      >   
>       <https://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileLock.html>.
>      >      >
>      >      > The page does list that numerous special assumptions may
>     fail, for
>      >      > example that programs which /don't/ lock a file are
>     prevented from
>      >      > changing it when it is locked by another program.
>      >      >
>      >      > However, I only care about the basic feature, namely that one
>      >     local file
>      >      > cannot be locked by two Java programs at once and will
>     cause one
>      >     of them
>      >      > to throw an exception.
>      >      >
>      >      > Is this guaranteed to work everywhere?
>      >      >
>      >      > Many greetings
>      >      > Stefan
>      >      >
>      >      > --
>      >      > Stefan Reich
>      >      > BotCompany.de // Java-based operating systems
>      >
>      >
>      >
>      > --
>      > Stefan Reich
>      > BotCompany.de // Java-based operating systems
> 
> 
> 
> -- 
> Stefan Reich
> BotCompany.de // Java-based operating systems