File locking supported on all platforms?
Stefan Reich
stefan.reich.maker.of.eye at googlemail.com
Thu Sep 17 08:39:21 UTC 2020
There is also a test <http://code.botcompany.de/1029789>:
void regularTest {
lock1 = makeLock();
lock2 = makeLock();
assertTrueVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014401>("Locking
lock1", lock1.tryToLock());
assertFalseVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014402>("Locking
lock2", lock2.tryToLock());
print <http://code.botcompany.de:8081/tb/show-snippet.php?id=1000658>("Waiting
for timeout (" + (timeout+1) + "s)");
sleepSeconds
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1001116>(timeout+1);
assertFalseVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014402>("Locking
lock2", lock2.tryToLock());
close <http://code.botcompany.de:8081/tb/show-snippet.php?id=1013362>(lock1);
assertTrueVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014401>("Locking
lock2", lock2.tryToLock());
close <http://code.botcompany.de:8081/tb/show-snippet.php?id=1013362>(lock2);
print <http://code.botcompany.de:8081/tb/show-snippet.php?id=1000658>("\nRegular
test OK!\n");
}
void crashTest {
lock1 = makeLock();
lock2 = makeLock();
assertTrueVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014401>("Locking
lock1", lock1.tryToLock());
assertFalseVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014402>("Locking
lock2", lock2.tryToLock());
lock1._simulateCrash();
print <http://code.botcompany.de:8081/tb/show-snippet.php?id=1000658>("Waiting
for timeout (" + (timeout+1) + "s)");
sleepSeconds
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1001116>(timeout+1);
assertTrueVerbose
<http://code.botcompany.de:8081/tb/show-snippet.php?id=1014401>("Locking
lock2", lock2.tryToLock());
closeAll <http://code.botcompany.de:8081/tb/show-snippet.php?id=1026437>(lock1,
lock2);
print <http://code.botcompany.de:8081/tb/show-snippet.php?id=1000658>("\nCrash
test OK!\n");
}
I think this thing might be solid
On Thu, 17 Sep 2020 at 09:46, Stefan Reich <
stefan.reich.maker.of.eye at googlemail.com> wrote:
> Thanks to Peter's suggestion, here's my working code for a crash-safe
> FileBasedLock in my own fancy Java dialect (JavaX).
>
> http://code.botcompany.de/1029788
> (Transpiled Java version:
> http://code.botcompany.de:8081/tb-int/get-transpiled.php?id=1029788&raw=1)
>
> I'm happy with this result. I actually don't use NFS and probably never
> will, but it's still a good feeling knowing even this case will be
> supported. Unless the clocks on your machines are badly out of sync. Yes I
> worry too much.
>
> Greetings
> Stefan
>
>
>
>
> On Mon, 14 Sep 2020 at 14:27, Stefan Reich <
> stefan.reich.maker.of.eye at googlemail.com> wrote:
>
>> Thanks Peter, that sounds pretty cool actually.
>>
>> Stefan
>>
>> On Mon, 14 Sep 2020 at 11:58, Peter Levart <peter.levart at gmail.com>
>> wrote:
>>
>>> Hi Stefan,
>>>
>>> You can always use the "create file exclusively" feature that should
>>> work on any POSIX compliant filesystem (NFS included):
>>>
>>> Files.createFile(path)
>>>
>>> ...you could implement an "exclusive lock" mechanism with this by
>>> creating and then updating a lock file periodically so that its "last
>>> modified" time is kept recent. Unlock would just remove the file. Observing
>>> the file with expired timestamp will give you permission to remove it even
>>> if you are not the lock owner.
>>>
>>> Regards, Peter
>>>
>>> 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> 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>> 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
>>>
>>>
>>>
>>
>> --
>> Stefan Reich
>> BotCompany.de // Java-based operating systems
>>
>
>
> --
> Stefan Reich
> BotCompany.de // Java-based operating systems
>
--
Stefan Reich
BotCompany.de // Java-based operating systems
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