Question

I have two processes which I am to synchronize.

Process A is running as root and creates a Reentrant-Lock on a specific file.
Process B is running under a “normal” user and should wait for the lock being releases by process A.

I tried many ways but cannot get it working because of wrong file permissions.
Here is the code:
(removed the in-VM-synchronisation-stuff):

Lock:

FileChannel channel = new RandomAccessFile(pFile, "rw").getChannel();
lock = channel.tryLock();

hasLock:

RandomAccessFile file = new RandomAccessFile(pFile, "rw");
FileChannel channel = file.getChannel();
FileLock lock = channel.tryLock();
if (lock == null) {
 return true;
}
lock.release();

The problem I have is that the lock gets created as:

-rw-r--r--. 1 root root 0 May  7 21:42 lockfile.lock

if I try to check the lock (by process B running as normal user) I get a

java.io.FileNotFoundException: _lockfile_ (Permission denied)
    at java.io.RandomAccessFile.open(Native Method)
    at java.io.RandomAccessFile.<init>(RandomAccessFile.java:212)

I tried to umask the directory to all g=rwx,o=rwx but this seems to be ignored. I tried to create the file first, setWritable(true, false) but this seems to be resetted. I did not get any method to work. I tried to use mode “r” instead of “rw” in hasLock but this leads to a ChannelNotWritableException.

So the main question is: how can I influence the permissions of the created lockfile?

Does anyone has some suggestions?

Regards Michael

Answer 1

Finally I got a solution:

The way of first creating the file was the correct one. The mistake I made was that I set the permissions before file creation (expecting that the permissions are stored in the file object and used on creation). I have to first create the file, then set the permissions to all and then lock it:

file.createNewFile();
file.setWritable(true, false);
FileChannel channel = new RandomAccessFile(file, "rw").getChannel();
channel.tryLock()

the lockfile is now created with worlwide-writable (this may be a security issue but actually there is no problem visible as the file does not contain any content.

Thanks to everyone helping me to find a solution!

Regards Michael