Bruce Momjian writes:
> I have been thinking about this. First, we can easily use fopen(r+) to
> check to see if the file exists, and if it does read the pid and do a
> kill -0 to see if it is running. If no one else does it, I will take it
> on.
It is better to use open with the O_CREAT and O_EXCL set. If the file does not
exist it will be created and the PID can be written to it. If the file exists
then the call will fail, at which point it can be opened with fread, and the
PID it contains can be checked to see if it still exists with kill. The open
call has the added advantage that 'The check for the existence of the file and
the creation of the file if it does not exist is atomic with respect to other
processes executing open naming the same filename in the same directory with
O_EXCL and O_CREAT set.' [from the UnixAWare 7 man page, open(2)].
Also, you can't just delete the file, create it and write the your PID to it
and assume that you have the lock, you need to close the file, sleep some
small amount of time and then open and read the file to see if you still have
the lock. If you like, I can take this task on.
Oh, the postmaster must clear the PID when it exits.
>
> Second, where to put the pid file. There is reason to put in /tmp,
> because it will get cleared in a reboot, and because it is locking the
> port number 5432. There is also reason to put it in /data because you
> can't have more than one postmaster running on a single data directory.
>
> So, we really want to lock both places. If this is going to make it
> easier for people to run more than one postmaster, because it will
> prevent/warn administrators when they try and put two postmasters in the
> same data dir or port, I say create the pid lock files both places, and
> give the admin a clear description of what he is doing wrong in each
> case.
IHMO, the pid should be put in the data directory. The reasoning that it will get cleared in a reboot is not sufficent
sincethe logic used to create the PID file will delete it if the PID it contains is not a running process. Besides, I
haveused systems where /tmp was not cleared out on a re-boot (for various reasons). Also, I would rather have a script
thatexplicitly removes the PID locking file at system statup (if it exists), in which case, it doesn't matter where it
resides.
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