I discovered Parallel::ForkManager the other day, and rejoiced. Unfortunately, in using it I ended up tearing my hair out due to a somewhat unrelated problem: the interaction of flock() and dup'd file descriptors in child processes.
(perl 5.6, FreeBSD 4.1.1-stable, but the principles apply equally elsewhere)

The problem:

• Parent open()'s file.
• Parent fork()'s children.
• Children flock() the open filehandle.
• It doesn't work: one child isn't locked out if the other one gets a LOCK_EX on the open filehandle.

Why?
In retrospect, this is easy- straight out of the perldoc for flock(), and also in `man flock`:

On systems that support a real flock(), locks are inherited across fork() calls, whereas those that must resort to the more capricious fcntl() function lose the locks, making it harder to write servers.

I fall into the first category: since the lock exists per-file, and the filehandle only has a reference to the lock, and since flock()'ing a file you already have a lock on succeeds, the children all get locks whenever they ask for them.

My solution
This is also not hard. Instead of opening the file in the parent, and flocking then appending in the children, I perform a standard locking append in the child whenever I want to append to a file:

# Warning: untested.  If this were actual code, I would "use strict;"
open (F, ">>$my_file");
flock (F, LOCK_EX) or die "no lock!";
seek (F, 0, 2); # In case someone wrote after open but before I got th
+e lock
print F $my_stuff_to_print;
close F;
[download]

Why I don't like it
You can think of my particular case as similar to log file output. Multiple children need to append lines to one or more shared files without interrupting each other. Nothing is reading the files until it's known that all writing has completed and all child processes exit. I don't care about the order of the lines as long as no line is cut in half by any other line. In practice, the output is short enough not to cause any problems, but that's not good enough for those of us who are paranoid.

Because the reason I'm forking in the first place is to get better performance, I don't like the overhead of the open, flock, close calls every time I want to print a single short line to a file. Though I admit that this is a small portion of the total time spent in each child (as each child only outputs one line to any particular file).

Therefore,

The Question
Isn't there a better way? (read: more efficient than open, lock, close every time) If so, what is it?

Thank you for your time :)

Alan

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