[Vm-dev] Termination semaphore for Process

Max Leske maxleske at gmail.com
Fri Feb 12 14:59:37 UTC 2016

Hi all,

I have a proposal and I’d like to hear your opinion.

Right now, when I want to wait for a process to finish I use something like #forkAndWait or I write a block with semaphores myself. This works fine for processes that I spawn and then immediately want to wait for. But what if I spawn a process somewhere else and only later know that I want to wait for its termination? Currently I would construct a polling loop to do that.

Here’s a different idea (which I’ve tested):

1. add instance variable “terminationSemaphore” to Process
2. at the end of Process>>terminate #ensure: that all waiting processes are signaled
3. add #waitForTermination message to Process:

	terminationSemaphore ifNil: [ terminationSemaphore := Semaphore new ].
	self resume.
	terminationSemaphore wait

This enables code like the following:

result := 0.
p := [ result := 2. 2 seconds asDelay wait ] newProcess
	priority: 20;

“don’t run the new process yet"
[ p waitForTermination. result2 := 3. ] forkAt: 30.
“p is now running, the second process is waiting"

p waitForTermination.
“p is still running, two processes are waiting”

“because processes waiting on a semaphore are signaled in the order
they sent #wait we even achieve ordering (at least with equal process
priorities) and so ‘result2’ is not nil but 3”

{result. result2} “#(2 3)

I wrote this in 10 minutes, so there’s room for polishing. I just want to know if this is something other people might want and if there are potential problems with process termination and semaphores which I am not aware of.


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