2010/3/2 Levente Uzonyi <leves@elte.hu>:
On Tue, 2 Mar 2010, Henrik Sperre Johansen wrote:
PS. For the not-so-faint-of-heart, open a Transcript and try evaluating: |proc| proc := [[Transcript show: 'Start!'. ] ensure: [Transcript show: 'Finish!' Processor yield.]] newProcess. proc resume. Processor yield. proc suspend. proc terminate.
Why don't we get it printing?
Forgot a . there, supposed to be
|proc| proc := [[Transcript show: 'Start!'. ] ensure: [Transcript show: 'Finish!'. Processor yield.]] newProcess. proc resume. Processor yield. proc suspend. proc terminate.
on my machine it prints: *Start!Start!Finish!
The problem occurs because Transcript >> #endEntry (sent from #show:) takes a while to execute, so the process (proc) prints 'Start!', but it gets terminated before execution reaches #resetContents (#reset in Squeak). So 'Start!' is still in the stream. Then our process executes the #ensure: block and it prints 'Start!' and 'Finish!' too.
There's worse problem with #ensure: and #terminate is that, a process executing an #ensure: block can be terminated. If you evaluate this code:
| process stage1 stage2 stage3 counter | stage1 := stage2 := stage3 := false. counter := 0. process := [ Â Â Â Â [ stage1 := true ] ensure: [ Â Â Â Â Â Â Â Â stage2 := true. Â Â Â Â Â Â Â Â 1000000 timesRepeat: [ counter := counter + 1 ]. Â Â Â Â Â Â Â Â stage3 := true ] ] newProcess. process resume. Processor yield. process suspend. process terminate. 1000 milliSeconds asDelay wait. { stage1. stage2. stage3. counter } explore
you will find that stage1 and stage2 is reached as expected, but stage3 is not and counter is less than 1000000. That's because the forked process started evaluating the #ensure: block, but it was terminated by our process.
Is this the expected behavior when sending #terminate to a process which is evaluating an #ensure: block?
What you think is expected behavior here: a) process termination should start (be triggered) only when process is outside of any #ensure: closure? b) #ensure: block should always run to the end, despite anything? Since you can't predict, what code will run inside ensure block, the only guarantee that you having is actually that your process will always enters such blocks during the normal flow, or exception handling. But there is no guarantees that it will be able to run the code inside it, when you terminating it.
Cheers, Balázs & Levente
*yay Henry* *
-- Best regards, Igor Stasenko AKA sig.