Issue 3231 in pharo: Semaphore changes

Status: FixedWaitingToBePharoed
Owner: stephane.ducasse
CC: siguctua
Labels: Type-Squeak

New issue 3231 by stephane.ducasse: Semaphore changes
http://code.google.com/p/pharo/issues/detail?id=3231

Needs a really careful review....


A new version of Kernel was added to project The Inbox:
http://source.squeak.org/inbox/Kernel-ul.514.mcz

==================== Summary ====================

Name: Kernel-ul.514
Author: ul
Time: 7 November 2010, 4:21:51.529 pm
UUID: 5710b62a-4264-9947-acdc-11e0ce45021d
Ancestors: Kernel-ul.513

A few changes to Semaphore:
- save a block creation + activation + a possible context switch in  
#critical:ifCurtailed:
- save a block creation + activation in #critical:ifError:, this changes  
the behavior when mutuallyExcludedBlock doesn't understand #ifError:
- #critical:ifLocked: is guaranteed to not get locked. The earlier  
implementation sent #critical: which could cause a context change, so  
another process could enter the critical section before the original.
- added a new method #passIfLocked: which is like #wait, but it doesn't  
wait for a signal if there's none, but evaluates the argument instead.

=============== Diff against Kernel-ul.513 ===============

Item was changed:
  ----- Method: Semaphore>>critical:ifCurtailed: (in category 'mutual  
exclusion') -----
  critical: mutuallyExcludedBlock ifCurtailed: terminationBlock
        "Evaluate mutuallyExcludedBlock only if the receiver is not  
currently in
        the process of running the critical: message. If the receiver is,  
evaluate
        mutuallyExcludedBlock after the other critical: message is finished."
+       ^self critical: [ mutuallyExcludedBlock ifCurtailed:  
terminationBlock ]
-       ^self critical:[[mutuallyExcludedBlock value] ifCurtailed:  
terminationBlock]
  !

Item was changed:
  ----- Method: Semaphore>>critical:ifError: (in category 'mutual  
exclusion') -----
  critical: mutuallyExcludedBlock ifError: errorBlock
        "Evaluate mutuallyExcludedBlock only if the receiver is not  
currently in
        the process of running the critical: message. If the receiver is,  
evaluate
        mutuallyExcludedBlock after the other critical: message is finished."
        | blockValue hasError errMsg errRcvr |
        hasError := false.
        blockValue := self critical:[
+               mutuallyExcludedBlock ifError: [ :msg :rcvr |
-               [mutuallyExcludedBlock value] ifError:[:msg :rcvr|
                        hasError := true.
                        errMsg := msg.
                        errRcvr := rcvr
                ].
        ].
        hasError ifTrue:[ ^errorBlock value: errMsg value: errRcvr].
        ^blockValue!

Item was changed:
  ----- Method: Semaphore>>critical:ifLocked: (in category 'mutual  
exclusion') -----
  critical: mutuallyExcludedBlock ifLocked: alternativeBlock
        "Evaluate mutuallyExcludedBlock only if the receiver is not  
currently in
        the process of running the critical: message. If the receiver is,  
evaluate
        mutuallyExcludedBlock after the other critical: message is finished."
-
        "Note: The following is tricky and depends on the fact that the VM  
will not switch between processes while executing byte codes (process  
switches happen only in real sends). The following test is written  
carefully so that it will result in bytecodes only."
+
+       | caught |
+       caught := false.
+       ^[
+               excessSignals = 0
+                       ifTrue: [ alternativeBlock value ]
+                       ifFalse: [
+                               excessSignals := excessSignals - 1.
+                               caught := true.
+                               mutuallyExcludedBlock value ] ]
+               ensure: [ caught ifTrue: [ self signal ] ]!
-       excessSignals == 0 ifTrue:[
-               "If we come here, then the semaphore was locked when the  
test executed.
-               Evaluate the alternative block and answer its result."
-               ^alternativeBlock value
-       ].
-       ^self critical: mutuallyExcludedBlock!

Item was added:
+ ----- Method: Semaphore>>passIfLocked: (in category 'communication') -----
+ passIfLocked: aBlock
+
+       excessSignals = 0 ifTrue: [ ^aBlock value ].
+       excessSignals := excessSignals - 1!

Comment #1 on issue 3231 by gazzaguru2: Semaphore changes
http://code.google.com/p/pharo/issues/detail?id=3231

We can check perhaps (on old Squeak 3.9) with our rather process/semaphore  
intensive exchange software...