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<div class="moz-cite-prefix">Hi Johan,<br>
<br>
This does sound very much like our internal issue 50024. It was
just a couple of months ago that I first suspected such a scenario
was possible, and just a couple of weeks ago that I filed the
issue. I'm very sorry to hear that it is not just theoretical.<br>
<br>
From what you've said, what may be happening is a sequence like
this:<br>
1) MFC completes, and voting starts<br>
2) Gem A votes<br>
3) Gem A retrieves disconnected object B with objectForOop:.<br>
4) Object B is promoted to dead, since it was disconnected and not
voted down by any session. Object C referenced by object B is also
promoted to dead at the same time.<br>
5) The reclaim gem commits the removal of object C (and possibly
also object B) from the object table.<br>
6) Gem A aborts or commits. Object C no longer exists in Gem A's
view of the object table. However, object B still exists in Gem
A's private memory, but object C does not.<br>
7) Gem A once again retrieves object B with objectForOop:. This
succeeds because it is still Gem A's private memory.<br>
8) Gem A sends a message to object B. <br>
9) The resulting Smalltalk execution retrieves an instance
variable of object B that refers to object C. Object C does not
exist in Gem A's private memory or in Gem A's view of the object
table, so the result is an ObjDoesNotExistError.<br>
<br>
This specific scenario can be avoided by sending _oopIsDead:
before sending objectForOop: in step 7, and ending the sequence
there if the object has been promoted to dead. Sending _oopIsDead:
only after _objectForOop: answers nil may not help -- I haven't
tested this but I think that in the case where the object is still
in the gem's memory the _objectForOop: will succeed even if the
object is dead. As is documented, _oopIsDead: is slower than
objectForOop: since it must consult the stone. Whether this will
noticeably affect your application depends on how often you are
executing objectForOop:. Solutions that we're considering will
also have a similar slowdown for objectForOop:, though probably
only during the period between the end of MFC and end of reclaim.<br>
<br>
However, sending _oopIsDead: before each objectForOop: does not
make it *entirely* safe. You could leave step 7 out of the
scenario above and still have the same outcome. At step 3,
_oopIsDead: would answer false, and the objectForOop: would
succeed. If that reference to object B was stored in a transient
object or a stack temporary variable that is used to send the
message in step 8, you could still get the ObjDoesNotExist error.<br>
To avoid this, you could, besides sending _oopIsDead: before each
objectForOop:, ensure that no references to an object retrieved
through objectForOop: are retained on the stack or in a transient
object across a commit or abort.<br>
<br>
I hope this helps you reach a solution that works in your
application. We're still discussing ways to fix this in future
GemStone versions.<br>
<br>
Regards,<br>
-Martin<br>
<br>
On 7/22/22 05:29, Johan Brichau via GemStone-Smalltalk wrote:<br>
</div>
<blockquote type="cite"
cite="mid:86A8C450-0E6E-4150-869D-EFD2EE2B21FF@yesplan.be">
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Hi,
<div class=""><br class="">
</div>
<div class="">We use the objectForOop: method to implement weak
references but are hitting some errors while using it. </div>
<div class="">I want to better understand what we are seeing.</div>
<div class="">The following is in Gemstone 3.4.5</div>
<div class=""><br class="">
</div>
<div class="">First off: we understand the dangers mentioned in
the method's comment, well… to some extent otherwise I would not
ask this question:-). To counter trouble with oop reuse, we also
store the object creation time together with the oop (and in the
referenced object). A check on class and creation timestamp
allows us to guarantee when we get an object back from
objectForOop: that it was the one we originally referenced or
not.</div>
<div class=""><br class="">
</div>
<div class="">The reason we use this concept is to implement weak
references. We store log items in the database which reference a
series of objects. The log item will stay in the database for a
very long time but should not keep the referenced objects from
being garbage collected. Hence, we store the oop (+ the creation
timestamp, as mentioned) instead of the object in the log items,
which would otherwise keep these from being collected in an MFC.</div>
<div class=""><br class="">
</div>
<div class="">The trouble is we sometimes encounter an error after
an MFC cycle has concluded.</div>
<div class="">The _objectForOop: method throws a ‘primitive
failed’ that the object with object ID xxxx does not exist. See
stack trace below.</div>
<div class=""><br class="">
</div>
<div class="">What is interesting is that the oop in the error
message is _not_ the argument passed to _objectForOop:. Instead,
it is the oop of an object held in an instVar of the object we
try to retrieve.</div>
<div class="">It is an instance of DateAndTime. Thus we seem to
retrieve a dead object that can still be retrieved but of which
the objects referenced by it are already collected and that this
errors the primitive?</div>
<div class="">We tried to counter the problem of retrieving dead
objects by using _oopIsDead: but only do so _after_ invoking
objectForOop:. </div>
<div class="">Perhaps the straightforward solution is to first
check _oopIsDead: and only then use _objectForOop: ? For now, we
have wrapped it with an exception handler to handle the error.</div>
<div class=""><br class="">
</div>
<div class="">Next, we saw this problem in Gemstone 2.4.4.1 as
well but there it would crash the entire gem. Since our upgrade
to GemStone 3.4.5, this error no longer crashes the gem and is
captured by the error handler.</div>
<div class="">This, however, has now shown that because the gem
continues working, the problem may occur even many hours after
the MFC cycle. Is this to be expected? I understand how in
between a collect and a reclaim, this may happen but not how it
can still occur many hours later?</div>
<div class=""><br class="">
</div>
<div class="">And finally… we are aware this is all rather
hacking. Now that we finally are moving up towards the latest
version of GemStone: are there better ways to implement weak
references in GemStone? ;-)</div>
<div class=""><br class="">
</div>
<div class="">Thanks for any thoughts!</div>
<div class="">Johan</div>
<div class=""><br class="">
</div>
<div class=""><span style="color: rgb(68, 68, 68); font-family: Consolas, "Liberation Mono", Courier, monospace, EmojiFontFace; font-variant-ligatures: normal; orphans: 2; white-space: pre; widows: 2; background-color: rgb(248, 248, 248); text-decoration-thickness: initial;" class="">----------- Continuation saved to object log ERROR Encountered: 2022-07-22T07:29:21.16192007064819+02:00
a InternalError occurred (error 2101), The object with object ID 5754897665 does not exist.
1 GRGemStonePlatform >> logError:title:shouldCommit: @3 line 4 [GsNMethod 551003905]
2 GRGemStonePlatform >> logError:title: @2 line 3 [GsNMethod 550999041]
3 NPGemStoneErrorHandler (WAErrorHandler) >> saveExceptionContinuation: @9 line 6 [GsNMethod 603180545]
4 NPGemStoneErrorHandler >> handleDefault: @6 line 7 [GsNMethod 632646401]
5 NPGemStoneErrorHandler (WAErrorHandler) >> handleError: @2 line 2 [GsNMethod 603181313]
6 NPGemStoneErrorHandler (WAErrorHandler) >> handleGemStoneException: @5 line 4 [GsNMethod 603182081]
7 NPGemStoneErrorHandler (WAHtmlHaltAndErrorHandler) >> handleException: @2 line 2 [GsNMethod 613492481]
8 NPGemStoneErrorHandler >> handleException: @6 line 5 [GsNMethod 632646145]
9 [] in WAExceptionHandler >> handleExceptionsDuring: @11 line 5 [GsNMethod 1320949761]
10 ExecBlock0 (ExecBlock) >> on:do: @3 line 44 [GsNMethod 53734657]
11 [] in WAExceptionHandler >> handleExceptionsDuring: @7 line 8 [GsNMethod 968955649]
12 [] in ExecBlock >> on:do: @16 line 53 [GsNMethod 65134849]
13 InternalError (AbstractException) >> _executeHandler: @8 line 11 [GsNMethod 61046017]
14 InternalError (AbstractException) >> _signalFromPrimitive: @1 line 1 [GsNMethod 61048321]
15 NPLogEntry >> privateObjectAtUniqueIdentifier:in:ifAbsent: @1 line 1 [GsNMethod 651626241]
16 NPLogEntry >> originalObjectInDB:ifAbsent: @3 line 3 [GsNMethod 651630337]
17 NPLogEntry >> originalObjectInDB: @2 line 2 [GsNMethod 651613953]
18 NPLogEntry >> concernedObjectsInDB: @5 line 6 [GsNMethod 651637249]
19 NPLoggingManagerForGemStone (NPLoggingManager) >> isCurrentUserFollowingEntry: @3 line 3 [GsNMethod 623065857]
…</span></div>
<div class=""><span style="color: rgb(68, 68, 68); font-family: Consolas, "Liberation Mono", Courier, monospace, EmojiFontFace; font-variant-ligatures: normal; orphans: 2; white-space: pre; widows: 2; background-color: rgb(248, 248, 248); text-decoration-thickness: initial;" class="">
</span></div>
<br>
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