[GemStone-Smalltalk] objectForOop: question about some errors we get + how to implement a weak reference?

[Johan Brichau]

Hi,

We use the objectForOop: method to implement weak references but are hitting some errors while using it. 
I want to better understand what we are seeing.
The following is in Gemstone 3.4.5

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.

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.

The trouble is we sometimes encounter an error after an MFC cycle has concluded.
The _objectForOop: method throws a ‘primitive failed’ that the object with object ID xxxx does not exist. See stack trace below.

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.
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?
We tried to counter the problem of retrieving dead objects by using _oopIsDead: but only do so _after_ invoking objectForOop:. 
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.

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.
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?

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? ;-)

Thanks for any thoughts!
Johan

----------- 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]
…

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