Interesting, your example was subtle enough that I had to read it a few times 
to understand the issue. (I was caught up on the ifNil and not the contents).

Actually, thinking about it - isn't the real issue the #ifNil - you want an 
atomic version.

It strikes me you could wrap the whole concept into something like an 
AtomicLazyVariable? E.g.

   fileTypes := AtomicLazyVariable using: [
       Dictionary new
           at: 'txt' put: 'Text File';
           at: 'html' put: 'Web Page';
           yourself ].

Then have something

     ^fileTypes value

Where you have a critical section in #value?


Sent from my iPhone

> On 11 Aug 2017, at 07:53, monty <> wrote:
> Here's a hypothetical broken class method that does lazy initialization of a 
> class inst var:
> fileTypes
>    fileTypes ifNil: [
>        fileTypes := Dictionary new.
>        fileTypes
>            at: 'txt' put: 'Text File';
>            at: 'html' put: 'Web Page';
>            at: 'pdf' put: 'Portable Document Format File';
>            at: 'doc' put: 'Microsoft Word Document'].
>    ^ fileTypes.
> Because the assignment is done first and the initialization is done after 
> with a cascade of interruptable sends of #at:put:, there's a window after the 
> assignment where 'fileTypes' is not nil but also not fully initialized--a 
> race condition.
> The fix is simple. Do the initialization before the atomic assignment takes 
> place, so the var is only ever bound to nil or a fully initialized object:
> fileTypes
>    fileTypes ifNil: [
>        fileTypes :=
>            Dictionary new
>                at: 'txt' put: 'Text File';
>                at: 'html' put: 'Web Page';
>                at: 'pdf' put: 'Portable Document Format File';
>                at: 'doc' put: 'Microsoft Word Document';
>                yourself].
>    ^ fileTypes.
> The fixed code is still vulnerable to duplicate initialization, because the 
> initialization sequence is interruptable and 'fileTypes' is nil during it, 
> but as long as the initialization is cheap enough, has no side effects that 
> restrict how often it can be done, and it's enough that the initialized 
> objects are equal (but not identical), that's OK.
> If it's too complex for a single statement, you can use a temp vars or put it 
> in a separate factory method:
> fileTypes
>    fileTypes ifNil: [
>        fileTypes := self newFileTypes].
>    ^ fileTypes.
> Similar precautions (given how easy) might as well be taken with explicit 
> initialization of class state too. Of course if the object is mutated later 
> (in other methods), then Mutexes or other constructs are needed to guard 
> access. But for immutable class state, ensuring initialization is done before 
> assignment should be enough.
>> Sent: Tuesday, August 01, 2017 at 7:36 AM
>> From: "Stephane Ducasse" <>
>> To: "Any question about pharo is welcome" <>
>> Subject: Re: [Pharo-users] Threads safety in Pharo
>> I would love to have an analysis of assumptions made in some code.
>> Because my impression is that the concurrent code is sometimes defined
>> knowing the underlying logic of scheduler and this is not good.
>> As I said to abdel privately in french it would be great to start from
>> my french squeak book (Yes I wrote one long time ago) chapter on
>> concurrent programming and turn it into a pharo chapter.
>> Stef
>>> On Tue, Aug 1, 2017 at 1:31 PM, Ben Coman <> wrote:
>>> Not sure I'll have what you're looking for, but to start, do you mean
>>> Pharo's green threads or vm native threads?
>>> cheers -ben
>>> On Mon, Jul 31, 2017 at 7:38 AM, Alidra Abdelghani via Pharo-users
>>> <> wrote:
>>>> ---------- Forwarded message ----------
>>>> From: Alidra Abdelghani <>
>>>> To:
>>>> Cc: "St├ęphane Ducasse" <>, farid arfi
>>>> <>
>>>> Bcc:
>>>> Date: Mon, 31 Jul 2017 01:38:58 +0200
>>>> Subject: Threads safety in Pharo
>>>> Hi,
>>>> Somebody once evoked the problem of threads safety in Pharo. With a friend
>>>> of mine who is expert in formal methods and process scheduling, we would
>>>> like to have a look on it.
>>>> Does anyone knows a good document describing the problem of Pharo with
>>>> threads safety or at least any document that we can start with?
>>>> Thanks in advance,
>>>> Abdelghani

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