[MacRuby-devel] Return from thread context

[Perry Smith]

Hi Laurent,

You used the term "block" below.  I don't know enough about the code  
yet but I would assume that "block" means either a basic block or  
something between curly braces.  Those entities do not have "ensure"  
-- (oh, gee, you used the term block here too...)  Anyway.  My  
thought / suggestions / question is perhaps making the things between  
curly braces simple and the things between "begin" and "end" (which  
can have ensure statements) more complex.  The point is that curly  
braces come up much more often and keeping them simple and fast would  
be nice.

I'm also worried that the use of exceptions to implement often used  
paths like "return" may be very expensive.  Depending upon the  
implementation, exceptions can cost 0 for the non-exception case but  
the processing of an actually exception is very expensive.  It would  
be nice if that cost would only be incurred in very rare cases.

On Jul 9, 2009, at 2:50 PM, Laurent Sansonetti wrote:

> I was planning to rewrite return-from-a-block today (or maybe  
> tomorrow). The current SjLj-based implementation is not correct  
> because it does not call ensure blocks, so I was thinking of using a  
> specialized C++ exception instead, which should do the trick.
>
> # This was a long-standing issue but yesterday night I found a use  
> case where it's problematic: Mutex#synchronize won't unlock if  
> return is called from it.
>
> Normally with the new implementation we should be able to catch the  
> specialized exception inside rb_vm_thread_run() and appropriately  
> raise a LocalJumpError.
>
> Thanks for your preliminary investigation on this!
>
> Laurent
>
> On Jul 9, 2009, at 12:43 PM, Perry Smith wrote:
>
>>
>> The file is:
>> ./spec/frozen/language/return_spec.rb
>>
>> I don't know how to describe which one other than the first  
>> describe that starts "in a Thread"
>>
>> Currently it does:
>>
>> The return keyword in a Thread
>> - raises a LocalJumpError if used to exit a threadSEGV recieved in  
>> SEGV handler
>> unknown: [BUG] Segmentation fault
>>
>> Perry
>> Ease Software, Inc. ( http://www.easesoftware.com )
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>>
>> On Jul 9, 2009, at 2:35 PM, Laurent Sansonetti wrote:
>>
>>> Hi Perry,
>>>
>>> Which spec are you talking about specifically?
>>>
>>> Laurent
>>>
>>> On Jul 9, 2009, at 5:56 AM, Perry Smith wrote:
>>>
>>>> The spec says that a 'return' from a thread should raise a  
>>>> LocalJumpError.
>>>>
>>>> Looking at the code for RETURN_NODE in compiler.cpp, the question  
>>>> of "Is this a thread" is never asked.  And, I guess this  
>>>> exception is only for the top level block of the thread since a  
>>>> function called from the block could do a return.
>>>>
>>>> I looked briefly at the Thread create process and I didn't see  
>>>> anything that flagged the block passed as the top level block for  
>>>> the thread.
>>>>
>>>> The testcase causes a segmentation fault.  I looked at the signal  
>>>> handler too and it appears as if it is "lets do this for now and  
>>>> address it later" code.  The SEGV fault handler simply sets a  
>>>> flag and returns.  The return will put us back where we were just  
>>>> at so we immediately seg fault again and then we do an exit --  
>>>> with no core file.
>>>>
>>>> I don't see the value of catching SEGV in the first place unless  
>>>> the underlying Ruby code has asked us to do that.  I think that  
>>>> would generally apply to all signals.
>>>>
>>>> Perry
>>>>
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