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redsock



Joined: 09 Oct 2009
Posts: 357
Location: Australia
redsock
Hey all, thought I'd share with you guys the post I did earlier this morning, haha, mainly because it was so interesting (IMO anyway).

The short and sweet: Got a bug report on rwasa the other day, and if I looked at it with strace, it disappeared! hahah

Fun stuff: https://2ton.com.au/rants_and_musings/rwasa_hhvm.html

Smile

Cheers
Post 12 Jun 2015, 02:57
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17247
Location: In your JS exploiting you and your system
revolution
Before I even read the description I thought this might have something to do with timing. Any time we use a tool to examine something we change things in subtle, and not so subtle, ways. And of all the things that can go wrong timing bugs can be one of the hardest to reproduce and monitor. Even dedicated researchers of OS/threading design have a lot of difficulty guaranteeing no deadlocks.
Post 12 Jun 2015, 04:20
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redsock



Joined: 09 Oct 2009
Posts: 357
Location: Australia
redsock
Yep, still chuckling about this one... we all know that these types of things "lurk in the shadows", but to have such a clear-cut example of inspection tools themselves causing unexpected/totally different results (especially completely unrelated so-called "system inspection tools" like strace) was pretty neat IMO.

Would be an excellent exercise for learning/investigating and proving said point nicely to the young brains Smile
Post 12 Jun 2015, 04:54
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JohnFound



Joined: 16 Jun 2003
Posts: 3500
Location: Bulgaria
JohnFound
It is known type of bugs, named Heisenbug. Very painful for debugging.
Post 12 Jun 2015, 04:58
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redsock



Joined: 09 Oct 2009
Posts: 357
Location: Australia
redsock
JohnFound wrote:
It is known type of bugs, named Heisenbug. Very painful for debugging.
Hey thanks for that! I didn't know it had a proper name, hahaha, this quote from the wikipedia page sums it up nicely:
Wikipedia wrote:
Time can also be a factor in heisenbugs, particularly with multi-threaded applications. Executing a program under control of a debugger can change the execution timing of the program as compared to normal execution.
Though I am not sure "strace" qualifies as a debugger normally... hmm, will ponder that some more.

Maybe I should update my post to include said fancy term for it Smile
Post 12 Jun 2015, 05:04
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redsock



Joined: 09 Oct 2009
Posts: 357
Location: Australia
redsock
JohnFound wrote:
It is known type of bugs, named Heisenbug. Very painful for debugging.
Updated at the bottom of my original page, hope you don't mind my including your name Smile thx again for teaching me a new word.
Post 12 Jun 2015, 05:22
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JohnFound



Joined: 16 Jun 2003
Posts: 3500
Location: Bulgaria
JohnFound
redsock wrote:
thx again for teaching me a new word.


You are welcome. Smile

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Post 12 Jun 2015, 06:21
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1408
Location: Toronto, Canada
AsmGuru62
We had multi-threaded bugs when system is ran on a PC with some more apps running in the background.
These were a PITA to debug!..
Post 12 Jun 2015, 14:05
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