[webkit-dev] Proposal to limit the size of the captured exception stack

Brian Burg bburg at apple.com
Thu Mar 16 22:25:07 PDT 2017

> On Mar 16, 2017, at 10:09 PM, Mark Lam <mark.lam at apple.com> wrote:
> Hi folks,
> Currently, if we have an exception stack that is incredibly deep (especially for a StackOverflowError), JSC may end up thrashing memory just to capture the large stack trace in memory.    This is bad for many reasons:
> 1. the captured stack will take a lot of memory.

How much? What would be the new footprint with your suggestion below?

> 2. capturing the stack may take a long time (due to memory thrashing) and makes for a bad user experience.
> 3. if memory availability is low, capturing such a large stack may result in an OutOfMemoryError being thrown in its place.
>    The OutOfMemoryError thrown there will also have the same problem with capturing such a large stack.
> 4. most of the time, no one will look at the captured Error.stack anyway.
> Since there isn’t a standard on what we really need to capture for Error.stack, I propose that we limit how much stack we capture to a practical size.  How about an Error.stack that consists of (1) the top N frames, (2) an ellipses, and (3) the bottom M frames?  If the number of frames on the stack at the time of capture  is less or equal to than N + M frames, then Error.stack will just show the whole stack with no ellipses.  For example, if N is 4 and M is 2, the captured stack will look something like this:
>      foo10001
>      foo10000
>      foo9999
>      foo9998
>>      foo1
>      foo0
> If we pick a sufficient large number for N and M (I suggest 100 each), I think this should provide sufficient context for debugging uses of Error.stack, while keeping an upper bound on how much memory and time we throw at capturing the exception stack.

Web Inspector’s interface wouldn’t look too good with such a large call stack anyway, so this doesn’t degrade the current experience IMO. Maybe even 50+50 would be fine. I’ve never seen a call stack that large in JS code (but then again, I don’t use fancy frameworks).

> My plan for implementing this is:
> 1. change Exception::finishCreation() to only capture the N and M frames, plus possibly 1 ellipses placeholder in the between them.
> 2. change all clients of Exception::stack() to be able to recognize and render the ellipses.
> Does anyone object to doing this or have a compelling reason why this should not be done?
> Thanks.
> Mark
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