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.
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:


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.

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?



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