Space leak with :trace

[mnislaih]

There is some ugly space behaviour going on with :trace.

I noticed while during a long running debugging session. My program runs for about 30 seconds (under ghci) and then throws a stack overflow exception. It does run in around 64M constant space. While launched under :trace, the space usage grows constantly up to 3.5G, and at this point my system dies, never reaching the exception.

I have been able to replicate using a smaller example.

import Data.List

loop x = if x == 0 then 0 : loop 1 else 0 : loop 0

main =  print (foldl' (+) 0 (loop 1))

Under ghci 6.8.2, 1) :main runs in 30M constant space, 2) :trace main does not.

Launching ghci with a heap space limit, e.g. +RTS -M300M, makes 2) eventually crash.

[igloo]

Fixed in HEAD and 6.8:

Sun Mar 16 14:17:48 PDT 2008  Ian Lynagh <igloo@earth.li>
  * Fix a space leak in :trace (trac #2128)
  We were doing lots of cons'ing and tail'ing without forcing the tails,
  so were building up lots of thunks.