Nice case - I think you're onto something. /Henrik
2009/6/2 <hpa...@fhcrc.org>:
> In fact reg.finalizer() looks like a dangerous feature.
>
> If the finalizer itself triggers (implicitely or
> explicitely) garbage collection, then bad things happen.
> In the following example, garbage collection is triggered
> explicitely (using R-2.9.0):
>
> setClass("B", representation(bb="environment"))
>
> newB <- function()
> {
> ans <- new("B", bb=new.env())
> reg.finalizer(a...@bb,
> function(e)
> {
> gc()
> cat("cleaning", class(ans), "object...\n")
> }
> )
> return(ans)
> }
>
> > for (i in 1:500) {cat(i, "\n"); b1 <- newB()}
> 1
> 2
> 3
> 4
> 5
> 6
> ...
> 13
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> 14
> ...
> 169
> 170
> 171
> Error: not a weak reference
> Error: not a weak reference
> [repeat the above line thousands of times]
> ...
> Error: not a weak reference
> Error: not a weak reference
> cleaning B object...
> Error: SET_VECTOR_ELT() can only be applied to a 'list', not a 'integer'
> Error: SET_VECTOR_ELT() can only be applied to a 'list', not a 'integer'
> [repeat the above line thousands of times]
> ...
> Error: SET_VECTOR_ELT() can only be applied to a 'list', not a 'integer'
> Error: SET_VECTOR_ELT() can only be applied to a 'list', not a 'integer'
> 172
> ...
> 246
> 247
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
> cleaning B object...
>
> *** caught segfault ***
> address 0x41, cause 'memory not mapped'
>
> Traceback:
> 1: gc()
> 2: function (e) { gc() cat("cleaning", class(ans),
> "object...\n")}(<environment>)
>
> Possible actions:
> 1: abort (with core dump, if enabled)
> 2: normal R exit
> 3: exit R without saving workspace
> 4: exit R saving workspace
> Selection: 2
> Save workspace image? [y/n/c]: n
> Segmentation fault
>
> So apparently, if the finalizer triggers garbage collection,
> then we can end up with a corrupted session. Then anything can
> happen, from the strange 'formal argument "envir" matched by
> multiple actual arguments' error I reported in the previous post,
> to a segfault. In the worse case, nothing apparently happens but
> the output produced by the code is wrong.
>
> Maybe garbage collection requests should be ignored during the
> execution of the finalizer? (and more generally during garbbage
> collection itself)
>
> Cheers,
> H.
>
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>
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