C/C++ 'assert' should not be used in R packages

On 10/11/2007 1:00 PM, Duncan Temple Lang wrote:

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Duncan Murdoch wrote:

Prof Brian Ripley wrote:

Please don't use 'assert' in R packages.  If called, this means  
that an
error in your code aborts the whole R process, including your  
user's work.
I see several R packages doing this, and one of them called  
'assert' on me
earlier in the week.

I partly disagree about this.  If assert() is triggered, it clearly
indicates a bug in the package.  If it just generated an R error,  
most
users would ignore it, and not report it to the package maintainer.

It may well be that when an assertion fails, none of the subsequent
calculations are reliable, in which case returning control to the  
user
could result in data corruption.  That's worse than losing a  
session,
because at least when you lose a session, you know it.

Could we write our own implementation of assert() that displays an R
error and unloads the package?  I think I could do something like  
that
in Windows by calling FreeLibrary to unload the DLL, but I'd  
prefer a
cross-platform solution.

I am not sure why you think we need to discard the DLL.

The package author has asserted that it is no longer safe to run.   
They are shutting down R, and Brian finds that too extreme.

What if we want to access variables and routines merely to find out
the resulting state after the assert().
And it is not clear whether you mean any "subsequent calculations"
or any "subsequent calculations using code within this DLL"
might be unreliable.

How could I know why a package author decided a shutdown is  
needed?  It
depends on the author.

But regardless of that and the assumption that users will ignore this
error, why would you want to call FreeLibrary? Since R has induced  
the
DLL to be loaded either directly or indirectly, R holds a handle  
to the
DLL. If you don't use R's cross-platform facilities for releasing the
DLL (either at the R or C-level), you will corrupt the session,
specifically in the list of assumed-live DLLs.

As I said, I'd prefer a cross-platform solution.

Duncan Murdoch

 D.

We provide 'error': please do use it to return control to the  
user when
your code misbehaves.

Similarly 'exit' and 'abort' should never be used in R packages.


Sometimes it is not under your control: I sometimes see an rgl  
failure at

R: indirect_vertex_array.c:659: emit_DrawArrays_old: Assertion
`elements_per_request >= count' failed.

that is coming from the Mesa GL libraries.

I'd say that's a bug, either in Mesa GL or in rgl.  If you can  
make it
reproducible, I'll try to track it down.

Duncan Murdoch

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