Windows/R issue with FFI

[Dominick Samperi]

Thanks Ben, I'll see what I can do to reliably reproduce and open a ticket.

One theory I'm investigating is that this might have something to do
with my anti-virus software (AVG), since it sometimes interacts with
Windows in strange ways (for example, an extra instance of a terminal app
pops up, then disappears after a few seconds). But disabling this software
does not seem to solve the problem.

On Sat, Mar 25, 2023 at 11:18 PM Ben Gamari <ben at smart-cactus.org> wrote:

> This sounds like a bug. Could you open a ticket, ideally with a fairly
> standalone reproducer?
>
> Cheer,
>
> - Ben
>
> On March 25, 2023 6:49:09 PM EDT, Dominick Samperi <djsamperi at gmail.com>
> wrote:
>>
>> Hello,
>> FFI code that used to work now fails under Windows (still seems to work
>> under Ubuntu), and I wonder if anybody has seen anything like this and
>> can provide some pointers...
>>
>> The code uses FFI to fetch information from the R side like R_NilValue,
>> using something like this;
>>
>> -- Fetch R's R_NilValue...
>> foreign import ccall unsafe "&R_NilValue" r_NilValue_ptr :: Ptr R_EXP
>> r_NilValue :: IO R_EXP
>> r_NilValue = peek r_NilValue_ptr
>> rNilValue1 :: IO REXP
>> rNilValue1 = do
>>     x <- r_NilValue
>>     traceShow("addr=",x) extREXP x
>>
>> Under Windows the address displayed is obviously bad, and this causes
>> the app to crash. This does not happen under Linux (Ubuntu).
>>
>> Now, replace the line containing peek with
>>
>> r_NilValue = trace "PEEK" peek r_NilValue_ptr
>>
>> The address is now valid! It seems that adding the trace "PEEK" adds
>> some delay and somehow resolves the problem.
>>
>> This problem is intermittent, so it is hard to come up with a
>> simple example that fails every time.
>>
>> A little background: R_NilValue is a pointer to a SEXP that is not
>> initialized until an embedded instance of R is initialized, and the
>> code above is not triggered until this happens. Perhaps there is
>> a race condition between the time R initializes itself and Haskell
>> performs the peek? I don't think R_NilValue is garbage collected
>> once initialized.
>>
>> Any tips would be appreciated.
>> Dominick
>>
>>
>>
>>
>>
>>
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