R devel - Jun 2018 - DOCUMENTATION(?): parallel::mcparallel() gives various types of "Error in unserialize(r) : ..." errors if value is of type raw

I stumbled upon the following:

f <- parallel::mcparallel(raw(0L))
parallel::mccollect(f)
# $`77083`
# NULL

but

f <- parallel::mcparallel(raw(1L))
parallel::mccollect(f)
# Error in unserialize(r) : read error
traceback()
# 2: unserialize(r)
# 1: parallel::mccollect(f)

(restarting because the above appears to corrupt the R session)

f <- parallel::mcparallel(raw(2L))
parallel::mccollect(f)
# Error in unserialize(r) : unknown input format

I can reproduce this on Linux using R 3.5.0 all the way back to R
3.3.2 (didn't try further) and R development (unstable) (2018-06-19
r74919).

Diving into the code of parallel::mccollect(), it looks that the 'raw'
data type has a special purpose:

                  r <- readChild(pid)
                  if (is.integer(r) || is.null(r))
                    fin[pid == pids] <- TRUE
                  if (is.raw(r))
                    res[which(pid == pids)] <- list(unserialize(r))

However, I don't see this behavior document in ?parallel::mcparallel.
The closest I get is that under the 'Value' section it says:

   "mccollect returns any results that are available in a list. [...]"

Not sure if that should be interpreted as only values that are of type
'list' should be used.  OTH, the example use values of type double.

/Henrik

Thanks for the report, fixed in documentation of mcparallel. Raw vectors 
are reserved for internal use and cannot be returned by expressions 
passed to mcparallel. But one can indeed wrap a raw vector into a list.

FYI this internal use is not in readChild/mccollect, but in 
sendMaster/mcparallel.

Best
Tomas

On 06/21/2018 05:05 AM, Henrik Bengtsson wrote:
> I stumbled upon the following:
>
> f <- parallel::mcparallel(raw(0L))
> parallel::mccollect(f)
> # $`77083`
> # NULL
>
> but
>
> f <- parallel::mcparallel(raw(1L))
> parallel::mccollect(f)
> # Error in unserialize(r) : read error
> traceback()
> # 2: unserialize(r)
> # 1: parallel::mccollect(f)
>
> (restarting because the above appears to corrupt the R session)
>
> f <- parallel::mcparallel(raw(2L))
> parallel::mccollect(f)
> # Error in unserialize(r) : unknown input format
>
> I can reproduce this on Linux using R 3.5.0 all the way back to R
> 3.3.2 (didn't try further) and R development (unstable) (2018-06-19
> r74919).
>
> Diving into the code of parallel::mccollect(), it looks that the
'raw'
> data type has a special purpose:
>
>                    r <- readChild(pid)
>                    if (is.integer(r) || is.null(r))
>                      fin[pid == pids] <- TRUE
>                    if (is.raw(r))
>                      res[which(pid == pids)] <- list(unserialize(r))
>
> However, I don't see this behavior document in ?parallel::mcparallel.
> The closest I get is that under the 'Value' section it says:
>
>     "mccollect returns any results that are available in a list.
[...]"
>
> Not sure if that should be interpreted as only values that are of type
> 'list' should be used.  OTH, the example use values of type double.
>
> /Henrik
>
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