segfault when comparing a `NULL` column

[HughParsonage]

I experience a segfault when running the below code on Windows, Mac, and Linux. While I've used the unexported data.table:::null.data.table for the minimal reprex, I first encountered the crash when testing a package which Imports data.table.

library(data.table)
# data.table 1.10.5 IN DEVELOPMENT built 2017-08-15 16:41:46 UTC; appveyor
# The fastest way to learn (by data.table authors): https://www.datacamp.com/courses/data-analysis-the-data-table-way
# Documentation: ?data.table, example(data.table) and browseVignettes("data.table")
# Release notes, videos and slides: http://r-datatable.com
# Warning message:
#   package ‘data.table’ was built under R version 3.4.1 
sessionInfo()
# R version 3.4.0 (2017-04-21)
# Platform: x86_64-w64-mingw32/x64 (64-bit)
# Running under: Windows >= 8 x64 (build 9200)
# 
# Matrix products: default
# 
# locale:
#   [1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252    LC_MONETARY=English_Australia.1252
# [4] LC_NUMERIC=C                       LC_TIME=English_Australia.1252    
# 
# attached base packages:
#   [1] stats     graphics  grDevices utils     datasets  methods   base     
# 
# other attached packages:
#   [1] data.table_1.10.5    RevoUtilsMath_10.0.0
# 
# loaded via a namespace (and not attached):
#   [1] compiler_3.4.0   RevoUtils_10.0.4 tools_3.4.0 

x <- NULL
DT <- data.table:::null.data.table()
DT2 <- DT[, .(x)]
DT2[x > 1]

*** caught segfault ***
address (nil), cause 'memory not mapped'
Segmentation fault (core dumped)

[HughParsonage]

No longer causing a seg.fault.

[st-pasha]

Trying on Mac with the DT freshly pulled from master (on R 3.4.2):

> require(data.table)
Loading required package: data.table
data.table 1.11.5 IN DEVELOPMENT built 2018-08-11 01:45:12 UTC; pasha  Latest news: r-datatable.com
> x <- NULL
> DT <- data.table:::null.data.table()
> DT2 <- DT[, .(x)]
> DT2[x > 1]

 *** caught segfault ***
address 0x0, cause 'memory not mapped'

Interestingly that R process continues to spin in the background, consuming 62% CPU. Not sure what it's doing...

[st-pasha]

It fails on line subset.c:317 :

...
copyMostAttrib(x, ans);
...

[HughParsonage]

OK must be partially an R 3.5 v R 3.4 issue.