fwrite crashes for tables with large columns

[logworthy]

Bit of an edge case here, but when trying to fwrite a table with a very large column R crashes every time.

Potentially related to #2612.

Minimal reproducible example:

huge_vector <- paste(rep('x', 30e6), collapse='')

foo <- data.table::data.table(x=huge_vector)

tf <- tempfile()

data.table::tables()
# NAME NROW NCOL MB COLS KEY
# 1:  foo    1    1 29    x    
# Total: 29MB

# this works fine
write.table(foo, file=tf)

# this reliably crashes my R session
data.table::fwrite(foo, file=tf)

Output of sessionInfo():

R version 3.4.3 (2017-11-30)
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     

loaded via a namespace (and not attached):
[1] compiler_3.4.3    tools_3.4.3       yaml_2.1.19       data.table_1.11.0

For what it's worth, I have 16GB RAM on the system with ~8GB free before running the above example.

[philippechataignon]

fwrite first attempt to compute max line length from a sample. By default, buffer is 8MB. Here line is 30M. In this PR #3288, I use strlen and a message error indicates that buffer is too small and give an advice to increase it.

> library(data.table)
> huge_vector <- paste(rep('x', 30e6), collapse='')
> foo <- data.table::data.table(x=factor(huge_vector))
> tf <- tempfile()
> 
> # this reliably crashes my R session
> data.table::fwrite(foo, file=tf)
Error in data.table::fwrite(foo, file = tf) : 
  Error : max line length is greater than buffer secure limit. Try to increase buffMB option. Example 'buffMB = 58'

# OK
> data.table::fwrite(foo, file=tf, buffMB=58)

[mattdowle]

Awesome PR @philippechataignon! I made a few minor changes to PR #3288 and now this example works without needing user to increase buffMB. The test is not huge really, but to keep resources to a minimum on CRAN I haven't added it to the test suite.