empty column positions produce segfault in read_fwf()

[jamesdunham]

I've found that read_fwf() will accept bad input in the form of numeric(0) column positions. This produces a segfault.

Are you able to replicate this?

library(readr)
sessionInfo()
# R version 3.3.0 (2016-05-03)
# Platform: x86_64-apple-darwin13.4.0 (64-bit)
# Running under: OS X 10.11.6 (El Capitan)
#
# locale:
# [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
#
# attached base packages:
# [1] stats     graphics  grDevices utils     datasets  methods   base     
#
# other attached packages:
# [1] readr_1.0.0.9000
#
# loaded via a namespace (and not attached):
# [1] assertthat_0.1 tools_3.3.0    tibble_1.1     Rcpp_0.12.6   

txt = "foo\n"
pos = fwf_positions(start = numeric(0), end = numeric(0))
read_fwf(txt, pos)

#  *** caught segfault ***
# address 0xfffffffffffffffc, cause 'memory not mapped'
# 
# Traceback:
#  1: .Call("readr_guess_types_", PACKAGE = "readr", sourceSpec, tokenizerSpec,     locale_, n)
#  2: guess_types_(datasource, tokenizer, locale, n = n)
#  3: guess_types(ds, tokenizer, locale, n = n)
#  4: col_spec_standardise(file, skip = skip, n = guess_max, tokenizer = tokenizer,     locale = locale, col_names = col_positions$col_names, col_types = col_types,     drop_skipped_names = TRUE)
#  5: read_fwf(txt, pos)
# An irrecoverable exception occurred. R is aborting now ...

Thanks.

[hadley]

Related to #534 - need better input validation