setDT doesn't work within on.exit

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by running the reported R reproduction with data.table 1.13.0 and the shown session information, focusing on setDT(d) inside on.exit. Done means the function returns a data.table without the invalid .internal.selfref warning and the following := operation works by reference.

Written by the indexing model from the issue text.

Description

bug

Example:

library(data.table)
testonexit <-function(d) {
  on.exit(setDT(d))
  d
}
d <- data.frame(a=1:2)
d <- testonexit(d)
d[, c:=1:2]

Gives the dreaded:

Warning message:
In [.data.table(d, , :=(c, 1:2)) :
Invalid .internal.selfref detected and fixed by taking a (shallow) copy of the data.table so that := can add this new column by reference. At an earlier point, this data.table has been copied by R (or was created manually using structure() or similar). Avoid names<- and attr<- which in R currently (and oddly) may copy the whole data.table. Use set* syntax instead to avoid copying: ?set, ?setnames and ?setattr. If this message doesn't help, please report your use case to the data.table issue tracker so the root cause can be fixed or this message improved.

# Output of sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19042)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] magrittr_1.5 data.table_1.13.0

loaded via a namespace (and not attached):
[1] bit_4.0.4 compiler_3.6.1 tools_3.6.1 bit64_4.0.5

Dominant language
R
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