Bug with `[<-.POSIXlt` on specific OSes

Davis Vaughan writes:
I've got a bit more information about this one. It seems like it
(only? not sure) appears when `TZ = "UTC"`, which is why I didn't see
it before on my Mac, which defaults to `TZ = ""`. I think this is at
least explainable by the fact that those "optional" fields aren't
technically needed when the time zone is UTC.
Exactly.  Debugging `[<-.POSIlt` with

x <- as.POSIXlt(as.POSIXct("2013-01-31", tz = "America/Chicago"))
Sys.setenv(TZ = "UTC")
x[1] <- NA

shows we get into

    value <- unclass(as.POSIXlt(value))
    if (ici) {
        for (n in names(x)) names(x[[n]]) <- nms
    }
    for (n in names(x)) x[[n]][i] <- value[[n]]

where

Browse[2]> names(value)
[1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"  "isdst"
Browse[2]> names(x)
 [1] "sec"    "min"    "hour"   "mday"   "mon"    "year"   "wday"   "yday"  
 [9] "isdst"  "zone"   "gmtoff"

Without having looked at the code, the docs say

     ?zone? (Optional.) The abbreviation for the time zone in force at
          that time: ?""? if unknown (but ?""? might also be used for
          UTC).

     ?gmtoff? (Optional.) The offset in seconds from GMT: positive
          values are East of the meridian.  Usually ?NA? if unknown,
          but ?0? could mean unknown.

so perhaps we should fill with the values for the unknown case?

-k
I can reproduce this now on my personal Mac:
```
x <- as.POSIXlt(as.POSIXct("2013-01-31", tz = "America/Chicago"))
Sys.setenv(TZ = "")
x[1] <- NA
x
#> [1] NA
x <- as.POSIXlt(as.POSIXct("2013-01-31", tz = "America/Chicago"))
Sys.setenv(TZ = "America/New_York")
x[1] <- NA
x
#> [1] NA
x <- as.POSIXlt(as.POSIXct("2013-01-31", tz = "America/Chicago"))
Sys.setenv(TZ = "UTC")
x[1] <- NA
#> Error in x[[n]][i] <- value[[n]] : replacement has length zero
x
#> [1] "2013-01-31 CST"
```
Here are `sessionInfo()` and `Sys.getenv("TZ")` outputs for 3 GitHub
Actions platforms where the bug exists (note they all set `TZ = "UTC"`!):
Linux:
```
sessionInfo()
R version 4.2.1 (2022-06-23)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.6 LTS
Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/openblas/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/libopenblasp-r0.2.20.so
locale:
 [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8
 [4] LC_COLLATE=C.UTF-8     LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8
 [7] LC_PAPER=C.UTF-8       LC_NAME=C              LC_ADDRESS=C
[10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base
loaded via a namespace (and not attached):
[1] compiler_4.2.1
Sys.getenv("TZ")
[1] "UTC"
```
Mac:
```
sessionInfo()
R version 4.2.1 (2022-06-23)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Big Sur ... 10.16
Matrix products: default
BLAS:
/Library/Frameworks/R.framework/Versions/4.2/Resources/lib/libRblas.0.dylib
LAPACK:
/Library/Frameworks/R.framework/Versions/4.2/Resources/lib/libRlapack.dylib
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
loaded via a namespace (and not attached):
[1] compiler_4.2.1
Sys.getenv("TZ")
[1] "UTC"
```
Windows:
This is the best I can get you, sorry (remote worker issues), but note that
it does also say `tz UTC` like the others.
```
version R version 4.2.1 (2022-06-23 ucrt)
os Windows Server x64 (build 20348)
system x86_64, mingw32
ui RTerm
language (EN)
collate English_United States.utf8
ctype English_United States.utf8
tz UTC
date 2022-10-11
```
And here is my Mac where the bug doesn't show up by default because `TZ =
""`:
```
sessionInfo()
R version 4.2.1 (2022-06-23)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Big Sur ... 10.16
Matrix products: default
BLAS:
/Library/Frameworks/R.framework/Versions/4.2/Resources/lib/libRblas.0.dylib
LAPACK:
/Library/Frameworks/R.framework/Versions/4.2/Resources/lib/libRlapack.dylib
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
loaded via a namespace (and not attached):
[1] compiler_4.2.1
Sys.getenv("TZ")
[1] ""
Sys.timezone()
[1] "America/New_York"
```
-Davis
On Thu, Oct 6, 2022 at 9:33 AM Davis Vaughan <davis at rstudio.com> wrote:
Hi all,

I have found another POSIXlt bug while I've been fiddling around with it.
This one only appears on specific OSes, because it has to do with the fact
that the `gmtoff` field is optional, and isn't always used on all OSes. It
also doesn't seem to be specific to r-devel, I think it has been there
awhile.

Here is the bug:

```
x <- as.POSIXlt(as.POSIXct("2013-01-31", tz = "America/Chicago"))

# Oh no!
x[1] <- NA
#> Error in x[[n]][i] <- value[[n]] : replacement has length zero
```

If you look at the objects, you can see that `x` has a `gmtoff` field, but
`NA` (when converted to POSIXlt, which is what `[<-.POSIXlt` does) does not:

```
unclass(x)
#> $sec
#> [1] 0
#>
#> $min
#> [1] 0
#>
#> $hour
#> [1] 0
#>
#> $mday
#> [1] 31
#>
#> $mon
#> [1] 0
#>
#> $year
#> [1] 113
#>
#> $wday
#> [1] 4
#>
#> $yday
#> [1] 30
#>
#> $isdst
#> [1] 0
#>
#> $zone
#> [1] "CST"
#>
#> $gmtoff
#> [1] -21600
#>
#> attr(,"tzone")
#> [1] "America/Chicago" "CST"             "CDT"

unclass(as.POSIXlt(NA))
#> $sec
#> [1] NA
#>
#> $min
#> [1] NA
#>
#> $hour
#> [1] NA
#>
#> $mday
#> [1] NA
#>
#> $mon
#> [1] NA
#>
#> $year
#> [1] NA
#>
#> $wday
#> [1] NA
#>
#> $yday
#> [1] NA
#>
#> $isdst
#> [1] -1
#>
#> attr(,"tzone")
#> [1] "UTC"
```

The problem seems to be that `[<-.POSIXlt` assumes that if the field was
there in `x` then it must also be there in `value`:

https://github.com/wch/r-source/blob/e10a971dee6a0ab851279c183cc21954d66b3be4/src/library/base/R/datetime.R#L1303-L1304

But this isn't the case for the `NA` value that was converted to POSIXlt.

I can't reproduce this on my personal Mac, but it affects the Linux, Mac,
and Windows machines we use for the lubridate CI checks through GitHub
Actions.

Thanks,
Davis

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Bug with `[<-.POSIXlt` on specific OSes

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