[Rd] Sys.getenv(): Error in substring(x, m + 1L) : invalid multibyte string at '<ff>' if an environment variable contains \xFF

[Martin Maechler]

>>>>> Tomas Kalibera 
>>>>>     on Tue, 31 Jan 2023 10:53:21 +0100 writes:

    > On 1/31/23 09:48, Ivan Krylov wrote:
    >> Can we use the "bytes" encoding for such environment variables invalid
    >> in the current locale? The following patch preserves CE_NATIVE for
    >> strings valid in the current UTF-8 or multibyte locale (or
    >> non-multibyte strings) but sets CE_BYTES for those that are invalid:
    >> 
    >> Index: src/main/sysutils.c
    >> ===================================================================
    >> --- src/main/sysutils.c	(revision 83731)
.....
    >> 
    >> Here are the potential problems with this approach:
    >> 
    >> * I don't know whether known_to_be_utf8 can disagree with utf8locale.
    >> known_to_be_utf8 was the original condition for setting CE_UTF8 on
    >> the string. I also need to detect non-UTF-8 multibyte locales, so
    >> I'm checking for utf8locale and mbcslocale. Perhaps I should be more
    >> careful and test for (enc == CE_UTF8) || (utf8locale && enc ==
    >> CE_NATIVE) instead of just utf8locale.
    >> 
    >> * I have verified that Sys.getenv() doesn't crash with UTF-8-invalid
    >> strings in the environment with this patch applied, but now
    >> print.Dlist does, because formatDL wants to compute the width of the
    >> string which has the 'bytes' encoding. If this is a good way to
    >> solve the problem, I can work on suggesting a fix for formatDL to
    >> avoid the error.

    > Thanks, indeed, type instability is a big problem of the approach "turn 
    > invalid strings to bytes". It is something what is historically being 
    > done in regular expression operations, but it is brittle and not user 
    > friendly: writing code to be agnostic to whether we are dealing with 
    > "bytes" or a regular string is very tedious. Pieces of type instability 
    > come also from that ASCII strings are always flagged "native" (never 
    > "bytes"). Last year I had to revert a big change which broke existing 
    > code by introducing some more of this instability due to better dealing 
    > with invalid strings in regular expressions. I've made some additions to 
    > R-devel allowing to better deal with such instability but it is still a 
    > pain and existing code has to be changed (and made more complicated).

    > So, I don't think this is the way to go.

    > Tomas

hmm.., that's a pity; I had hoped it was a pragmatic and valid strategy,
but of course you are right that type stability is really a
valid goal....

In general, what about behaving close to "old R" and replace all such
strings by  NA_character_  (and typically raising one warning)?
This would keep the result a valid character vector, just with some NA entries.

Specifically for  Sys.getenv(),  I still think Simon has a very
valid point of "requiring" (of our design) that 
Sys.getenv()[["BOOM"]]  {double `[[`} should be the same as
Sys.getenv("BOOM")  

Also, as typical R user, I'd definitely want to be able to get all the valid
environment variables, even if there are one or more invalid
ones. ... and similarly in other cases, it may be a cheap
strategy to replace invalid strings ("string" in the sense of 
length 1 STRSXP, i.e., in R, a "character" of length 1) by
NA_character_  and keep all valid parts of the character vector
in a valid encoding.

- - - - - -

In addition to the above cheap "replace by NA"-strategy, 
and at least once R is "all UTF-8", we could
also consider a more expensive strategy that would try to
replace invalid characters/byte-sequences by one specific valid UTF-8
character, i.e., glyph (think of a special version of "?") that
we would designate as replacement for "invalid-in-current-encoding".

Probably such a glyph already exists and we have seen it used in
some console's output when having to print such things.