[Rd] Operations with long altrep vectors cause segfaults on Windows

[Martin Maechler]

>>>>> Hugh Parsonage 
>>>>>     on Tue, 8 Sep 2020 18:08:11 +1000 writes:

    > I can only reproduce on Windows, but reliably (both 4.0.0 and 4.0.2):

    > $> R --vanilla
    > x <- c(0L, -2e9:2e9)

    > # > Segmentation fault

    > Tried to reproduce on Linux but the above worked as expected. Not an
    > issue merely with the length of the vector; for example, x <-
    > rep_len(1:10, 1e10) works, though the altrep vector must be long to
    > reproduce:

    > x <- c(0L, -1e9:1e9)  #ok

    > Segmentation faults occur with the following too:

    > x <- (-2e9:2e9) + 1L

Your operation would "need" (not in theory, but in practice)
to go from altrep to regular vectors.
I guess the segfault occurs because of something like this :

 R asks Windows to hand it a huge amount of memory and Windows replies
 "ok, here is the memory pointer"
 and then R tries to write to there, but illegally (because
 Windows should have told R that it does not really have enough
 memory for that ..). 
 
I cannot reproduce the segmentation fault .. but I can confirm
there is a bug there that shows for me on Windows but not on
Linux:

"My" Windows is on a terminalserver not with too many GB of memory
(but then in a version of Windows that recognizes that it cannot
 get so much memory):

------------------------- Here some transcript (thanks to
                          using Emacs w/ ESS also on Windows) ------------------

R Under development (unstable) (2020-08-24 r79074) -- "Unsuffered Consequences"
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> x <- (-2e9:2e9) + 1L
Fehler: kann Vektor der Größe 14.9 GB nicht allozieren
> y <- c(0L, -2e9:2e9)
Fehler: kann Vektor der Größe 14.9 GB nicht allozieren
> Sys.setenv(LANGUAGE="en")
> y <- c(0L, -2e9:2e9)
Error: cannot allocate vector of size 14.9 Gb
> y <- -1e9:4e9
> .Internal(inspect(y))
@0x00000000195a6808 14 REALSXP g0c0 [REF(65535)]  -1000000000 : -294967296 (compact)
> .Machine$integer.max / 1e9
[1] 2.147484
> y <- -1e6:2.2e9
> .Internal(inspect(y))
@0x000000000a11a5d8 14 REALSXP g0c0 [REF(65535)]  -1000000 : -2094967296 (compact)
> y <- -1e6:2e9
> .Internal(inspect(y))
@0x000000000a13adf0 13 INTSXP g0c0 [REF(65535)]  -1000000 : 2000000000 (compact)
> 
------------------------- end of transcript -----------------------------------

So indeed, no seg.fault, R notices that it can't get 15 GB of
memory.

But the bug is bad news:  We have *silent* integer overflow happening
according to what  .Internal(inspect(y)) shows...

 .... less bad new: Probably the bug is only in the 'internal inspect' code
 where a format specifier is used in C's printf() that does not work
 correctly on Windows, at least the way it is currently compiled ..


On (64-bit) Linux, I get

> y <- -1e9:4e9 ; .Internal(inspect(y))
@7d86388 14 REALSXP g0c0 [REF(65535)]  -1000000000 : 4000000000 (compact)

> y <- c(0L, y)
Error: cannot allocate vector of size 37.3 Gb

which seems much better ... until I do find a bug, may again
only in the C code underlying .Internal(inspect(.)) :

> y <- -1e9:2e9 ; .Internal(inspect(y))
@7d86ac0 13 INTSXP g0c0 [REF(65535)] Error: long vectors not supported yet: ../../../R/src/main/altclasses.c:139
>