R devel - May 2018 - possible internal (un)tar bug

This is a not too old R-devel on Linux, it already fails in R 3.4.4, and on
macOS as well.

The tar file seems valid, external tar can untar it, so maybe an untar()
bug.

setwd(tempdir())
dir.create("pkg")
cat("foobar\n",  file = file.path("pkg",
"NAMESPACE"))
cat("this: that\n", file = file.path("pkg",
"DESCRIPTION"))

tar("pkg_1.0.tar.gz", "pkg", compression = "gzip",
tar = "internal")
unlink("pkg", recursive = TRUE)

con <- file("pkg_1.0.tar.gz", open = "rb")
ex <- tempfile()
untar(con, files = "pkg/DESCRIPTION", exdir = ex)

#> Error in untar2(tarfile, files, list, exdir, restore_times) :
#>   incomplete block on file

>>>>> G?bor Cs?rdi <csardi.gabor at gmail.com>
>>>>>     on Tue, 1 May 2018 12:05:32 +0000 writes:
    > This is a not too old R-devel on Linux, it already fails
    > in R 3.4.4, and on macOS as well.

and fails in considerably older R versions, too.

Basically  untar() seems to fail on a connection, but works fine
on a plain file name.

This is a bug --> Thank you for the report, G?bor !

I'm investigating.
Martin

----------- my version of your reprex :

setwd(tempdir())
dir.create("pkg")
cat("this: that\n", file = file.path("pkg",
"DESCRIPTION"))
tf <- "pkg_1.0.tar.gz"
tar(tf, "pkg", compression = "gzip", tar =
"internal")
unlink("pkg", recursive = TRUE)

## MM: tar *file* is good
stopifnot(identical(untar(tf, list=TRUE), "pkg/DESCRIPTION"))
untar(tf, files = (f <- "pkg/DESCRIPTION")) # no problem
stopifnot(file.exists(f))
unlink("pkg", recursive = TRUE)

## Now with a connection -- "nothing works":
con <- file(tf, open = "rb"); try( untar(con, list = TRUE) ) ##
-> Error
con <- file(tf, open = "rb"); try( untar(con, files =
"pkg/DESCRIPTION") )
## The error message is the same in both cases:
'
Error in untar2(tarfile, files, list, exdir, restore_times) :
   incomplete block on file
'

>>>>> Martin Maechler <maechler at stat.math.ethz.ch>
>>>>>     on Tue, 1 May 2018 16:14:43 +0200 writes:
>>>>> G?bor Cs?rdi <csardi.gabor at gmail.com>
>>>>>     on Tue, 1 May 2018 12:05:32 +0000 writes:
    >> This is a not too old R-devel on Linux, it already fails
    >> in R 3.4.4, and on macOS as well.

    > and fails in considerably older R versions, too.

    > Basically untar() seems to fail on a connection, but works
    > fine on a plain file name.

Well, there's an easy workaround:   If you want to use a
connection (instead of a simple filename) with  untar() and want
to use compression (as in the example), you
can currently  do that easily when you ensure the connection is
a "gzcon" one :

##=========>  Workaround for now:

## Create :
setwd(tempdir()) ; dir.create("pkg")
cat("this: that\n", file = file.path("pkg",
"DESCRIPTION"))
tf <- "pkg_1.0.tar.gz"
tar(tf, "pkg", compression = "gzip", tar =
"internal")
unlink("pkg", recursive = TRUE)

## As it is a compressed tar file, use it via a gzcon() connection,
## and both cases work fine:
con <- gzcon(file(tf, open = "rb")) ; (f <- untar(con, list =
TRUE))
##     ~~~~~
con <- gzcon(file(tf, open = "rb")) ; untar(con, files = f)
stopifnot(identical(f, "pkg/DESCRIPTION"),
          file.exists(f))
unlink(c(tf,"pkg"), recursive = TRUE) # clean after me


------------

Of course, ideally untar() should do that for us and I'm testing
a simple patch to do that.

Martin