[R] Bug in internal 'tar' implementation?

[Bert Gunter]

This post should probably go to R-devel rather than here.

-- Bert

On Tue, Jan 31, 2023 at 9:47 AM David Engster <deng using randomsample.de> wrote:

> I think I found a bug in the internal implementation of 'tar', but
> before bothering the R maintainers, I was advised to ask here to make
> sure I'm not missing something.
>
> Fortunately, it can be very easily reproduced on a Linux system. In an
> empty temporary directory, execute the following code:
>
> cat("foobar", file="test.txt")
> file.symlink("test.txt", "test_link.txt")
> tar("test.tar", c("test_link.txt", "test.txt"), tar="internal")
> system2("tar", c("tf", "test.tar"))
>
> This file create a file "test.txt" and a symbolic link "test_link.txt"
> pointing to that file. Those two are then put into "test.tar" using R's
> internal tar implementation, and then the system's 'tar' binary (usually
> GNU tar) will be used to display the contents of that archive.
>
> On my system (Debian 11, GNU tar 1.34), this gives me the following
> output:
>
> [1] TRUE
> test_link.txt
> tar: Skipping to next header
> tar: Exiting with failure status due to previous errors
>
> Not that *extracting* the archive with 'tar xf' (fortunately) works
> fine, it's just displaying its contents that fails. After looking into
> the hexdump of 'test.tar' and R's internal tar() code, I found out the
> reason for this is that a wrong size for the link is put into the tar
> header: it should be zero, but the size of the linked file is put in
> there instead. This leads to 'tar tf' jumping over too many blocks after
> displaying the link filename and hence aborting.
>
> While I'm aware the 'tar()' help says to avoid links for portability
> reasons, it also says that it supports symbolic links on OSes that
> support them, which Linux of course does, so do you agree this should be
> fixed? (It's a very simple one-line change.)
>
> Best,
> David
>
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