[R] Bug in internal 'tar' implementation?

[Duncan Murdoch]

Or directly to bugs.r-project.org .  It definitely looks to me like a bug.

Instructions for bug reports are here: 
https://www.r-project.org/bugs.html .  David might need to set up a 
Bugzilla account according to those instructions before reporting.

Duncan Murdoch

On 31/01/2023 1:40 p.m., Bert Gunter wrote:
> 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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>> and provide commented, minimal, self-contained, reproducible code.
>>
> 
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> 
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.