[Rd] package installation fails when symlink of same name exists

[Martin Maechler]

>>>>> Jeroen Ooms <jeroenooms at gmail.com>
>>>>>     on Wed, 20 Jul 2016 10:26:19 +0200 writes:

    > On Tue, Jul 19, 2016 at 6:46 PM, Kevin Ushey <kevinushey at gmail.com> wrote:
    >> R fails to install a package from source over a pre-existing package
    >> when the path to that package is a symlink, rather than a directory.
    >> ...
    >> I don't think anyone's reported this being an issue before

    > I ran into this as well a while back:
    >   https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=16725

I've now at least "acknowledged" that bug report.
and have looked into changing the  is_subdir() function so it
returns TRUE in the case of a symlink [on those platforms where
Sys.readlink() "works", i.e., supposedly not on Windows; however
that maybe sufficient to close that bug report and also Kevin's
issue, right ?]

However, Kevin, in his posting, continues 

    > I guess my wish here would be that R would check if any file already
    > existed at the 'instdir' path, and if it existed and was a symlink, R
    > would remove that symlink before install. 

are you sure?
I think ... and from what you mention below ("packrat") it would
rather be important to *keep* the symlink, and install to
whereever the symlink is pointing,  no ?


    > It could happen before creating the directory, e.g. here:

    > https://github.com/wch/r-source/blob/62f5acbdbdf36e1fc618510312125d1677d79941/src/library/tools/R/install.R#L277-L281

    > One thing that was a bit surprising to me -- R does not remove a
    > pre-existing package installation if it exists (when installing from
    > source), it merely installs over it, so files / artifacts from a
    > previous package installation could be left over after installing a
    > new package. It seems this is not a problem in practice since I don't
    > think anyone's reported this being an issue before, but for hygiene it
    > seems like a pre-existing directory could / should be removed when
    > installing a new package. (It appears that R does clear out a
    > pre-existing directory when downloading and installing a package
    > binary directly from CRAN.)

Well, at least with  update.packages()  it seems natural to me
that R would not just remove all previous parts there ..

    > For motivation: I bumped into this when attempting to implement a
    > package caching feature with packrat. A packrat project using a global
    > cache will have a (private) R library containing symlinks to R package
    > installations in a separate, global library. This allows projects to
    > effectively be isolated from one another, while avoiding duplication
    > of packages used across multiple projects.

Yes, I found this a nice feature when I heard about packrat.

But then, really R should *not* remove the symlink and create a
regular subdirectory in that library there !

    > Unfortunately, some packrat
    > users bump into this when attempting to update a package that has
    > entered the cache (and so is a symlink in their R library).

    > Thanks for your time,
    > Kevin