bug#23723: patch-shebang phase breaks symlinks

[Jelle Licht <jlicht@fsfe.org>]

Hi

Ludovic Courtès <ludo@gnu.org> writes:
> Hello!
>
> Jelle Licht <jlicht@fsfe.org> skribis:
>
>> It seems that the patch-shebang functionality does not deal gracefully
>> with symlinks: it just overwrites them!
>>
>> After struggling somewhat with getting the recently packaged node 6.0.0
>> to behave, I found out that `patch-shebang' in (guix build
>> gnu-build-system) does not work properly on symlinks.
>
> There’s ‘patch-shebangs’ (plural) in this file, but it explicitly
> touches only regular files (see ‘list-of-files’).
>

It seems I made a mistake when writing the bug report; I am talking
about the `patch-shebang' defined in (guix build utils). My apologies.

Also, seeing as my experience with the stat utility and similarly styled
programming libraries was lacking, I decided to play around with the
definition of `list-of-files': It actually does include symlinks, as
(stat:type (stat "some-symlinked-file")) gives us a plain old 'regular.
Looking into this a bit more, it seems that calling `stat' gives the
exact same results on both the linked-to-file and the symlink to that
file.

For the particular problem I ran into to be fixed, it is imperative that
`list-of-files' of `patch-shebangs' includes the symlink; it does after
all need to be patched. The way this patching currently happens just
clobbers symlinks.

> However, ‘patch-source-shebangs’ indeed overwrites symlinks.  Is it the
> one that’s causing problems?
>
>> To illustrate, in this specific case, there was an executable
>> script included with the node tarball, namely
>> `lib/node-modules/npm/bin/npm-cli.js' with an env-based shebang:
>> `/usr/bin/env node'.
>>
>> As the `node' executable will only be available after the `install'
>> phase of the build system, it is not patched before hand. During the
>> `install' phase, a symlink is created from `bin/npm' to
>> `lib/node-modules/npm/bin/npm-cli.js' (in the store output directory
>> this time, of course). Then the `patch-shebangs' phase finds this
>> symlink and proceeds to patch it. Instead of transparently following the
>> symlink and patching the `npm-cli.js' script, the `npm' symlink is
>> overwritten with a shebang-patched copy of `npm-cli.js'.
>
> I don’t think ‘patch-shebangs’ is to blame; could it be
> ‘patch-source-shebangs’?

AFAIK, it is definitely the 'patch-shebangs' phase that is to blame for
my woes; removing it using modify-phases makes the issue disappear, when
looking at the build dir in the store.

>
> Thanks!
>
> Ludo’.

Looking forward to your thoughts on the matter

- Jelle