pack: Add '--relocatable'.

* gnu/packages/aux-files/run-in-namespace.c: New file.
* Makefile.am (AUX_FILES): Add it.
* guix/scripts/pack.scm (<c-compiler>): New record type.
(c-compiler, bootstrap-c-compiler, c-compiler-compiler): New procedures.
(self-contained-tarball): Use
'relative-file-name' for the SOURCE -> TARGET symlink.
(docker-image): Add 'defmod' to please Geiser.
(wrapped-package, map-manifest-entries): New procedures.
(%options, show-help): Add --relocatable.
(guix-pack): Honor it.

1 files changed, 42 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 8b9f8721ba..6aeb313773 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -2834,6 +2834,15 @@ guix pack -S /opt/gnu/bin=bin guile emacs geiser
 @noindent
 That way, users can happily type @file{/opt/gnu/bin/guile} and enjoy.
 
+@cindex relocatable binaries, with @command{guix pack}
+What if the recipient of your pack does not have root privileges on
+their machine, and thus cannot unpack it in the root file system?  In
+that case, you will want to use the @code{--relocatable} option (see
+below).  This option produces @dfn{relocatable binaries}, meaning they
+they can be placed anywhere in the file system hierarchy: in the example
+above, users can unpack your tarball in their home directory and
+directly run @file{./opt/gnu/bin/guile}.
+
 Alternatively, you can produce a pack in the Docker image format using
 the following command:
 
@@ -2867,6 +2876,39 @@ This produces a tarball that follows the
 Docker Image Specification}.
 @end table
 
+@item --relocatable
+@itemx -R
+Produce @dfn{relocatable binaries}---i.e., binaries that can be placed
+anywhere in the file system hierarchy and run from there.  For example,
+if you create a pack containing Bash with:
+
+@example
+guix pack -R -S /mybin=bin bash
+@end example
+
+@noindent
+... you can copy that pack to a machine that lacks Guix, and from your
+home directory as a normal user, run:
+
+@example
+tar xf pack.tar.gz
+./mybin/sh
+@end example
+
+@noindent
+In that shell, if you type @code{ls /gnu/store}, you'll notice that
+@file{/gnu/store} shows up and contains all the dependencies of
+@code{bash}, even though the machine actually lacks @file{/gnu/store}
+altogether!  That is probably the simplest way to deploy Guix-built
+software on a non-Guix machine.
+
+There's a gotcha though: this technique relies on the @dfn{user
+namespace} feature of the kernel Linux, which allows unprivileged users
+to mount or change root.  Old versions of Linux did not support it, and
+some GNU/Linux distributions turn it off; on these systems, programs
+from the pack @emph{will fail to run}, unless they are unpacked in the
+root file system.
+
 @item --expression=@var{expr}
 @itemx -e @var{expr}
 Consider the package @var{expr} evaluates to.