packages: Add a 'snippet' field to <origin>.

* guix/packages.scm (<origin>): Add 'snippet', 'modules', and
  'imported-modules' fields.
  (patch-and-repack): Make 'inputs' a keyword parameter.  Add 'snippet',
  'modules', and 'imported-modules' parameters.  Accept SOURCE as a raw
  file name.  Insert SNIPPET in BUILDER.  Pass IMPORTED-MODULES to
  'build-expression->derivation'.
  (package-source-derivation): Pass the extra arguments to
  'patch-and-repack'.
* tests/packages.scm ("package-source-derivation, snippet"): New test.
* doc/guix.texi (Defining Packages): Mention the 'patches' and 'snippet'
  fields.
  (Invoking guix build): Tell that --source has patches and snippets
  applied.
  (Software Freedom): Mention packages that contain non-free code.

1 files changed, 17 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 43e7935b4c..4fb14063d0 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -985,6 +985,11 @@ base32 representation of the hash.  You can obtain this information with
 @code{guix download} (@pxref{Invoking guix download}) and @code{guix
 hash} (@pxref{Invoking guix hash}).
 
+@cindex patches
+When needed, the @code{origin} form can also have a @code{patches} field
+listing patches to be applied, and a @code{snippet} field giving a
+Scheme expression to modify the source code.
+
 @item
 @cindex GNU Build System
 The @code{build-system} field is set to @var{gnu-build-system}.  The
@@ -1479,6 +1484,10 @@ themselves.
 For instance, @code{guix build -S gcc} returns something like
 @file{/nix/store/@dots{}-gcc-4.7.2.tar.bz2}, which is GCC's source tarball.
 
+The returned source tarball is the result of applying any patches and
+code snippets specified in the package's @code{origin} (@pxref{Defining
+Packages}).
+
 @item --system=@var{system}
 @itemx -s @var{system}
 Attempt to build for @var{system}---e.g., @code{i686-linux}---instead of
@@ -1878,6 +1887,14 @@ software distribution guidelines}.  Among other things, these guidelines
 reject non-free firmware, recommendations of non-free software, and
 discuss ways to deal with trademarks and patents.
 
+Some packages contain a small and optional subset that violates the
+above guidelines, for instance because this subset is itself non-free
+code.  When that happens, the offending items are removed with
+appropriate patches or code snippets in the package definition's
+@code{origin} form (@pxref{Defining Packages}).  That way, @code{guix
+build --source} returns the ``freed'' source rather than the unmodified
+upstream source.
+
 
 @node Package Naming
 @subsection Package Naming