build: ruby: Add support for tarball and directory sources.

Previously, the Ruby build system only knew how to work with gem archives,
which made it difficult to build unreleased gems from a Git repository or
released gems in tarball form.

* gnu/build/ruby-build-system.scm (gnu:unpack, gem-archive?): New procedures.
  (unpack): Use GNU build system unpack phase for non-gem sources.
  (build): Rebuild the gemspec iff the source is a gem archive.
* guix.texi ("ruby-build-system"): Mention that tarballs and directories are
  acceptable.

doc/guix.texi

@@ -2506,12 +2506,13 @@ This variable is exported by @code{(guix build-system ruby)}.  It
 implements the RubyGems build procedure used by Ruby packages, which
 involves running @code{gem build} followed by @code{gem install}.
 
-The @code{source} field of a package that uses this build system is
-expected to reference a gem archive instead of a traditional tarball,
-since this is the format that all Ruby developers use when releasing
-their software.  The build system unpacks the gem archive, potentially
-patches the source, runs the test suite, repackages the gem, and
-installs it.
+The @code{source} field of a package that uses this build system
+typically references a gem archive, since this is the format that Ruby
+developers use when releasing their software.  The build system unpacks
+the gem archive, potentially patches the source, runs the test suite,
+repackages the gem, and installs it.  Additionally, directories and
+tarballs may be referenced to allow building unreleased gems from Git or
+a traditional source release tarball.
 
 Which Ruby package is used can be specified with the @code{#:ruby}
 parameter.  A list of additional flags to be passed to the @command{gem}

guix/build/ruby-build-system.scm

@@ -41,29 +41,39 @@ directory."
     ((file-name . _) file-name)
     (() (error "No files matching pattern: " pattern))))
 
+(define gnu:unpack (assq-ref gnu:%standard-phases 'unpack))
+
+(define (gem-archive? file-name)
+  (string-match "^.*\\.gem$" file-name))
+
 (define* (unpack #:key source #:allow-other-keys)
   "Unpack the gem SOURCE and enter the resulting directory."
+  (if (gem-archive? source)
       (and (zero? (system* "gem" "unpack" source))
-       ;; The unpacked gem directory is named the same as the archive, sans
-       ;; the ".gem" extension.  It is renamed to simply "gem" in an effort to
-       ;; keep file names shorter to avoid UNIX-domain socket file names and
-       ;; shebangs that exceed the system's fixed maximum length when running
-       ;; test suites.
+           ;; The unpacked gem directory is named the same as the archive,
+           ;; sans the ".gem" extension.  It is renamed to simply "gem" in an
+           ;; effort to keep file names shorter to avoid UNIX-domain socket
+           ;; file names and shebangs that exceed the system's fixed maximum
+           ;; length when running test suites.
            (let ((dir (match:substring (string-match "^(.*)\\.gem$"
                                                      (basename source))
                                        1)))
              (rename-file dir "gem")
              (chdir "gem")
-         #t)))
+             #t))
+      ;; Use GNU unpack strategy for things that aren't gem archives.
+      (gnu:unpack #:source source)))
 
 (define* (build #:key source #:allow-other-keys)
   "Build a new gem using the gemspec from the SOURCE gem."
+  (define (first-gemspec)
+    (first-matching-file "\\.gemspec$"))
 
   ;; Remove the original gemspec, if present, and replace it with a new one.
   ;; This avoids issues with upstream gemspecs requiring tools such as git to
   ;; generate the files list.
-  (let ((gemspec (or (false-if-exception
-                      (first-matching-file "\\.gemspec$"))
+  (when (gem-archive? source)
+    (let ((gemspec (or (false-if-exception (first-gemspec))
                        ;; Make new gemspec if one wasn't shipped.
                        ".gemspec")))
 
@@ -82,12 +92,12 @@ directory."
                   (write-char (read-char pipe) out))))
             #t)
           (lambda ()
-          (close-pipe pipe))))
+            (close-pipe pipe))))))
 
   ;; Build a new gem from the current working directory.  This also allows any
   ;; dynamic patching done in previous phases to be present in the installed
   ;; gem.
-    (zero? (system* "gem" "build" gemspec))))
+  (zero? (system* "gem" "build" (first-gemspec))))
 
 (define* (check #:key tests? test-target #:allow-other-keys)
   "Run the gem's test suite rake task TEST-TARGET.  Skip the tests if TESTS?