packages: Factorize and document 'computed-origin-method'.

The 'computed-origin-method' had been introduced to work around
limitations of the 'snippet' mechanism.  The procedure was duplicated,
which made it hard to automatically detect packages using it.

* guix/packages.scm (computed-origin-method): Move procedure from...
* gnu/packages/gnuzilla.scm: ...here and...
* gnu/packages/gnuzilla.scm: ...there.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

3 files changed, 26 insertions, 25 deletions

diff --git a/gnu/packages/gnuzilla.scm b/gnu/packages/gnuzilla.scm
index 431b487fd0..9f6e1f24e1 100644
--- a/gnu/packages/gnuzilla.scm
+++ b/gnu/packages/gnuzilla.scm
@@ -682,18 +682,8 @@ in C/C++.")
    ("1j6l66v1xw27z8w78mpsnmqgv8m277mf4r0hgqcrb4zx7xc2vqyy" "527e5e090608" "zh-CN")
    ("1frwx35klpyz3sdwrkz7945ivb2dwaawhhyfnz4092h9hn7rc4ky" "6cd366ad2947" "zh-TW")))
 
-(define* (computed-origin-method gexp-promise hash-algo hash
-                                 #:optional (name "source")
-                                 #:key (system (%current-system))
-                                 (guile (default-guile)))
-  "Return a derivation that executes the G-expression that results
-from forcing GEXP-PROMISE."
-  (mlet %store-monad ((guile (package->derivation guile system)))
-    (gexp->derivation (or name "computed-origin")
-                      (force gexp-promise)
-                      #:graft? #f       ;nothing to graft
-                      #:system system
-                      #:guile-for-build guile)))
+;; XXXX: Workaround 'snippet' limitations.
+(define computed-origin-method (@@ (guix packages) computed-origin-method))
 
 (define %icecat-version "78.14.0-guix0-preview1")
 (define %icecat-build-id "20210907000000") ;must be of the form YYYYMMDDhhmmss
diff --git a/gnu/packages/linux.scm b/gnu/packages/linux.scm
index 41b6dfd022..a853b9c2fc 100644
--- a/gnu/packages/linux.scm
+++ b/gnu/packages/linux.scm
@@ -215,18 +215,8 @@ defconfig.  Return the appropriate make target if applicable, otherwise return
           (file-name (string-append "linux-libre-deblob-check-" version "-" gnu-revision))
           (sha256 deblob-check-hash))))
 
-(define* (computed-origin-method gexp-promise hash-algo hash
-                                 #:optional (name "source")
-                                 #:key (system (%current-system))
-                                 (guile (default-guile)))
-  "Return a derivation that executes the G-expression that results
-from forcing GEXP-PROMISE."
-  (mlet %store-monad ((guile (package->derivation guile system)))
-    (gexp->derivation (or name "computed-origin")
-                      (force gexp-promise)
-                      #:graft? #f       ;nothing to graft
-                      #:system system
-                      #:guile-for-build guile)))
+;; XXXX: Workaround 'snippet' limitations
+(define computed-origin-method (@@ (guix packages) computed-origin-method))
 
 (define (make-linux-libre-source version
                                  upstream-source
diff --git a/guix/packages.scm b/guix/packages.scm
index ad7937b4fb..8c3a0b0b7b 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
-;;; Copyright © 2014, 2015, 2017, 2018 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2014, 2015, 2017, 2018, 2019 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015 Eric Bavier <bavier@member.fsf.org>
 ;;; Copyright © 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2017, 2019, 2020 Efraim Flashner <efraim@flashner.co.il>
@@ -344,6 +344,27 @@ name of its URI."
          ;; git, svn, cvs, etc. reference
          #f))))
 
+;; Work around limitations in the 'snippet' mechanism.  It is not possible for
+;; a 'snippet' to produce a tarball with a different base name than the
+;; original downloaded source.  Moreover, cherry picking dozens of upsteam
+;; patches and applying them suddenly is often impractical; especially when a
+;; comprehensive code reformatting is done upstream.  Mainly designed for
+;; Linux and IceCat packages.
+;; XXXX: do not make part of public API (export) such radical capability
+;; before a detailed review process.
+(define* (computed-origin-method gexp-promise hash-algo hash
+                                 #:optional (name "source")
+                                 #:key (system (%current-system))
+                                 (guile (default-guile)))
+  "Return a derivation that executes the G-expression that results
+from forcing GEXP-PROMISE."
+  (mlet %store-monad ((guile (package->derivation guile system)))
+    (gexp->derivation (or name "computed-origin")
+                      (force gexp-promise)
+                      #:graft? #f       ;nothing to graft
+                      #:system system
+                      #:guile-for-build guile)))
+
 
 (define %supported-systems
   ;; This is the list of system types that are supported.  By default, we