system: Add 'locale-libcs' field.

* gnu/system/locale.scm (localedef-command)[maybe-version-directory]:
  New procedure.
  Use it.
  (locale-directory): Rename to...
  (single-locale-directory): ... this.  Check the version of LIBC to
  determine whether to create a "X.Y" sub-directory or to make it a
  symlink to ".".  Add the version number in the derivation name.
  (locale-directory): New procedure.
  (%default-locale-libcs): New variable.
* gnu/system.scm (<operating-system>)[locale-libcs]: New field.
  (operating-system-locale-directory): Pass it to 'locale-directory'.
* doc/guix.texi (operating-system Reference): Document 'locale-libcs'.
  (Locales)[Locale Data Compatibility Considerations]: New section.

2 files changed, 62 insertions, 6 deletions

diff --git a/gnu/system.scm b/gnu/system.scm
index 3d570c0d1f..8fed857b39 100644
--- a/gnu/system.scm
+++ b/gnu/system.scm
@@ -76,6 +76,7 @@
             operating-system-timezone
             operating-system-locale
             operating-system-locale-definitions
+            operating-system-locale-libcs
             operating-system-mapped-devices
             operating-system-file-systems
             operating-system-activation-script
@@ -144,6 +145,8 @@
             (default "en_US.utf8"))
   (locale-definitions operating-system-locale-definitions ; list of <locale-definition>
                       (default %default-locale-definitions))
+  (locale-libcs operating-system-locale-libcs     ; list of <packages>
+                (default %default-locale-libcs))
   (name-service-switch operating-system-name-service-switch ; <name-service-switch>
                        (default %default-nss))
 
@@ -643,7 +646,8 @@ listed in OS.  The C library expects to find it under
     (raise (condition
             (&message (message "system locale lacks a definition")))))
 
-  (locale-directory (operating-system-locale-definitions os)))
+  (locale-directory (operating-system-locale-definitions os)
+                    #:libcs (operating-system-locale-libcs os)))
 
 (define (kernel->grub-label kernel)
   "Return a label for the GRUB menu entry that boots KERNEL."
diff --git a/gnu/system/locale.scm b/gnu/system/locale.scm
index 010fb45272..e798827a01 100644
--- a/gnu/system/locale.scm
+++ b/gnu/system/locale.scm
@@ -18,11 +18,15 @@
 
 (define-module (gnu system locale)
   #:use-module (guix gexp)
+  #:use-module (guix store)
+  #:use-module (guix monads)
   #:use-module (guix records)
   #:use-module (guix packages)
+  #:use-module (guix utils)
   #:use-module (gnu packages base)
   #:use-module (gnu packages compression)
   #:use-module (srfi srfi-26)
+  #:use-module (ice-9 match)
   #:export (locale-definition
             locale-definition?
             locale-definition-name
@@ -31,6 +35,7 @@
 
             locale-directory
 
+            %default-locale-libcs
             %default-locale-definitions))
 
 ;;; Commentary:
@@ -50,6 +55,15 @@
 (define* (localedef-command locale
                             #:key (libc (canonical-package glibc)))
   "Return a gexp that runs 'localedef' from LIBC to build LOCALE."
+  (define (maybe-version-directory)
+    ;; XXX: For libc prior to 2.22, GuixSD did not store locale data in a
+    ;; version-specific sub-directory.  Check whether this is the case.
+    ;; TODO: Remove this hack once libc 2.21 is buried.
+    (let ((version (package-version libc)))
+      (if (version>=? version "2.22")
+          (list version "/")
+          '())))
+
   #~(begin
       (format #t "building locale '~a'...~%"
               #$(locale-definition-name locale))
@@ -58,20 +72,29 @@
                       "-i" #$(locale-definition-source locale)
                       "-f" #$(locale-definition-charset locale)
                       (string-append #$output "/"
-                                     #$(package-version libc) "/"
+                                     #$@(maybe-version-directory)
                                      #$(locale-definition-name locale))))))
 
-(define* (locale-directory locales
-                           #:key (libc (canonical-package glibc)))
+(define* (single-locale-directory locales
+                                  #:key (libc (canonical-package glibc)))
   "Return a directory containing all of LOCALES for LIBC compiled.
 
 Because locale data formats are incompatible when switching from one libc to
 another, locale data is put in a sub-directory named after the 'version' field
 of LIBC."
+  (define version
+    (package-version libc))
+
   (define build
     #~(begin
         (mkdir #$output)
-        (mkdir (string-append #$output "/" #$(package-version libc)))
+
+        ;; XXX: For libcs < 2.22, locale data is stored in the top-level
+        ;; directory.
+        ;; TODO: Remove this hack once libc 2.21 is buried.
+        #$(if (version>=? version "2.22")
+              #~(mkdir (string-append #$output "/" #$version))
+              #~(symlink "." (string-append #$output "/" #$version)))
 
         ;; 'localedef' executes 'gzip' to access compressed locale sources.
         (setenv "PATH" (string-append #$gzip "/bin"))
@@ -80,9 +103,38 @@ of LIBC."
          (and #$@(map (cut localedef-command <> #:libc libc)
                       locales)))))
 
-  (gexp->derivation "locale" build
+  (gexp->derivation (string-append "locale-" version) build
                     #:local-build? #t))
 
+(define* (locale-directory locales
+                           #:key (libcs %default-locale-libcs))
+  "Return a locale directory containing all of LOCALES for each libc package
+listed in LIBCS.
+
+It is useful to list more than one libc when willing to support
+already-installed packages built against a different libc since the locale
+data format changes between libc versions."
+  (match libcs
+    ((libc)
+     (single-locale-directory locales #:libc libc))
+    ((libcs ..1)
+     (mlet %store-monad ((dirs (mapm %store-monad
+                                     (lambda (libc)
+                                       (single-locale-directory locales
+                                                                #:libc libc))
+                                     libcs)))
+       (gexp->derivation "locale-multiple-versions"
+                         #~(begin
+                             (use-modules (guix build union))
+                             (union-build #$output (list #$@dirs)))
+                         #:modules '((guix build union))
+                         #:local-build? #t
+                         #:substitutable? #f)))))
+
+(define %default-locale-libcs
+  ;; The libcs for which we build locales by default.
+  (list (canonical-package glibc)))
+
 (define %default-locale-definitions
   ;; Arbitrary set of locales that are built by default.  They are here mostly
   ;; to facilitate first-time use to some people, while others may have to add