build-system/meson: Support cross-compilation.

For cross-compilation, meson needs to be passed a
‘cross file’ with information on the architecture,
CPU type, endianness and operating system, and the
name of the cross-compiler binaries.

The new module (guix build meson-configuration) has
some utilities for writing these cross files, used
by 'make-cross-file' in a G-exp.  The values for
the cross file are generated by 'make-machine-alist'
and 'make-binaries-alist'.

'make-machine-alist' and 'make-binaries-alist' live
on the host side, such that new architectures and operating
systems can be added without causing rebuilds for old
architectures.

All operating systems and targets supported by Guix are
theoretically supported, but only aarch64-linux-gnu,
powerpc64le-linux-gnu and arm-linux-gnueabihf have been
tested.  i686-linux-gnu has also been tested with a previous
version of this patch series but required some changes
to 'cross-base.scm'.

This has been tested with:

$ ./pre-inst-env guix build glib --target=TARGET

* guix/build/meson-configuration.scm
  (write-section-header): New procedure.
  (write-assignment): New procedure.
  (write-assignments): New procedure.
* guix/build-system/meson.scm
  (target-hurd?): New predicate.
  (make-machine-alist): New procedure.
  (make-binaries-alist): New procedure.
  (make-cross-file): New procedure.
  (meson-cross-build): New procedure.
  (lower)[build-inputs]: Add standard cross packages when cross-compiling.
  Do not include regular 'inputs' when cross-compiling.
  (lower)[host-inputs]: Include 'inputs' when cross-compiling.
  (lower)[target-inputs]: Add cross packages when cross-compiling.
  (lower)[build]: Call 'meson-cross-build' instead of 'cross-build'
  when cross-compiling.
  (lower)[target]: Set it.
  (lower)[private-keywords]: Do not remove #:target when cross-compiling.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

3 files changed, 247 insertions, 19 deletions

diff --git a/Makefile.am b/Makefile.am
index 7d5f6a7fa2..8f44c888e3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -234,6 +234,7 @@ MODULES =					\
   guix/build/emacs-utils.scm			\
   guix/build/java-utils.scm			\
   guix/build/lisp-utils.scm			\
+  guix/build/meson-configuration.scm		\
   guix/build/maven/java.scm			\
   guix/build/maven/plugin.scm			\
   guix/build/maven/pom.scm			\
diff --git a/guix/build-system/meson.scm b/guix/build-system/meson.scm
index 5adc0f92c8..dae0abde94 100644
--- a/guix/build-system/meson.scm
+++ b/guix/build-system/meson.scm
@@ -2,6 +2,7 @@
 ;;; Copyright © 2017 Peter Mikkelsen <petermikkelsen10@gmail.com>
 ;;; Copyright © 2018, 2019 Marius Bakke <mbakke@fastmail.com>
 ;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -30,7 +31,8 @@
   #:use-module (guix packages)
   #:use-module (ice-9 match)
   #:export (%meson-build-system-modules
-            meson-build-system))
+            meson-build-system
+            make-cross-file))
 
 ;; Commentary:
 ;;
@@ -40,6 +42,68 @@
 ;;
 ;; Code:
 
+(define (make-machine-alist triplet)
+  "Make an association list describing what should go into
+the ‘host_machine’ section of the cross file when cross-compiling
+for TRIPLET."
+  `((system . ,(cond ((target-hurd? triplet) "gnu")
+                     ((target-linux? triplet) "linux")
+                     ((target-mingw? triplet) "windows")
+                     (#t (error "meson: unknown operating system"))))
+    (cpu_family . ,(cond ((target-x86-32? triplet) "x86")
+                         ((target-x86-64? triplet) "x86_64")
+                         ((target-arm32? triplet) "arm")
+                         ((target-aarch64? triplet) "aarch64")
+                         ((target-powerpc? triplet)
+                          (if (target-64bit? triplet)
+                              "ppc64"
+                              "ppc"))
+                         (#t (error "meson: unknown architecture"))))
+    (cpu . ,(cond ((target-x86-32? triplet) ; i386, ..., i686
+                   (substring triplet 0 4))
+                  ((target-x86-64? triplet) "x86_64")
+                  ((target-aarch64? triplet) "armv8-a")
+                  ((target-arm32? triplet) "armv7")
+                  ;; According to #mesonbuild on OFTC, there does not appear
+                  ;; to be an official-ish list of CPU types recognised by
+                  ;; Meson, the "cpu" field is not used by Meson itself and
+                  ;; most software doesn't look at this field, except perhaps
+                  ;; for selecting optimisations, so set it to something
+                  ;; arbitrary.
+                  (#t "strawberries")))
+    (endian . ,(cond ((string-prefix? "powerpc64le-" triplet) "little")
+                     ((string-prefix? "mips64el-" triplet) "little")
+                     ((target-x86-32? triplet) "little")
+                     ((target-x86-64? triplet) "little")
+                     ;; At least in Guix.  Aarch64 and 32-bit arm
+                     ;; have a big-endian mode as well.
+                     ((target-arm? triplet) "little")
+                     (#t (error "meson: unknown architecture"))))))
+
+(define (make-binaries-alist triplet)
+  "Make an associatoin list describing what should go into
+the ‘binaries’ section of the cross file when cross-compiling for
+TRIPLET."
+  `((c . ,(cc-for-target triplet))
+    (cpp . ,(cxx-for-target triplet))
+    (pkgconfig . ,(pkg-config-for-target triplet))
+    (objcopy . ,(string-append triplet "-objcopy"))
+    (ar . ,(string-append triplet "-ar"))
+    (ld . ,(string-append triplet "-ld"))
+    (strip . ,(string-append triplet "-strip"))))
+
+(define (make-cross-file triplet)
+  (computed-file "cross-file"
+    (with-imported-modules '((guix build meson-configuration))
+      #~(begin
+          (use-modules (guix build meson-configuration))
+          (call-with-output-file #$output
+            (lambda (port)
+              (write-section-header port "host_machine")
+              (write-assignments port '#$(make-machine-alist triplet))
+              (write-section-header port "binaries")
+              (write-assignments port '#$(make-binaries-alist triplet))))))))
+
 (define %meson-build-system-modules
   ;; Build-side modules imported by default.
   `((guix build meson-build-system)
@@ -68,24 +132,34 @@
                 #:rest arguments)
   "Return a bag for NAME."
   (define private-keywords
-    `(#:meson #:ninja #:inputs #:native-inputs #:outputs #:target))
-
-  (and (not target) ;; TODO: add support for cross-compilation.
-       (bag
-         (name name)
-         (system system)
-         (build-inputs `(("meson" ,meson)
-                         ("ninja" ,ninja)
-                         ,@native-inputs
-                         ,@inputs
-                         ;; Keep the standard inputs of 'gnu-build-system'.
-                         ,@(standard-packages)))
-         (host-inputs (if source
-                          `(("source" ,source))
-                          '()))
-         (outputs outputs)
-         (build meson-build)
-         (arguments (strip-keyword-arguments private-keywords arguments)))))
+    `(#:meson #:ninja #:inputs #:native-inputs #:outputs
+      ,@(if target
+            '()
+            '(#:target))))
+
+  (bag
+    (name name)
+    (system system) (target target)
+    (build-inputs `(("meson" ,meson)
+                    ("ninja" ,ninja)
+                    ,@native-inputs
+                    ,@(if target '() inputs)
+                    ;; Keep the standard inputs of 'gnu-build-system'.
+                    ,@(if target
+                          (standard-cross-packages target 'host)
+                          '())
+                    ,@(standard-packages)))
+    (host-inputs `(,@(if source
+                         `(("source" ,source))
+                         '())
+                   ,@(if target inputs '())))
+    ;; Keep the standard inputs of 'gnu-buid-system'.
+    (target-inputs (if target
+                       (standard-cross-packages target 'target)
+                       '()))
+    (outputs outputs)
+    (build (if target meson-cross-build meson-build))
+    (arguments (strip-keyword-arguments private-keywords arguments))))
 
 (define* (meson-build name inputs
                       #:key
@@ -161,6 +235,103 @@ has a 'meson.build' file."
                       #:disallowed-references disallowed-references
                       #:guile-for-build guile)))
 
+(define* (meson-cross-build name
+                            #:key
+                            target
+                            build-inputs host-inputs target-inputs
+                            guile source
+                            (outputs '("out"))
+                            (configure-flags ''())
+                            (search-paths '())
+                            (native-search-paths '())
+
+                            (build-type "debugoptimized")
+                            (tests? #f)
+                            (test-target "test")
+                            (glib-or-gtk? #f)
+                            (parallel-build? #t)
+                            (parallel-tests? #f)
+                            (validate-runpath? #t)
+                            (patch-shebangs? #t)
+                            (strip-binaries? #t)
+                            (strip-flags ''("--strip-debug"))
+                            (strip-directories ''("lib" "lib64" "libexec"
+                                                  "bin" "sbin"))
+                            (elf-directories ''("lib" "lib64" "libexec"
+                                                "bin" "sbin"))
+                            ;; See 'gnu-cross-build' for why this needs to be
+                            ;; disabled when cross-compiling.
+                            (make-dynamic-linker-cache? #f)
+                            (phases '%standard-phases)
+                            (system (%current-system))
+                            (imported-modules %meson-build-system-modules)
+                            (modules '((guix build meson-build-system)
+                                       (guix build utils)))
+                            allowed-references
+                            disallowed-references)
+  "Cross-build SOURCE for TARGET using MESON, and with INPUTS, assuming that
+SOURCE has a 'meson.build' file."
+  (define cross-file
+    (make-cross-file target))
+  (define inputs
+    (if (null? target-inputs)
+        (input-tuples->gexp host-inputs)
+        #~(append #$(input-tuples->gexp host-inputs)
+                  #+(input-tuples->gexp target-inputs))))
+  (define builder
+    (with-imported-modules imported-modules
+      #~(begin
+          (use-modules #$@(sexp->gexp modules))
+
+          (define build-phases
+            #$(let ((phases (if (pair? phases) (sexp->gexp phases) phases)))
+                (if glib-or-gtk?
+                    phases
+                    #~(modify-phases #$phases
+                        (delete 'glib-or-gtk-compile-schemas)
+                        (delete 'glib-or-gtk-wrap)))))
+
+          ;; Do not use 'with-build-variables', as there should be
+          ;; no reason to use %build-inputs and friends.
+          (meson-build #:source #+source
+                       #:system #$system
+                       #:build #$(nix-system->gnu-triplet system)
+                       #:target #$target
+                       #:outputs #$(outputs->gexp outputs)
+                       #:inputs #$inputs
+                       #:native-inputs #+(input-tuples->gexp build-inputs)
+                       #:search-paths '#$(sexp->gexp
+                                          (map search-path-specification->sexp
+                                                     search-paths))
+                       #:native-search-paths '#$(sexp->gexp
+                                                 (map search-path-specification->sexp
+                                                      native-search-paths))
+                       #:phases build-phases
+                       #:make-dynamic-linker-cache? #$make-dynamic-linker-cache?
+                       #:configure-flags `("--cross-file" #+cross-file
+                                           ,@#$(sexp->gexp configure-flags))
+                       #:build-type #$build-type
+                       #:tests? #$tests?
+                       #:test-target #$test-target
+                       #:parallel-build? #$parallel-build?
+                       #:parallel-tests? #$parallel-tests?
+                       #:validate-runpath? #$validate-runpath?
+                       #:patch-shebangs? #$patch-shebangs?
+                       #:strip-binaries? #$strip-binaries?
+                       #:strip-flags #$(sexp->gexp strip-flags)
+                       #:strip-directories #$(sexp->gexp strip-directories)
+                       #:elf-directories #$(sexp->gexp elf-directories)))))
+
+  (mlet %store-monad ((guile (package->derivation (or guile (default-guile))
+                                                  system #:graft? #f)))
+    (gexp->derivation name builder
+                      #:system system
+                      #:target target
+                      #:substitutable? substitutable?
+                      #:allowed-references allowed-references
+                      #:disallowed-references disallowed-references
+                      #:guile-for-build guile)))
+
 (define meson-build-system
   (build-system
     (name 'meson)
diff --git a/guix/build/meson-configuration.scm b/guix/build/meson-configuration.scm
new file mode 100644
index 0000000000..1aac5f8f0a
--- /dev/null
+++ b/guix/build/meson-configuration.scm
@@ -0,0 +1,56 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix build meson-configuration)
+  #:use-module (ice-9 match)
+  #:export (write-section-header write-assignment write-assignments))
+
+;; Commentary:
+;;
+;; Utilities for generating a ‘Cross build definition file’ for
+;; the Meson build system.  Configuration values are currently
+;; never escaped.  In practice this is unlikely to be a problem
+;; in the build environment.
+;;
+;; Code:
+
+(define (write-section-header port section-name)
+  "Write a section header for a section named SECTION-NAME to PORT."
+  (format port "[~a]~%" section-name))
+
+(define (write-assignment port key value)
+  "Write an assignment of VALUE to KEY to PORT.
+
+VALUE must be a string (without any special characters such as quotes),
+a boolean or an integer.  Lists are currently not supported"
+  (match value
+    ((? string?)
+     (format port "~a = '~a'~%" key value))
+    ((? integer?)
+     (format port "~a = ~a~%" key value))
+    (#f
+     (format port "~a = true~%" key))
+    (#t
+     (format port "~a = false~%" key))))
+
+(define* (write-assignments port alist)
+  "Write the assignments in ALIST, an association list, to PORT."
+  (for-each (match-lambda
+              ((key . value)
+               (write-assignment port key value)))
+            alist))