bug#20239: [PATCH 3/4] services: qemu-binfmt: Extend guix-daemon with extra chroot directories.

[Ludovic Courtès]

Fixes <https://bugs.gnu.org/20239>.

* gnu/services/virtualization.scm (<qemu-binfmt-configuration>)[guix-support?]:
New field.
(qemu-binfmt-guix-chroot): New procedure.
(qemu-binfmt-service-type)[extensions]: Add GUIX-SERVICE-TYPE.
* doc/guix.texi (Virtualization Services): Document 'guix-support?'.
---
 doc/guix.texi                   | 29 +++++++++++++++++++++++++++++
 gnu/services/virtualization.scm | 16 ++++++++++++++--
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 5c836ae96..85d95c4da 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -17698,6 +17698,35 @@ This is the configuration for the @code{qemu-binfmt} 
service.
 The list of emulated QEMU platforms.  Each item must be a @dfn{platform
 object} as returned by @code{lookup-qemu-platforms} (see below).
 
address@hidden @code{guix-support?} (default: @code{#f})
+When it is true, QEMU and all its dependencies are added to the build
+environment of @command{guix-daemon} (@pxref{Invoking guix-daemon,
address@hidden option}).  This allows the @code{binfmt_misc}
+handlers to be used within the build environment, which in turn means
+that you can transparently build programs for another architecture.
+
+For example, let's suppose you're on an x86_64 machine and you have this
+service:
+
address@hidden
+(service qemu-binfmt-service-type
+         (qemu-binfmt-configuration
+           (platforms (lookup-qemu-platforms "arm"))
+           (qemu-support? #t)))
address@hidden example
+
+You can run:
+
address@hidden
+guix build -s armhf-linux inkscape
address@hidden example
+
address@hidden
+and it will build Inkscape for ARMv7 @emph{as if it were a native
+build}, transparently using QEMU to emulate the ARMv7 CPU.  Pretty handy
+if you'd like to test a package build for an architecture you don't have
+access to!
+
 @item @code{qemu} (default: @code{qemu})
 The QEMU package to use.
 @end table
diff --git a/gnu/services/virtualization.scm b/gnu/services/virtualization.scm
index ac6afe043..f716de622 100644
--- a/gnu/services/virtualization.scm
+++ b/gnu/services/virtualization.scm
@@ -662,7 +662,9 @@ potential infinite waits blocking libvirt."))
   (qemu        qemu-binfmt-configuration-qemu
                (default qemu))
   (platforms   qemu-binfmt-configuration-platforms
-               (default '())))                    ;safest default
+               (default '()))                     ;safest default
+  (guix-support? qemu-binfmt-configuration-guix-support?
+                 (default #f)))
 
 (define (qemu-platform->binfmt qemu platform)
   "Return a gexp that evaluates to a binfmt string for PLATFORM, using the
@@ -724,6 +726,14 @@ given QEMU package."
                                 '#$(map qemu-platform-name platforms))
                       #f)))))))
 
+(define qemu-binfmt-guix-chroot
+  (match-lambda
+    ;; Add QEMU and its dependencies to the guix-daemon chroot so that our
+    ;; binfmt_misc handlers work in the chroot (otherwise 'execve' would fail
+    ;; with ENOENT.)
+    (($ <qemu-binfmt-configuration> qemu platforms guix?)
+     (if guix? (list qemu) '()))))
+
 (define qemu-binfmt-service-type
   ;; TODO: Make a separate binfmt_misc service out of this?
   (service-type (name 'qemu-binfmt)
@@ -731,7 +741,9 @@ given QEMU package."
                  (list (service-extension file-system-service-type
                                           (const (list %binfmt-file-system)))
                        (service-extension shepherd-root-service-type
-                                          qemu-binfmt-shepherd-services)))
+                                          qemu-binfmt-shepherd-services)
+                       (service-extension guix-service-type
+                                          qemu-binfmt-guix-chroot)))
                 (default-value (qemu-binfmt-configuration))
                 (description
                  "This service supports transparent emulation of binaries
-- 
2.15.1