services: qemu-binfmt: Extend guix-daemon with extra chroot directories.

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?'.
(Additional Build Options): Mention binfmt_misc and offloading under
'--system'.

doc/guix.texi

@@ -5621,11 +5621,26 @@ The following derivations will be built:
 Attempt to build for @var{system}---e.g., @code{i686-linux}---instead of
 the system type of the build host.
 
+@quotation Note
+The @code{--system} flag is for @emph{native} compilation and must not
+be confused with cross-compilation.  See @code{--target} below for
+information on cross-compilation.
+@end quotation
+
 An example use of this is on Linux-based systems, which can emulate
 different personalities.  For instance, passing
-@code{--system=i686-linux} on an @code{x86_64-linux} system allows users
+@code{--system=i686-linux} on an @code{x86_64-linux} system allows you
 to build packages in a complete 32-bit environment.
 
+Similarly, when transparent emulation with QEMU and @code{binfmt_misc}
+is enabled (@pxref{Virtualization Services,
+@code{qemu-binfmt-service-type}}), you can build for any system for
+which a QEMU @code{binfmt_misc} handler is installed.
+
+Builds for a system other than that of the machine you are using can
+also be offloaded to a remote machine of the right architecture.
+@xref{Daemon Offload Setup}, for more information on offloading.
+
 @item --target=@var{triplet}
 @cindex cross-compilation
 Cross-build for @var{triplet}, which must be a valid GNU triplet, such
@@ -17698,6 +17713,35 @@ 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).
 
+@item @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,
+@code{--chroot-directory} 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:
+
+@example
+(service qemu-binfmt-service-type
+         (qemu-binfmt-configuration
+           (platforms (lookup-qemu-platforms "arm"))
+           (qemu-support? #t)))
+@end example
+
+You can run:
+
+@example
+guix build -s armhf-linux inkscape
+@end example
+
+@noindent
+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

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 +726,19 @@ 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.)
+    ;;
+    ;; The 'F' flag of binfmt_misc is meant to address this problem by loading
+    ;; the interpreter upfront rather than lazily, but apparently that is
+    ;; insufficient (perhaps it loads the 'qemu-ARCH' binary upfront but looks
+    ;; up its dependencies lazily?).
+    (($ <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)
@@ -732,7 +747,9 @@ given QEMU package."
                                           (const
                                            (list %binary-format-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