channels: Build channel modules in an inferior.

This ensures that channel modules are compiled with the right Guile,
that they get to see the right modules, and so on.  IOW, it avoids bugs
such as those addressed by commits
3c0e1639 and
cb341c12.

* guix/channels.scm (standard-module-derivation): Add 'core'
parameter.  Rewrite in terms of 'gexp->derivation-in-inferior'.
(build-from-source): Add #:core parameter and pass it to
'standard-module-derivation'.
(build-channel-instance): Add 'core' parameter and pass it on.
(channel-instance-derivations)[dependencies]: Remove.
Adjust 'build-channel-instance' call.

guix/channels.scm

@@ -218,45 +218,48 @@ (define %pull-version
   ;; place a set of compiled Guile modules in ~/.config/guix/latest.
   1)
 
-(define (standard-module-derivation name source dependencies)
-  "Return a derivation that builds the Scheme modules in SOURCE and that
-depend on DEPENDENCIES, a list of lowerable objects.  The assumption is that
-SOURCE contains package modules to be added to '%package-module-path'."
-  (define modules
-    (scheme-modules* source))
-
+(define (standard-module-derivation name source core dependencies)
+  "Return a derivation that builds with CORE, a Guix instance, the Scheme
+modules in SOURCE and that depend on DEPENDENCIES, a list of lowerable
+objects.  The assumption is that SOURCE contains package modules to be added
+to '%package-module-path'."
   ;; FIXME: We should load, say SOURCE/.guix-channel.scm, which would allow
   ;; channel publishers to specify things such as the sub-directory where .scm
   ;; files live, files to exclude from the channel, preferred substitute URLs,
   ;; etc.
-  (mlet* %store-monad ((compiled
-                        (compiled-modules modules
-                                          #:name name
-                                          #:module-path (list source)
-                                          #:extensions dependencies)))
-
-    (gexp->derivation name
-                      (with-extensions dependencies
-                        (with-imported-modules '((guix build utils))
-                          #~(begin
-                              (use-modules (guix build utils))
-
-                              (let ((go  (string-append #$output "/lib/guile/"
-                                                        (effective-version)
-                                                        "/site-ccache"))
-                                    (scm (string-append #$output
-                                                        "/share/guile/site/"
-                                                        (effective-version))))
-                                (mkdir-p (dirname go))
-                                (symlink #$compiled go)
-                                (mkdir-p (dirname scm))
-                                (symlink #$source scm))))))))
+
+  (define build
+    ;; This is code that we'll run in CORE, a Guix instance, with its own
+    ;; modules and so on.  That way, we make sure these modules are built for
+    ;; the right Guile version, with the right dependencies, and that they get
+    ;; to see the right (gnu packages …) modules.
+    (with-extensions dependencies
+      #~(begin
+          (use-modules (guix build compile)
+                       (guix build utils)
+                       (srfi srfi-26))
+
+          (define go
+            (string-append #$output "/lib/guile/" (effective-version)
+                           "/site-ccache"))
+          (define scm
+            (string-append #$output "/share/guile/site/"
+                           (effective-version)))
+
+          (compile-files #$source go
+                         (find-files #$source "\\.scm$"))
+          (mkdir-p (dirname scm))
+          (symlink #$source scm)
+          scm)))
+
+  (gexp->derivation-in-inferior name build core))
 
 (define* (build-from-source name source
-                            #:key verbose? commit
+                            #:key core verbose? commit
                             (dependencies '()))
   "Return a derivation to build Guix from SOURCE, using the self-build script
-contained therein.  Use COMMIT as the version string."
+contained therein; use COMMIT as the version string.  When CORE is true, build
+package modules under SOURCE using CORE, an instance of Guix."
   ;; Running the self-build script makes it easier to update the build
   ;; procedure: the self-build script of the Guix-to-be-installed contains the
   ;; right dependencies, build procedure, etc., which the Guix-in-use may not
@@ -278,9 +281,10 @@ (define script
                #:pull-version %pull-version))
 
       ;; Build a set of modules that extend Guix using the standard method.
-      (standard-module-derivation name source dependencies)))
+      (standard-module-derivation name source core dependencies)))
 
-(define* (build-channel-instance instance #:optional (dependencies '()))
+(define* (build-channel-instance instance
+                                 #:optional core (dependencies '()))
   "Return, as a monadic value, the derivation for INSTANCE, a channel
 instance.  DEPENDENCIES is a list of extensions providing Guile modules that
 INSTANCE depends on."
@@ -288,6 +292,7 @@ (define* (build-channel-instance instance #:optional (dependencies '()))
                       (channel-name (channel-instance-channel instance)))
                      (channel-instance-checkout instance)
                      #:commit (channel-instance-commit instance)
+                     #:core core
                      #:dependencies dependencies))
 
 (define (resolve-dependencies instances)
@@ -328,17 +333,6 @@ (define core-instance
             (guix-channel? (channel-instance-channel instance)))
           instances))
 
-  (define dependencies
-    ;; Dependencies of CORE-INSTANCE.
-    ;; FIXME: It would be best not to hard-wire this information here and
-    ;; instead query it to CORE-INSTANCE.
-    (list (module-ref (resolve-interface '(gnu packages gnupg))
-                      'guile-gcrypt)
-          (module-ref (resolve-interface '(gnu packages guile))
-                      'guile-git)
-          (module-ref (resolve-interface '(gnu packages guile))
-                      'guile-bytestructures)))
-
   (define edges
     (resolve-dependencies instances))
 
@@ -348,10 +342,7 @@ (define (instance->derivation instance)
                  (mlet %store-monad ((core (instance->derivation core-instance))
                                      (deps (mapm %store-monad instance->derivation
                                                  (edges instance))))
-                   (build-channel-instance instance
-                                           (cons core
-                                                 (append deps
-                                                         dependencies)))))
+                   (build-channel-instance instance core deps)))
              instance))
 
   (mapm %store-monad instance->derivation instances))