packages: Add support for system-dependent inputs.

* guix/packages.scm (package-derivation)[intern]: New procedure.  Pass
  #t as the `recursive?' argument, instead of #f.
  [expand-input]: New procedure, with code formerly in the body.
  Support inputs where the input is a procedure returning a file name or
  an <origin>.
  Use `expand-input' in the body.

* tests/packages.scm ("trivial with system-dependent input"): New test.

guix/packages.scm

@@ -227,6 +227,51 @@ recursively."
 (define* (package-derivation store package
                              #:optional (system (%current-system)))
   "Return the derivation of PACKAGE for SYSTEM."
+  (define (intern file)
+    ;; Add FILE to the store.  Set the `recursive?' bit to #t, so that
+    ;; file permissions are preserved.
+    (add-to-store store (basename file)
+                  #t #t "sha256" file))
+
+  (define expand-input
+    ;; Expand the given input tuple such that it contains only
+    ;; references to derivation paths or store paths.
+    (match-lambda
+     (((? string? name) (? package? package))
+      (list name (package-derivation store package)))
+     (((? string? name) (? package? package)
+       (? string? sub-drv))
+      (list name (package-derivation store package)
+            sub-drv))
+     (((? string? name)
+       (and (? string?) (? derivation-path?) drv))
+      (list name drv))
+     (((? string? name)
+       (and (? string?) (? file-exists? file)))
+      ;; Add FILE to the store.  When FILE is in the sub-directory of a
+      ;; store path, it needs to be added anyway, so it can be used as a
+      ;; source.
+      (list name (intern file)))
+     (((? string? name) (? origin? source))
+      (list name (package-source-derivation store source)))
+     ((and i ((? string? name) (? procedure? proc) sub-drv ...))
+      ;; This form allows PROC to make a SYSTEM-dependent choice.
+
+      ;; XXX: Currently PROC must return a .drv, a store path, a local
+      ;; file name, or an <origin>.  If it were allowed to return a
+      ;; package, then `transitive-inputs' and co. would need to be
+      ;; adjusted.
+      (let ((input (proc system)))
+        (if (or (string? input) (origin? input))
+            (expand-input (cons* name input sub-drv))
+            (raise (condition (&package-input-error
+                               (package package)
+                               (input   i)))))))
+     (x
+      (raise (condition (&package-input-error
+                         (package package)
+                         (input   x)))))))
+
   (or (cached-derivation package system)
 
       ;; Compute the derivation and cache the result.  Caching is
@@ -241,31 +286,7 @@ recursively."
              outputs)
           ;; TODO: For `search-paths', add a builder prologue that calls
           ;; `set-path-environment-variable'.
-          (let ((inputs (map (match-lambda
-                              (((? string? name) (? package? package))
-                               (list name (package-derivation store package)))
-                              (((? string? name) (? package? package)
-                                (? string? sub-drv))
-                               (list name (package-derivation store package)
-                                     sub-drv))
-                              (((? string? name)
-                                (and (? string?) (? derivation-path?) drv))
-                               (list name drv))
-                              (((? string? name)
-                                (and (? string?) (? file-exists? file)))
-                               ;; Add FILE to the store.  When FILE is in the
-                               ;; sub-directory of a store path, it needs to be
-                               ;; added anyway, so it can be used as a source.
-                               (list name
-                                     (add-to-store store (basename file)
-                                                   #t #f "sha256" file)))
-                              (((? string? name) (? origin? source))
-                               (list name
-                                     (package-source-derivation store source)))
-                              (x
-                               (raise (condition (&package-input-error
-                                                  (package package)
-                                                  (input   x))))))
+          (let ((inputs (map expand-input
                              (package-transitive-inputs package))))
 
             (apply builder

tests/packages.scm

@@ -95,6 +95,25 @@
            (equal? '(hello guix)
                    (call-with-input-file (string-append p "/test") read))))))
 
+(test-assert "trivial with system-dependent input"
+  (let* ((p (package (inherit (dummy-package "trivial-system-dependent-input"))
+              (build-system trivial-build-system)
+              (source #f)
+              (arguments
+               `(#:guile ,%bootstrap-guile
+                 #:builder
+                 (let ((out  (assoc-ref %outputs "out"))
+                       (bash (assoc-ref %build-inputs "bash")))
+                   (zero? (system* bash "-c"
+                                   (format #f "echo hello > ~a" out))))))
+              (inputs `(("bash" ,(lambda (system)
+                                   (search-bootstrap-binary "bash"
+                                                            system)))))))
+         (d (package-derivation %store p)))
+    (and (build-derivations %store (list d))
+         (let ((p (pk 'drv d (derivation-path->output-path d))))
+           (eq? 'hello (call-with-input-file p read))))))
+
 (test-assert "GNU Hello"
   (let ((hello (package-with-explicit-inputs hello %bootstrap-inputs
                                              #:guile %bootstrap-guile)))