derivations: Introduce 'read-derivation-from-file'.

This avoids the open/fstat/close syscalls upon a cache hit that we had
with the previous idiom:

  (call-with-input-file file read-derivation)

where caching happened in 'read-derivation' itself.

* guix/derivations.scm (%read-derivation): Rename to...
(read-derivation): ... this.
(read-derivation-from-file): New procedure.
(derivation-prerequisites, substitution-oracle)
(derivation-prerequisites-to-build):
(derivation-path->output-path, derivation-path->output-paths):
(derivation-path->base16-hash, map-derivation): Use
'read-derivation-from-file' instead of (call-with-input-file …
read-derivation).
* guix/grafts.scm (item->deriver): Likewise.
* guix/scripts/build.scm (log-url, options->things-to-build): Likewise.
* guix/scripts/graph.scm (file->derivation): Remove.
(derivation-dependencies, %derivation-node-type): Use
'read-derivation-from-file' instead.
* guix/scripts/offload.scm (guix-offload): Likewise.
* guix/scripts/perform-download.scm (guix-perform-download): Likewise.
* guix/scripts/publish.scm (load-derivation): Remove.
(narinfo-string): Use 'read-derivation-from-file'.

1 files changed, 2 insertions, 6 deletions

diff --git a/guix/scripts/graph.scm b/guix/scripts/graph.scm
index 0af1fa3ad3..d5be442884 100644
--- a/guix/scripts/graph.scm
+++ b/guix/scripts/graph.scm
@@ -221,15 +221,11 @@ GNU-BUILD-SYSTEM have zero dependencies."
 ;;; Derivation DAG.
 ;;;
 
-(define (file->derivation file)
-  "Read the derivation from FILE and return it."
-  (call-with-input-file file read-derivation))
-
 (define (derivation-dependencies obj)
   "Return the <derivation> objects and store items corresponding to the
 dependencies of OBJ, a <derivation> or store item."
   (if (derivation? obj)
-      (append (map (compose file->derivation derivation-input-path)
+      (append (map (compose read-derivation-from-file derivation-input-path)
                    (derivation-inputs obj))
               (derivation-sources obj))
       '()))
@@ -263,7 +259,7 @@ a plain store file."
               ((? derivation-path? item)
                (mbegin %store-monad
                  ((store-lift add-temp-root) item)
-                 (return (list (file->derivation item)))))
+                 (return (list (read-derivation-from-file item)))))
               (x
                (raise
                 (condition (&message (message "unsupported argument for \