guix hash: Extract file hashing procedures.

* guix/scripts/hash.scm (guix-hash)[vcs-file?] (nar-hash, default-hash):
  Extract hashing logic to...
* guix/hash.scm (vcs-file?, file-hash*): ... these new procedures in this
  new file.

Modified-by: Maxime Devos <maximedevos@telenet.be>
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

Makefile.am

@@ -100,6 +100,7 @@ MODULES =					\
   guix/extracting-download.scm			\
   guix/git-download.scm				\
   guix/hg-download.scm				\
+  guix/hash.scm					\
   guix/swh.scm					\
   guix/monads.scm				\
   guix/monad-repl.scm				\

guix/hash.scm

@@ -0,0 +1,73 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
+;;; Copyright © 2022 Maxime Devos <maximedevos@telenet.be>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix hash)
+  #:use-module (gcrypt hash)
+  #:use-module (guix serialization)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-11)
+  #:export (vcs-file?
+            file-hash*))
+
+(define (vcs-file? file stat)
+  "Returns true if FILE is a version control system file."
+  (case (stat:type stat)
+    ((directory)
+     (member (basename file) '(".bzr" ".git" ".hg" ".svn" "CVS")))
+    ((regular)
+     ;; Git sub-modules have a '.git' file that is a regular text file.
+     (string=? (basename file) ".git"))
+    (else
+     #f)))
+
+(define* (file-hash* file #:key
+                     (algorithm (hash-algorithm sha256))
+                     (recursive? 'auto)
+                     (select? (negate vcs-file?)))
+  "Compute the hash of FILE with ALGORITHM.
+
+Symbolic links are only dereferenced if RECURSIVE? is false.
+Directories are only supported if RECURSIVE? is #true or 'auto'.
+The executable bit is only recorded if RECURSIVE? is #true.
+If FILE is a symbolic link, it is only followed if RECURSIVE? is false.
+
+For regular files, there are two different hashes when the executable
+hash isn't recorded: the regular hash and the nar hash. In most situations,
+the regular hash is desired and setting RECURSIVE? to 'auto' does the right
+thing for both regular files and directories.
+
+This procedure must only be used under controlled circumstances;
+the detection of symbolic links in FILE is racy.
+
+When FILE is a directory, the procedure SELECT? called as (SELECT? FILE STAT)
+decides which files to include. By default, version control files are
+excluded. To include everything, SELECT? can be set to (const #true)."
+  (if (or (eq? recursive? #true)
+          (and (eq? recursive? 'auto)
+               ;; Don't change this to (eq? 'directory ...), because otherwise
+               ;; if 'file' denotes a symbolic link, the 'file-hash' below
+               ;; would dereference it -- dereferencing symbolic links would
+               ;; open an avoidable can of potential worms.
+               (not (eq? 'regular (stat:type (lstat file))))))
+      (let-values (((port get-hash)
+                    (open-hash-port algorithm)))
+        (write-file file port #:select? select?)
+        (force-output port)
+        (get-hash))
+      (file-hash algorithm file)))

guix/scripts/hash.scm

@@ -4,6 +4,7 @@
 ;;; Copyright © 2016 Jan Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2018 Tim Gesthuizen <tim.gesthuizen@yahoo.de>
 ;;; Copyright © 2021 Simon Tournier <zimon.toutoune@gmail.com>
+;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -24,6 +25,7 @@
   #:use-module (gcrypt hash)
   #:use-module (guix serialization)
   #:use-module (guix ui)
+  #:use-module (guix hash)
   #:use-module (guix scripts)
   #:use-module (guix base16)
   #:use-module (guix base32)
@@ -46,20 +48,14 @@
 (define* (nar-hash file #:optional
                    (algorithm (assoc-ref %default-options 'hash-algorithm))
                    select?)
-  (let-values (((port get-hash)
-                (open-hash-port algorithm)))
-    (write-file file port #:select? select?)
-    (force-output port)
-    (get-hash)))
+  (file-hash* file #:algorithm algorithm #:select? select? #:recursive? #true))
 
 (define* (default-hash file #:optional
                        (algorithm (assoc-ref %default-options 'hash-algorithm))
                        select?)
   (match file
     ("-" (port-hash algorithm (current-input-port)))
-    (_
-     (call-with-input-file file
-       (cute port-hash algorithm <>)))))
+    (_ (file-hash* file #:algorithm algorithm #:recursive? #false))))
 
 (define* (git-hash file #:optional
                        (algorithm (assoc-ref %default-options 'hash-algorithm))
@@ -181,16 +177,6 @@ use '--serializer' instead~%"))
     (parse-command-line args %options (list %default-options)
                         #:build-options? #f))
 
-  (define (vcs-file? file stat)
-    (case (stat:type stat)
-      ((directory)
-       (member (basename file) '(".bzr" ".git" ".hg" ".svn" "CVS")))
-      ((regular)
-       ;; Git sub-modules have a '.git' file that is a regular text file.
-       (string=? (basename file) ".git"))
-      (else
-       #f)))
-
   (let* ((opts (parse-options))
          (args (filter-map (match-lambda
                             (('argument . value)