pack: Prevent duplicate files in tar archives.

Tar translate duplicate files in the archive into hard links.  These can cause
problems, as not every tool support them; for example dpkg doesn't.

* gnu/system/file-systems.scm (reduce-directories): New procedure.
(file-prefix?): Lift the restriction on file prefix.  The procedure can be
useful for comparing relative file names.  Adjust doc.
(file-name-depth): New procedure, extracted from ...
(btrfs-store-subvolume-file-name): ... here.
* guix/scripts/pack.scm (self-contained-tarball/builder): Use
reduce-directories.
* tests/file-systems.scm ("reduce-directories"): New test.

gnu/system/file-systems.scm

@@ -55,6 +55,7 @@ (define-module (gnu system file-systems)
             file-system-dependencies
             file-system-location
 
+            reduce-directories
             file-system-type-predicate
             btrfs-subvolume?
             btrfs-store-subvolume-file-name
@@ -231,8 +232,8 @@ (define %not-slash
   (char-set-complement (char-set #\/)))
 
 (define (file-prefix? file1 file2)
-  "Return #t if FILE1 denotes the name of a file that is a parent of FILE2,
-where both FILE1 and FILE2 are absolute file name.  For example:
+  "Return #t if FILE1 denotes the name of a file that is a parent of FILE2.
+For example:
 
   (file-prefix? \"/gnu\" \"/gnu/store\")
   => #t
@@ -240,8 +241,6 @@ (define (file-prefix? file1 file2)
   (file-prefix? \"/gn\" \"/gnu/store\")
   => #f
 "
-  (and (string-prefix? "/" file1)
-       (string-prefix? "/" file2)
   (let loop ((file1 (string-tokenize file1 %not-slash))
              (file2 (string-tokenize file2 %not-slash)))
     (match file1
@@ -252,7 +251,31 @@ (define (file-prefix? file1 file2)
          ((head2 tail2 ...)
           (and (string=? head1 head2) (loop tail1 tail2)))
          (()
-               #f)))))))
+          #f))))))
+
+(define (file-name-depth file-name)
+  (length (string-tokenize file-name %not-slash)))
+
+(define (reduce-directories file-names)
+  "Eliminate entries in FILE-NAMES that are children of other entries in
+FILE-NAMES.  This is for example useful when passing a list of files to GNU
+tar, which would otherwise descend into each directory passed and archive the
+duplicate files as hard links, which can be undesirable."
+  (let* ((file-names/sorted
+          ;; Ascending sort by file hierarchy depth, then by file name length.
+          (stable-sort (delete-duplicates file-names)
+                       (lambda (f1 f2)
+                         (let ((depth1 (file-name-depth f1))
+                               (depth2 (file-name-depth f2)))
+                           (if (= depth1 depth2)
+                               (string< f1 f2)
+                               (< depth1 depth2)))))))
+    (reverse (fold (lambda (file-name results)
+                     (if (find (cut file-prefix? <> file-name) results)
+                         results        ;parent found -- skipping
+                         (cons file-name results)))
+                   '()
+                   file-names/sorted))))
 
 (define* (file-system-device->string device #:key uuid-type)
   "Return the string representations of the DEVICE field of a <file-system>
@@ -624,9 +647,6 @@ (define (prepend-slash/maybe s)
         s
         (string-append "/" s)))
 
-  (define (file-name-depth file-name)
-    (length (string-tokenize file-name %not-slash)))
-
   (and-let* ((btrfs-subvolume-fs (filter btrfs-subvolume? file-systems))
              (btrfs-subvolume-fs*
               (sort btrfs-subvolume-fs

guix/scripts/pack.scm

@@ -230,13 +230,15 @@ (define (import-module? module)
                           `((guix build pack)
                             (guix build utils)
                             (guix build union)
-                            (gnu build install))
+                            (gnu build install)
+                            (gnu system file-systems))
                           #:select? import-module?)
     #~(begin
         (use-modules (guix build pack)
                      (guix build utils)
                      ((guix build union) #:select (relative-file-name))
                      (gnu build install)
+                     ((gnu system file-systems) #:select (reduce-directories))
                      (srfi srfi-1)
                      (srfi srfi-26)
                      (ice-9 match))
@@ -303,7 +305,7 @@ (define tar #+(file-append archiver "/bin/tar"))
 
                    ,(string-append "." (%store-directory))
 
-                   ,@(delete-duplicates
+                   ,@(reduce-directories
                       (filter-map (match-lambda
                                     (('directory directory)
                                      (string-append "." directory))

tests/file-systems.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2015, 2017 Ludovic Courtès <ludo@gnu.org>
-;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
+;;; Copyright © 2020, 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -50,6 +50,11 @@ (define-module (test-file-systems)
                    (device "/foo")
                    (flags '(bind-mount read-only)))))))))
 
+(test-equal "reduce-directories"
+  '("./opt/gnu/" "./opt/gnuism" "a/b/c")
+  (reduce-directories '("./opt/gnu/etc" "./opt/gnu/" "./opt/gnu/bin"
+                        "./opt/gnu/lib/debug" "./opt/gnuism" "a/b/c" "a/b/c")))
+
 (test-assert "does not pull (guix config)"
   ;; This module is meant both for the host side and "build side", so make
   ;; sure it doesn't pull in (guix config), which depends on the user's