file-systems: 'file-system-needed-for-boot?' is #t for parents of the store.

Suggested by John Darrington <john@darrington.wattle.id.au>.

* gnu/system/file-systems.scm (%not-slash): New variable.
(file-prefix?): New procedure.
(file-system-needed-for-boot?): Use it to check whether FS holds the
store.
* tests/file-systems.scm ("file-system-needed-for-boot?"): New test.
* gnu/tests/install.scm (%separate-store-os)[file-systems]: Remove
'needed-for-boot?' field for "/gnu".

gnu/system/file-systems.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -95,11 +95,39 @@
   (dependencies     file-system-dependencies      ; list of <file-system>
                     (default '())))               ; or <mapped-device>
 
-(define-inlinable (file-system-needed-for-boot? fs)
-  "Return true if FS has the 'needed-for-boot?' flag set, or if it's the root
-file system."
+(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:
+
+  (file-prefix? \"/gnu\" \"/gnu/store\")
+  => #t
+
+  (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
+           (()
+            #t)
+           ((head1 tail1 ...)
+            (match file2
+              ((head2 tail2 ...)
+               (and (string=? head1 head2) (loop tail1 tail2)))
+              (()
+               #f)))))))
+
+(define (file-system-needed-for-boot? fs)
+  "Return true if FS has the 'needed-for-boot?' flag set, or if it holds the
+store--e.g., if FS is the root file system."
   (or (%file-system-needed-for-boot? fs)
-      (string=? "/" (file-system-mount-point fs))))
+      (and (file-prefix? (file-system-mount-point fs) (%store-prefix))
+           (not (memq 'bind-mount (file-system-flags fs))))))
 
 (define (file-system->spec fs)
   "Return a list corresponding to file-system FS that can be passed to the

gnu/tests/install.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -257,8 +257,7 @@ build (current-guix) and then store a couple of full system images.")
                            (device "store-fs")
                            (title 'label)
                            (mount-point "/gnu")
-                           (type "ext4")
-                           (needed-for-boot? #t)) ;definitely!
+                           (type "ext4"))
                          %base-file-systems))
     (users %base-user-accounts)
     (services (cons (service marionette-service-type

tests/file-systems.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -17,6 +17,7 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (test-file-systems)
+  #:use-module (guix store)
   #:use-module (gnu system file-systems)
   #:use-module (srfi srfi-64)
   #:use-module (rnrs bytevectors))
@@ -50,4 +51,25 @@
            (string-contains message "invalid UUID")
            (equal? form '(uuid "foobar"))))))
 
+(test-assert "file-system-needed-for-boot?"
+  (let-syntax ((dummy-fs (syntax-rules ()
+                           ((_ directory)
+                            (file-system
+                              (device "foo")
+                              (mount-point directory)
+                              (type "ext4"))))))
+    (parameterize ((%store-prefix "/gnu/guix/store"))
+      (and (file-system-needed-for-boot? (dummy-fs "/"))
+           (file-system-needed-for-boot? (dummy-fs "/gnu"))
+           (file-system-needed-for-boot? (dummy-fs "/gnu/guix"))
+           (file-system-needed-for-boot? (dummy-fs "/gnu/guix/store"))
+           (not (file-system-needed-for-boot?
+                 (dummy-fs "/gnu/guix/store/foo")))
+           (not (file-system-needed-for-boot? (dummy-fs "/gn")))
+           (not (file-system-needed-for-boot?
+                 (file-system
+                   (inherit (dummy-fs (%store-prefix)))
+                   (device "/foo")
+                   (flags '(bind-mount read-only)))))))))
+
 (test-end)