accounts: Add /etc/subid and /etc/subgid allocation logic.

This commit adds allocation logic for subid ranges. Subid ranges are
ranges of contiguous subids that are mapped to a user in the host
system. This patch implements a flexible allocation algorithm allowing
users that do not want (or need) to specify details of the subid ranges
that they are requesting to avoid doing so, while upholding requests of
users that need to have specific ranges.

* gnu/build/accounts.scm (%subordinate-id-min): New variable;
(%subordinate-id-max): new variable;
(%subordinate-id-count): new variable;
(subordinate-id?): new variable;
(&subordinate-id-error): new variable;
(&subordinate-id-overflow-error): new variable;
(&illegal-subid-range-error): new variable;
(&specific-subid-range-expected-error): new variable;
(&generic-subid-range-expected-error): new variable;
(within-interval?): new variable;
(allocate-unused-range): new variable;
(allocate-generic-range): new variable;
(allocate-specific-range): new variable;
(reserve-subids): new variable;
(range->entry): new variable;
(entry->range): new variable;
(allocate-subids): new variable;
(subuid+subgid-databases): new variable.

* gnu/system/accounts.scm (subid-range-end): New variable;
(subid-range-has-start?): new variable;
(subid-range-less): new variable.

* test/accounts.scm: Test them.

Change-Id: I8de1fd7cfe508b9c76408064d6f498471da0752d
Co-Authored-By: Ludovic Courtès <ludo@gnu.org>
Signed-off-by: Giacomo Leidi <goodoldpaul@autistici.org>
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/gnu/system/accounts.scm b/gnu/system/accounts.scm
index 9a006c188db..1b88ca301fc 100644
--- a/gnu/system/accounts.scm
+++ b/gnu/system/accounts.scm
@@ -45,6 +45,9 @@
             subid-range-name
             subid-range-start
             subid-range-count
+            subid-range-end
+            subid-range-has-start?
+            subid-range-less
 
             sexp->user-account
             sexp->user-group
@@ -102,6 +105,33 @@
                   ; find_new_sub_uids.c
                   (default 65536)))
 
+(define (subid-range-end range)
+  "Returns the last subid referenced in RANGE."
+  (and
+   (subid-range-has-start? range)
+   (+ (subid-range-start range)
+      (subid-range-count range)
+      -1)))
+
+(define (subid-range-has-start? range)
+  "Returns #t when RANGE's start is a number."
+  (number? (subid-range-start range)))
+
+(define (subid-range-less a b)
+  "Returns #t when subid range A either starts before, or is more specific
+than B.  When it is not possible to determine whether a range is more specific
+w.r.t. another range their names are compared alphabetically."
+  (define start-a (subid-range-start a))
+  (define start-b (subid-range-start b))
+  (cond ((and (not start-a) (not start-b))
+         (string< (subid-range-name a)
+                  (subid-range-name b)))
+        ((and start-a start-b)
+         (< start-a start-b))
+        (else
+         (and start-a
+              (not start-b)))))
+
 (define (default-home-directory account)
   "Return the default home directory for ACCOUNT."
   (string-append "/home/" (user-account-name account)))