Subject: [bug#71639] [PATCH v2 2/5] services: backup: Add password-command support to restic-service

From: Richard Sent <[email-masked]>
Date: Wed, 19 Jun 2024 23:44:13 -0400

* gnu/services/backup.scm (restic-backup-job): Add password-command.
(verify-restic-backup-job-configuration): Create.
(restic-backup-job-program): Set either RESTIC_PASSWORD or
RESTIC_PASSWORD_COMMAND depending on what is configured.
* doc/guix.texi (Miscellaneous Services): Document it.

Change-Id: Ice9cf85d1ee4485a2737f515c63c969918219df0
---
 doc/guix.texi           |  7 +++++++
 gnu/services/backup.scm | 41 ++++++++++++++++++++++++++++++++++++-----
 2 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 63c9cbd1a7..f22d679023 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -41344,6 +41344,13 @@ Miscellaneous Services
 that will be used to set the @env{RESTIC_PASSWORD} environment variable
 for the current job.
 
+@item @code{password-command} (type: file-like)
+String path or file-like object representing the executable file that
+prints password to stdout.  If a file-like object is used, it is placed
+in the store globally executable and in plain text.  The executable
+should be designed such that it does not compromise the password if an
+unauthorized user runs it.
+
 @item @code{schedule} (type: gexp-or-string)
 A string or a gexp that will be passed as time specification in the
 mcron job specification (@pxref{Syntax, mcron job specifications,,
diff --git a/gnu/services/backup.scm b/gnu/services/backup.scm
index 1279ece88f..fd904bc9a9 100644
--- a/gnu/services/backup.scm
+++ b/gnu/services/backup.scm
@@ -66,6 +66,9 @@ (define (lowerable? value)
 (define list-of-lowerables?
   (list-of lowerable?))
 
+(define-maybe/no-serialization string)
+(define-maybe/no-serialization file-like)
+
 (define-configuration/no-serialization restic-backup-job
   (restic
    (package restic)
@@ -80,10 +83,16 @@ (define-configuration/no-serialization restic-backup-job
    (string)
    "The restic repository target of this job.")
   (password-file
-   (string)
+   (maybe-string)
    "Name of the password file, readable by the configured @code{user}, that
 will be used to set the @code{RESTIC_PASSWORD} environment variable for the
 current job.")
+  (password-command
+   (maybe-file-like)
+   "An executable file who's path is stored in @code{RESTIC_PASSWORD_COMMAND}.
+When run, the file writes the password to standard output. Due to the nature
+of the store this command will be globally executable and should have external
+protections to ensure unauthorized users cannot retrieve the password.")
   (schedule
    (gexp-or-string)
    "A string or a gexp that will be passed as time specification in the mcron
@@ -104,6 +113,14 @@ (define-configuration/no-serialization restic-backup-job
    "A list of values that are lowered to strings.  These will be passed as
 command-line arguments to the current job @command{restic backup} invokation."))
 
+(define (verify-restic-backup-job-configuration config)
+  (unless (or (maybe-value-set? (restic-backup-job-password-file config))
+              (maybe-value-set? (restic-backup-job-password-command config)))
+    (error "either password-file or password-command must be configured."))
+  (when (and (maybe-value-set? (restic-backup-job-password-file config))
+             (maybe-value-set? (restic-backup-job-password-command config)))
+    (error "password-file and password-command can not be configured simultaneously.")))
+
 (define list-of-restic-backup-jobs?
   (list-of restic-backup-job?))
 
@@ -113,12 +130,21 @@ (define-configuration/no-serialization restic-backup-configuration
    "The list of backup jobs for the current system."))
 
 (define (restic-backup-job-program config)
+  (define (maybe-value-or-false maybe)
+    (if (maybe-value-set? maybe)
+        maybe
+        #f))
+
+  (verify-restic-backup-job-configuration config)
+
   (let ((restic
          (file-append (restic-backup-job-restic config) "/bin/restic"))
         (repository
          (restic-backup-job-repository config))
         (password-file
-         (restic-backup-job-password-file config))
+         (maybe-value-or-false (restic-backup-job-password-file config)))
+        (password-command
+         (maybe-value-or-false (restic-backup-job-password-command config)))
         (files
          (restic-backup-job-files config))
         (extra-flags
@@ -134,9 +160,14 @@ (define (restic-backup-job-program config)
      #~(begin
          (use-modules (ice-9 popen)
                       (ice-9 rdelim))
-         (setenv "RESTIC_PASSWORD"
-                 (with-input-from-file #$password-file read-line))
-
+         (or (and=> #$password-file (lambda (x)
+                                      (setenv "RESTIC_PASSWORD"
+                                              (with-input-from-file x read-line))))
+             (and=> #$password-command (lambda (x)
+                                         (setenv "RESTIC_PASSWORD_COMMAND" x)))
+             ;; Have a backup error message in case
+             ;; verify-restic-backup-job-configuration is messed with
+             (error "Neither password-file or password-command set"))
          (when #$init?
            ;; Use cat config to check if the repository exists. See
            ;; https://github.com/restic/restic/issues/1690 and
-- 
2.45.1