nixos/borgbackup: allow dump scripts as stdin inputs

borg is able to process stdin during backups when backing up the special path -,
which can be very useful for backing up things that can be streamed (eg database
dumps, zfs snapshots).

nixos/modules/services/backup/borgbackup.nix

···
       ${cfg.postInit}
     fi
   '' + ''
-    borg create $extraArgs \
-      --compression ${cfg.compression} \
-      --exclude-from ${mkExcludeFile cfg} \
-      $extraCreateArgs \
-      "::$archiveName$archiveSuffix" \
-      ${escapeShellArgs cfg.paths}
+    (
+      set -o pipefail
+      ${optionalString (cfg.dumpCommand != null) ''${escapeShellArg cfg.dumpCommand} | \''}
+      borg create $extraArgs \
+        --compression ${cfg.compression} \
+        --exclude-from ${mkExcludeFile cfg} \
+        $extraCreateArgs \
+        "::$archiveName$archiveSuffix" \
+        ${if cfg.paths == null then "-" else escapeShellArgs cfg.paths}
+    )
   '' + optionalString cfg.appendFailedSuffix ''
     borg rename $extraArgs \
       "::$archiveName$archiveSuffix" "$archiveName"
···
       + " without at least one public key";
   };
 
+  mkSourceAssertions = name: cfg: {
+    assertion = count isNull [ cfg.dumpCommand cfg.paths ] == 1;
+    message = ''
+      Exactly one of borgbackup.jobs.${name}.paths or borgbackup.jobs.${name}.dumpCommand
+      must be set.
+    '';
+  };
+
   mkRemovableDeviceAssertions = name: cfg: {
     assertion = !(isLocalPath cfg.repo) -> !cfg.removableDevice;
     message = ''
···
         options = {
 
           paths = mkOption {
-            type = with types; coercedTo str lib.singleton (listOf str);
-            description = "Path(s) to back up.";
+            type = with types; nullOr (coercedTo str lib.singleton (listOf str));
+            default = null;
+            description = ''
+              Path(s) to back up.
+              Mutually exclusive with <option>dumpCommand</option>.
+            '';
             example = "/home/user";
+          };
+
+          dumpCommand = mkOption {
+            type = with types; nullOr path;
+            default = null;
+            description = ''
+              Backup the stdout of this program instead of filesystem paths.
+              Mutually exclusive with <option>paths</option>.
+            '';
+            example = "/path/to/createZFSsend.sh";
           };
 
           repo = mkOption {
···
       assertions =
         mapAttrsToList mkPassAssertion jobs
         ++ mapAttrsToList mkKeysAssertion repos
+        ++ mapAttrsToList mkSourceAssertions jobs
         ++ mapAttrsToList mkRemovableDeviceAssertions jobs;
 
       system.activationScripts = mapAttrs' mkActivationScript jobs;

nixos/tests/borgbackup.nix

···
           environment.BORG_RSH = "ssh -oStrictHostKeyChecking=no -i /root/id_ed25519.appendOnly";
         };
 
+        commandSuccess = {
+          dumpCommand = pkgs.writeScript "commandSuccess" ''
+            echo -n test
+          '';
+          repo = remoteRepo;
+          encryption.mode = "none";
+          startAt = [ ];
+          environment.BORG_RSH = "ssh -oStrictHostKeyChecking=no -i /root/id_ed25519";
+        };
+
+        commandFail = {
+          dumpCommand = "${pkgs.coreutils}/bin/false";
+          repo = remoteRepo;
+          encryption.mode = "none";
+          startAt = [ ];
+          environment.BORG_RSH = "ssh -oStrictHostKeyChecking=no -i /root/id_ed25519";
+        };
+
       };
     };
 
···
         client.fail("{} list borg\@server:wrong".format(borg))
 
         # TODO: Make sure that data is not actually deleted
+
+    with subtest("commandSuccess"):
+        server.wait_for_unit("sshd.service")
+        client.wait_for_unit("network.target")
+        client.systemctl("start --wait borgbackup-job-commandSuccess")
+        client.fail("systemctl is-failed borgbackup-job-commandSuccess")
+        id = client.succeed("borg-job-commandSuccess list | tail -n1 | cut -d' ' -f1").strip()
+        client.succeed(f"borg-job-commandSuccess extract ::{id} stdin")
+        assert "test" == client.succeed("cat stdin")
+
+    with subtest("commandFail"):
+        server.wait_for_unit("sshd.service")
+        client.wait_for_unit("network.target")
+        client.systemctl("start --wait borgbackup-job-commandFail")
+        client.succeed("systemctl is-failed borgbackup-job-commandFail")
   '';
 })