Backups: a revision chain belongs to a PC, not just a machine

plugins/backups/api/routes.py

@@ -27,6 +27,7 @@ from shopdb.api import (
 from ..models import BackupRevision
 from ..models.backup import _utciso
 from ..services.registry import REGISTRY, getkind
+from ..services.retention import samesource
 
 backups_bp = Blueprint('backups', __name__)
 
@@ -278,9 +279,13 @@ def diff_revision(backuprevisionid):
     if againstid:
         other = db.session.get(BackupRevision, againstid)
     else:
+        # The previous revision FROM THE SAME PC. Several PCs can share a
+        # machine number, and diffing across them reported one device's COM port
+        # as a change on another, which is not a change at all.
         other = (db.session.query(BackupRevision)
                  .filter(BackupRevision.assetid == revision.assetid,
                          BackupRevision.backupkind == revision.backupkind,
+                         samesource(revision.sourcehostname),
                          BackupRevision.backuprevisionid < revision.backuprevisionid)
                  .order_by(BackupRevision.backuprevisionid.desc())
                  .first())