omd backup: exclude generated core helper config

Backing up a running site could produce an unrestorable archive: the
core rotates var/check_mk/core/helper_config/ generations during the
backup, and a vanished hardlinked file left a dangling hardlink in the
tar, making omd restore fail with a KeyError.

Change-Id: I4cd24b78c4a8b859c1400c97d9c116b82c99b36f
Jira: CMK-37169
Caused-by: Ic76bc983ebd221687e2f249bf6bcb3389f348aa2

Author: SoloJacobs

.werks/19719.md

@@ -0,0 +1,23 @@
+[//]: # (werk v2)
+# omd restore: Prevent 'KeyError: "linkname'
+
+key        | value
+---------- | ---
+date       | 2026-08-07T07:00:01.567466+00:00
+version    | 2.4.0p36
+class      | fix
+edition    | cre
+component  | omd
+level      | 2
+compatible | yes
+
+The backup of a running site could fail to restore.
+While the backup is created, the monitoring core could rotate the generated helper configuration below `var/check_mk/core/helper_config/`.
+This could corrupt the backup file and `omd restore` failed with an error like:
+
+```
+KeyError: "linkname 'var/check_mk/core/helper_config/89/autochecks/....mk' not found"
+```
+
+The helper configuration is now excluded from `omd backup` altogether.
+It is generated data and is recreated automatically (via `cmk -U`) when a backup is restored.

omd/packages/omd/omdlib/backup.py

@@ -198,6 +198,9 @@ def get_exclude_patterns(options: BackupExclusions) -> list[str]:
     excludes.append("var/check_mk/persisted/*")
     excludes.append("var/check_mk/persisted_sections/*")
 
+    # exclude the generated helper config, it is recreated by "cmk -U" during restore
+    excludes.append("var/check_mk/core/helper_config/*")
+
     if not options.rrds:
         excludes.append("var/pnp4nagios/perfdata/*")
         excludes.append("var/pnp4nagios/spool/*")

tests/unit/omdlib/test_omdlib_backup.py

@@ -104,6 +104,7 @@ def test_get_exclude_patterns_default() -> None:
     actual_excludes = get_exclude_patterns(BackupExclusions.from_options(options))
 
     assert "tmp/*" in actual_excludes
+    assert "var/check_mk/core/helper_config/*" in actual_excludes
     assert "var/check_mk/agents/*" not in actual_excludes
     assert "var/log/*/*" not in actual_excludes
     assert "var/check_mk/rrd/*" not in actual_excludes