Check group-vars Jinja syntax in pre-commit

Port the syntax-only checker from MASH PR #3613, including its hook triggers and YAML diagnostics. Check both group_vars/matrix_servers and group_vars/jitsi_jvb_servers when either file, the checker, or the hook configuration changes.

Source: https://github.com/mother-of-all-self-hosting/mash-playbook/pull/3613

Co-authored-by: QEDeD <pope-jellied-hull@duck.com>

.pre-commit-config.yaml

@@ -32,6 +32,14 @@ repos:
         pass_filenames: false
   - repo: local
     hooks:
+      - id: check-group-vars-jinja
+        name: Check the Jinja syntax in group_vars
+        entry: bin/check-group-vars-jinja.py
+        language: python
+        additional_dependencies: ['jinja2==3.1.6', 'pyyaml==6.0.3']
+        files: '^(group_vars/(matrix_servers|jitsi_jvb_servers)|bin/check-group-vars-jinja\.py|\.pre-commit-config\.yaml)$'
+        args: ['group_vars/matrix_servers', 'group_vars/jitsi_jvb_servers']
+        pass_filenames: false
       - id: check-examples-vars-migration-version
         name: Check examples/vars.yml migration version matches expected
         entry: bin/check-examples-vars-migration-version.sh

bin/check-group-vars-jinja.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2026 MASH project contributors
+# SPDX-FileCopyrightText: 2026 Slavi Pantaleev
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# Checks that the Jinja expressions in the playbook's group_vars files can be parsed.
+#
+# Ansible renders a `{{ ... }}` expression in group_vars only when something uses that value,
+# usually a role's task. A typo such as a missing `)` or `'` can therefore pass `--syntax-check`
+# and the ansible-lint hook (which only lints roles/custom), and the playbook run fails where the
+# value is first used.
+# This script reads the file as YAML, parses every string value with Jinja2 (as Ansible does before
+# evaluating it), and prints the variable name and line of each value that does not parse.
+# It checks syntax only: it does not know which variables or filters exist.
+#
+# Usage: bin/check-group-vars-jinja.py group_vars/matrix_servers group_vars/jitsi_jvb_servers
+
+import sys
+
+import jinja2
+import yaml
+
+
+def string_values(node, variable):
+    """Yields (variable, node) for every string value below a YAML node."""
+    if node.tag == "!unsafe":  # Ansible never evaluates values tagged !unsafe
+        return
+    if isinstance(node, yaml.MappingNode):
+        for key, value in node.value:
+            yield from string_values(value, variable or key.value)
+    elif isinstance(node, yaml.SequenceNode):
+        for value in node.value:
+            yield from string_values(value, variable)
+    else:
+        yield variable, node
+
+
+def main(paths):
+    environment = jinja2.Environment()
+    failed = False
+    for path in paths:
+        try:
+            with open(path, encoding="utf-8") as file:
+                document = yaml.compose(file, Loader=yaml.SafeLoader)
+        except yaml.YAMLError as error:
+            print(f"{path}: invalid YAML: {error}")
+            failed = True
+            continue
+        if document is None:
+            print(f"{path}:1: empty YAML document")
+            failed = True
+            continue
+        for variable, node in string_values(document, None):
+            try:
+                environment.parse(node.value)
+            except jinja2.TemplateSyntaxError as error:
+                line = node.start_mark.line + 1
+                print(f"{path}:{line}: {variable}: {error.message} (line {error.lineno} of the value)")
+                failed = True
+    return 1 if failed else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))