Implement environment variable interpolation to YAML dictionary keys

`podman-compose` currently does not support interpolating environment
variables in dictionary keys, despite the compose file specification
indicating this capability.
See the relevant compose-spec documentation:
https://github.com/compose-spec/compose-spec/blob/main/12-interpolation.md

This feature is useful in `labels` or `environment` sections, where keys
can be user-defined strings. To enable interpolation, an alternate equal
sign syntax must be used, e.g.:
services:
  foo:
    labels:
      - "$VAR_NAME=label_value"

After this PR `podman-compose` will align more closely with the compose
file specification, allowing for the interpolation of environment
variables in dictionary keys.

Signed-off-by: Monika Kairaityte <monika@kibit.lt>

newsfragments/env-var-interpolation-for-keys.feature

@@ -0,0 +1 @@
+Add support for environment variable interpolation for YAML keys.

podman_compose.py

@@ -290,7 +290,7 @@ def rec_subs(value: dict | str | Iterable, subs_dict: dict[str, Any]) -> dict |
             svc_envs = rec_subs(svc_envs, subs_dict)
             subs_dict.update(svc_envs)
 
-        value = {k: rec_subs(v, subs_dict) for k, v in value.items()}
+        value = {rec_subs(k, subs_dict): rec_subs(v, subs_dict) for k, v in value.items()}
     elif isinstance(value, str):
 
         def convert(m: re.Match) -> str:

tests/integration/interpolation/.env

@@ -1 +1,2 @@
 DOT_ENV_VARIABLE=This value is from the .env file
+TEST_LABELS=TEST

tests/integration/interpolation/docker-compose.yml

@@ -11,4 +11,10 @@ services:
             EXAMPLE_DOT_ENV: $DOT_ENV_VARIABLE
             EXAMPLE_LITERAL: This is a $$literal
             EXAMPLE_EMPTY: $NOT_A_VARIABLE
+    labels_test:
+        image: busybox
+        labels:
+            - "$TEST_LABELS=test_labels"
+            - test.${TEST_LABELS}=${TEST_LABELS}
+            - "${TEST_LABELS}.test2=test2(`${TEST_LABELS}`)"
 

tests/integration/interpolation/test_podman_compose_interpolation.py

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
+import json
 import os
 import unittest
 
@@ -36,6 +37,18 @@ class TestComposeInterpolation(unittest.TestCase, RunSubprocessMixin):
             self.assertIn("EXAMPLE_DOT_ENV='This value is from the .env file'", str(output))
             self.assertIn("EXAMPLE_EMPTY=''", str(output))
             self.assertIn("EXAMPLE_LITERAL='This is a $literal'", str(output))
+
+            output, _ = self.run_subprocess_assert_returncode([
+                "podman",
+                "inspect",
+                "interpolation_labels_test_1",
+            ])
+            inspect_output = json.loads(output)
+            labels_dict = inspect_output[0].get("Config", {}).get("Labels", {})
+            self.assertIn(('TEST', 'test_labels'), labels_dict.items())
+            self.assertIn(('TEST.test2', 'test2(`TEST`)'), labels_dict.items())
+            self.assertIn(('test.TEST', 'TEST'), labels_dict.items())
+
         finally:
             self.run_subprocess_assert_returncode([
                 podman_compose_path(),

tests/unit/test_rec_subs.py

@@ -71,3 +71,34 @@ class TestRecSubs(unittest.TestCase):
         sub_dict = {"v1": "high priority", "empty": ""}
         result = rec_subs(input, sub_dict)
         self.assertEqual(result, expected, msg=desc)
+
+    def test_env_var_substitution_in_dictionary_keys(self) -> None:
+        sub_dict = {"NAME": "TEST1", "NAME2": "TEST2"}
+        input = {
+            'services': {
+                'test': {
+                    'image': 'busybox',
+                    'labels': {
+                        '$NAME and ${NAME2}': '${NAME2} and $NAME',
+                        'test1.${NAME}': 'test1',
+                        '$NAME': '${NAME2}',
+                        '${NAME}.test2': 'Host(`${NAME2}`)',
+                    },
+                }
+            }
+        }
+        result = rec_subs(input, sub_dict)
+        expected = {
+            'services': {
+                'test': {
+                    'image': 'busybox',
+                    'labels': {
+                        'TEST1 and TEST2': 'TEST2 and TEST1',
+                        'test1.TEST1': 'test1',
+                        'TEST1': 'TEST2',
+                        'TEST1.test2': 'Host(`TEST2`)',
+                    },
+                }
+            }
+        }
+        self.assertEqual(result, expected)