feat(package): merge SAR Metadata exports back into LE child templates

Extend merge_language_extensions_s3_uris with a registry-driven Metadata
pass that copies rewritten property values (LicenseUrl, ReadmeUrl) from
the exported template back into the original (Fn::ForEach-preserving)
template after LE expansion.

Without this pass, when a child stack uses Transform: AWS::LanguageExtensions
and declares AWS::ServerlessRepo::Application metadata, sam package
silently dropped the License/Readme S3 URLs from the merged output —
they were uploaded but never wired into the template the user deploys.

Implementation iterates METADATA_EXPORTS (the registry added in the prior
commit) so future metadata types pick up merge support without touching
the merge walker.

Author: bnusunny

samcli/lib/package/language_extensions_packaging.py

@@ -27,6 +27,7 @@
     substitute_loop_variable,
 )
 from samcli.lib.cfn_language_extensions.utils import FOREACH_REQUIRED_ELEMENTS, is_foreach_key
+from samcli.lib.package.packageable_resources import METADATA_EXPORTS
 
 LOG = logging.getLogger(__name__)
 
@@ -82,6 +83,8 @@ def merge_language_extensions_s3_uris(
 
     _update_resources_with_s3_uris(original_resources, exported_resources, dynamic_prop_keys)
 
+    _merge_metadata(result.get("Metadata", {}), exported_template.get("Metadata", {}))
+
     return result
 
 
@@ -183,6 +186,24 @@ def _resolve_property_paths(prop_names: List[str], properties: Dict[str, Any]) -
 # ---------------------------------------------------------------------------
 
 
+def _merge_metadata(
+    original_metadata: Dict[str, Any],
+    exported_metadata: Dict[str, Any],
+) -> None:
+    """Copy rewritten property values from exported_metadata into
+    original_metadata for every (metadata_type, property_name) declared
+    in METADATA_EXPORTS. Mutates original_metadata in place.
+    """
+    for spec in METADATA_EXPORTS:
+        original_entry = original_metadata.get(spec.metadata_type)
+        exported_entry = exported_metadata.get(spec.metadata_type)
+        if not isinstance(original_entry, dict) or not isinstance(exported_entry, dict):
+            continue
+        for prop_name in spec.property_names:
+            if prop_name in exported_entry:
+                original_entry[prop_name] = exported_entry[prop_name]
+
+
 def _update_resources_with_s3_uris(
     original_resources: Dict[str, Any],
     exported_resources: Dict[str, Any],

tests/unit/lib/package/test_language_extensions_packaging.py

@@ -0,0 +1,104 @@
+"""Unit tests for samcli.lib.package.language_extensions_packaging.
+
+Resource-property merge tests live in test_artifact_exporter.py; this file
+focuses on the Metadata merge pass added for the registry-driven merge.
+"""
+
+from unittest import TestCase
+
+from samcli.lib.package.language_extensions_packaging import merge_language_extensions_s3_uris
+
+
+class TestMergeMetadata(TestCase):
+    def test_serverless_repo_license_url_is_merged(self):
+        original = {
+            "Transform": "AWS::LanguageExtensions",
+            "Metadata": {
+                "AWS::ServerlessRepo::Application": {
+                    "Name": "MyApp",
+                    "LicenseUrl": "./LICENSE.txt",
+                    "ReadmeUrl": "./README.md",
+                }
+            },
+            "Resources": {},
+        }
+        exported = {
+            "Metadata": {
+                "AWS::ServerlessRepo::Application": {
+                    "Name": "MyApp",
+                    "LicenseUrl": "s3://bucket/license-md5",
+                    "ReadmeUrl": "s3://bucket/readme-md5",
+                }
+            },
+            "Resources": {},
+        }
+
+        result = merge_language_extensions_s3_uris(original, exported)
+
+        sar = result["Metadata"]["AWS::ServerlessRepo::Application"]
+        self.assertEqual(sar["LicenseUrl"], "s3://bucket/license-md5")
+        self.assertEqual(sar["ReadmeUrl"], "s3://bucket/readme-md5")
+        self.assertEqual(sar["Name"], "MyApp")  # unrelated keys preserved
+
+    def test_metadata_without_serverless_repo_is_unchanged(self):
+        original = {
+            "Metadata": {"OtherKey": {"Foo": "./bar"}},
+            "Resources": {},
+        }
+        exported = {
+            "Metadata": {"OtherKey": {"Foo": "./bar"}},
+            "Resources": {},
+        }
+
+        result = merge_language_extensions_s3_uris(original, exported)
+
+        self.assertEqual(result["Metadata"], {"OtherKey": {"Foo": "./bar"}})
+
+    def test_missing_metadata_section_in_either_template_is_safe(self):
+        # No Metadata in original
+        result = merge_language_extensions_s3_uris(
+            {"Resources": {}},
+            {"Metadata": {"AWS::ServerlessRepo::Application": {"LicenseUrl": "s3://x"}}, "Resources": {}},
+        )
+        self.assertNotIn("Metadata", result)
+
+        # No Metadata in exported
+        original = {
+            "Metadata": {"AWS::ServerlessRepo::Application": {"LicenseUrl": "./LICENSE"}},
+            "Resources": {},
+        }
+        result = merge_language_extensions_s3_uris(original, {"Resources": {}})
+        # Original retained, since exporter never wrote anything
+        self.assertEqual(result["Metadata"]["AWS::ServerlessRepo::Application"]["LicenseUrl"], "./LICENSE")
+
+    def test_partial_serverless_repo_export_preserves_unwritten_properties(self):
+        """If the exported template has only LicenseUrl written (no ReadmeUrl),
+        the original's ReadmeUrl must be left untouched. Guards the
+        `if prop_name in exported_entry:` check in _merge_metadata.
+        """
+        original = {
+            "Metadata": {
+                "AWS::ServerlessRepo::Application": {
+                    "Name": "MyApp",
+                    "LicenseUrl": "./LICENSE.txt",
+                    "ReadmeUrl": "./README.md",
+                }
+            },
+            "Resources": {},
+        }
+        exported = {
+            "Metadata": {
+                "AWS::ServerlessRepo::Application": {
+                    "Name": "MyApp",
+                    "LicenseUrl": "s3://bucket/license-md5",
+                    # ReadmeUrl deliberately absent
+                }
+            },
+            "Resources": {},
+        }
+
+        result = merge_language_extensions_s3_uris(original, exported)
+
+        sar = result["Metadata"]["AWS::ServerlessRepo::Application"]
+        self.assertEqual(sar["LicenseUrl"], "s3://bucket/license-md5")
+        self.assertEqual(sar["ReadmeUrl"], "./README.md")  # untouched