feat: Implement backward compatibility in layers update. (#350)

* feat:  Implement backward compatibility in layers update.

Author: Sashwatdas123

newrelic_lambda_cli/layers.py

@@ -47,7 +47,18 @@ def index(region, runtime, architecture):
     ]
 
 
-def layer_selection(available_layers, runtime, architecture):
+def layer_selection(
+    available_layers, runtime, architecture, upgrade=False, existing_layer_arn=None
+):
+    if upgrade and existing_layer_arn:
+        base_arn = existing_layer_arn.rsplit(":", 1)[0]
+
+        for i, layer in enumerate(available_layers):
+            candidate_arn = layer["LatestMatchingVersion"]["LayerVersionArn"]
+            candidate_base_arn = candidate_arn.rsplit(":", 1)[0]
+            if candidate_base_arn == base_arn:
+                return candidate_arn
+
     if len(available_layers) == 1:
         return available_layers[0]["LatestMatchingVersion"]["LayerVersionArn"]
 
@@ -147,8 +158,16 @@ def _add_new_relic(input, config, nr_license_key):
                 % (config["Configuration"]["FunctionArn"], runtime, architecture)
             )
             return False
-
-        new_relic_layer = layer_selection(available_layers, runtime, architecture)
+        existing_layer_arn = (
+            existing_newrelic_layer[0] if existing_newrelic_layer else None
+        )
+        new_relic_layer = layer_selection(
+            available_layers,
+            runtime,
+            architecture,
+            upgrade=input.upgrade,
+            existing_layer_arn=existing_layer_arn,
+        )
 
     update_kwargs = {
         "FunctionName": config["Configuration"]["FunctionArn"],

tests/test_layers.py

@@ -143,7 +143,11 @@ def test_add_new_relic(aws_credentials, mock_function_config):
             )
 
             layer_selection_mock.assert_called_with(
-                mock_index.return_value, "java11", "x86_64"
+                mock_index.return_value,
+                "java11",
+                "x86_64",
+                upgrade=None,
+                existing_layer_arn=None,
             )
             assert "original_handler" in config["Configuration"]["Handler"]
 
@@ -417,7 +421,6 @@ def test_add_new_relic_nodejs(aws_credentials, mock_function_config):
 
     runtime = "nodejs20.x"
 
-    # --- Scenario 1: Standard Node.js Handler (ESM disabled) ---
     print(f"\nTesting Node.js ({runtime}) Standard Handler...")
     original_std_handler = "original_handler"
     config_std = mock_function_config(runtime)
@@ -430,11 +433,16 @@ def test_add_new_relic_nodejs(aws_credentials, mock_function_config):
         enable_extension_function_logs=True,
     )
 
-    update_kwargs_std = _add_new_relic(
-        install_opts_std,
-        config_std,
-        nr_license_key=nr_license_key,
-    )
+    with patch("sys.stdout.isatty") as mock_isatty, patch(
+        "newrelic_lambda_cli.layers.click.prompt"
+    ) as mock_prompt:
+        mock_isatty.return_value = True
+        mock_prompt.return_value = 0
+        update_kwargs_std = _add_new_relic(
+            install_opts_std,
+            config_std,
+            nr_license_key=nr_license_key,
+        )
 
     assert update_kwargs_std is not False, "Expected update_kwargs, not False"
     assert (
@@ -479,11 +487,16 @@ def test_add_new_relic_nodejs(aws_credentials, mock_function_config):
         esm=True,
     )
 
-    update_kwargs_esm = _add_new_relic(
-        install_opts_esm,
-        config_esm,
-        nr_license_key=nr_license_key,
-    )
+    with patch("sys.stdout.isatty") as mock_isatty, patch(
+        "newrelic_lambda_cli.layers.click.prompt"
+    ) as mock_prompt:
+        mock_isatty.return_value = True
+        mock_prompt.return_value = 0
+        update_kwargs_esm = _add_new_relic(
+            install_opts_esm,
+            config_esm,
+            nr_license_key=nr_license_key,
+        )
 
     assert update_kwargs_esm is not False, "Expected update_kwargs, not False"
     assert (