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fix(intrinsics): only resolve the selected Fn::If branch #9134

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d4c07ac
fix(intrinsics): resolve only the selected Fn::If branch
EPNOS Jul 19, 2026
16c70d0
fix(intrinsics): forward ignore_errors to Fn::GetAtt symbol resolution
EPNOS Jul 20, 2026
e16ec5b
test(intrinsics): add integration coverage for Fn::If with unresolvab…
EPNOS Jul 20, 2026
0a7b27d
Merge branch 'develop' into fix/4510-lazy-fn-if-resolution
EPNOS Aug 2, 2026
88ffaea
fix(intrinsics): stop forwarding ignore_errors into Fn::GetAtt symbol…
EPNOS Aug 3, 2026
57b4fbd
Merge branch 'fix/4510-lazy-fn-if-resolution' of https://github.com/E…
EPNOS Aug 3, 2026
b63826a
fix(providers): treat None Layers as empty list when parsing function…
EPNOS Aug 5, 2026
ace7482
fix(intrinsics): drop properties that resolve to AWS::NoValue via Fn::If
EPNOS Aug 5, 2026
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19 changes: 5 additions & 14 deletions samcli/lib/intrinsic_resolver/intrinsic_property_resolver.py
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Original file line number Diff line number Diff line change
Expand Up @@ -716,24 +716,14 @@ def handle_fn_if(self, intrinsic_value, ignore_errors):
-------
This will return value_if_true and value_if_false depending on how the condition is evaluated
"""
arguments = self.intrinsic_property_resolver(
intrinsic_value, ignore_errors, parent_function=IntrinsicResolver.FN_IF
)
verify_intrinsic_type_list(arguments, IntrinsicResolver.FN_IF)
verify_number_arguments(arguments, IntrinsicResolver.FN_IF, num=3)
verify_intrinsic_type_list(intrinsic_value, IntrinsicResolver.FN_IF)
verify_number_arguments(intrinsic_value, IntrinsicResolver.FN_IF, num=3)

condition_name = self.intrinsic_property_resolver(
arguments[0], ignore_errors, parent_function=IntrinsicResolver.FN_IF
intrinsic_value[0], ignore_errors, parent_function=IntrinsicResolver.FN_IF
)
verify_intrinsic_type_str(condition_name, IntrinsicResolver.FN_IF)

value_if_true = self.intrinsic_property_resolver(
arguments[1], ignore_errors, parent_function=IntrinsicResolver.FN_IF
)
value_if_false = self.intrinsic_property_resolver(
arguments[2], ignore_errors, parent_function=IntrinsicResolver.FN_IF
)

condition = self._conditions.get(condition_name)
verify_intrinsic_type_dict(
condition,
Expand All @@ -750,7 +740,8 @@ def handle_fn_if(self, intrinsic_value, ignore_errors):
message="The result of {} must evaluate to bool".format(IntrinsicResolver.FN_IF),
)

return value_if_true if condition_evaluated else value_if_false
selected_value = intrinsic_value[1] if condition_evaluated else intrinsic_value[2]
return self.intrinsic_property_resolver(selected_value, ignore_errors, parent_function=IntrinsicResolver.FN_IF)

@aws-sam-tooling-bot aws-sam-tooling-bot Bot Aug 3, 2026

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[BUG] handle_fn_if can now return None, which it could never do before, and at least one downstream consumer is not null-safe.

Before this change, a branch containing !Ref AWS::NoValue was pre-resolved to None by the outer intrinsic_property_resolver(intrinsic_value, ...) call, and the subsequent resolve(arguments[1|2]) hit the if intrinsic is None: raise InvalidIntrinsicException guard at the top of intrinsic_property_resolver. So Fn::If always raised, and with ignore_errors=True the enclosing dict loop left the property as the raw {"Fn::If": [...]} dict.

Now the selected branch is resolved directly, so !Ref AWS::NoValue reaches IntrinsicsSymbolTable.handle_pseudo_no_value() and None is returned and assigned as the property value (the generic dict branch does sanitized_dict[sanitized_key] = sanitized_val with no None filtering).

Concrete failure — the common "conditionally omit a property" idiom:

Properties:
 Layers: !If [UseLayers, [!Ref MyLayer], !Ref "AWS::NoValue"]

When UseLayers is false, Properties["Layers"] becomes None. In samcli/lib/providers/sam_function_provider.py:265, resource_properties.get("Layers", []) returns None (the default only applies when the key is absent), and _parse_layer_info then does for layer in list_of_layers → TypeError: 'NoneType' object is not iterable, an unhandled traceback instead of a domain error.

Note the element-level form Layers: [!If [Cond, !Ref MyLayer, !Ref "AWS::NoValue"]] is fine — it yields [None] and _parse_layer_info skips unrecognized entries. Only the whole-property form breaks, and that is exactly one of the scenarios this PR sets out to fix, so it is worth closing here rather than leaving it as a newly reachable crash.

Two options:

  • Make the resolver match CloudFormation's AWS::NoValue semantics by dropping keys whose resolved value is None in the generic dict branch of intrinsic_property_resolver. This is the semantically correct fix but has wider blast radius, so it needs its own tests.
  • Harden the consumer, e.g. resource_properties.get("Layers") or [] in both call sites in sam_function_provider.py.

Either way, please add a unit test covering an Fn::If whose selected branch is !Ref AWS::NoValue — the four new tests only cover unresolvable Fn::GetAtt in the unselected branch, so this path is currently untested.

Note on the previous review comment: the handle_fn_getatt change that forwarded ignore_errors into resolve_symbols is no longer in the diff, and test_template_ignore_errors_leaves_unresolvable_layer_getatt_as_dict was added as a guard for the nested-stack layer behavior. That finding is resolved and I did not re-raise it. The PR description still describes the handle_fn_getatt change, so it is now out of date.

@EPNOS EPNOS Aug 5, 2026

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Fixed
b63826a

@aws-sam-tooling-bot aws-sam-tooling-bot Bot Aug 5, 2026

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[BUG] handle_fn_if can now return None, and the or [] patches in sam_function_provider.py only cover two of the affected call sites.

When the selected branch is {"Ref": "AWS::NoValue"}, resolve_symbols routes to IntrinsicsSymbolTable.handle_pseudo_no_value(), which returns Python None. Before this change that None hit the if intrinsic is None: raise InvalidIntrinsicException guard, so handle_fn_if always raised and the enclosing dict branch (with ignore_errors=True) left the raw {"Fn::If": [...]} in place. Now None is returned and stored as the property value:

# intrinsic_property_resolver, dict branch
sanitized_val = self.intrinsic_property_resolver(val, ignore_errors, parent_function=parent_function)
...
sanitized_dict[sanitized_key] = sanitized_val   # key kept, value is None

The key is retained rather than dropped, which does not match CloudFormation's AWS::NoValue semantics (the property is removed). A concrete crash path is SamApiProvider, which reads Events off the intrinsic-resolved stack.resources:

Events:
 ApiEvent: !If [UseApi, {Type: Api, Properties: {Path: /, Method: get}}, !Ref "AWS::NoValue"]

With UseApi false this resolves to {"ApiEvent": None}, and sam local start-api then hits samcli/lib/providers/sam_api_provider.py:469:

for , event in serverlessfunction_events.items():
   event_type = event.get(self._EVENT_TYPE)   # AttributeError: 'NoneType' object has no attribute 'get'

Previously that entry stayed as the raw Fn::If dict and was silently skipped. The same shape applies to properties.get("Events", {}) / properties.get(AUTHORIZER_TYPE, "").lower() style lookups elsewhere — a .get(key, default) default never fires when the key is present with a None value, so patching individual consumers with or [] will keep surfacing new failures.

Normalizing at the resolver would fix the whole class at once, and there is already precedent for exactly this in the codebase — CfnLanguageExtensionsApi._remove_no_value in samcli/lib/cfn_language_extensions/api.py:482, whose docstring notes "When Fn::If returns AWS::NoValue, the resolver returns None" and which skips such keys and list items (while deliberately preserving AWS::NoValue inside intrinsic-function arguments).

@EPNOS EPNOS Aug 5, 2026

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Fixed
ace7482


def handle_fn_equals(self, intrinsic_value, ignore_errors):
"""
Expand Down
21 changes: 21 additions & 0 deletions tests/integration/local/invoke/test_integrations_cli.py
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Original file line number Diff line number Diff line change
Expand Up @@ -393,6 +393,27 @@ def test_invoke_with_env_using_parameters(self):
self.assertEqual(environ["MyRuntimeVersion"], "v0")
self.assertEqual(environ["EmptyDefaultParameter"], "")

@pytest.mark.flaky(reruns=3)
def test_invoke_with_env_using_fn_if_ignores_unresolvable_branch(self):
command_list = InvokeIntegBase.get_command_list(
"EchoEnvWithFnIf",
template_path=self.template_path,
event_path=self.event_path,
)

process = Popen(command_list, stdout=PIPE)
try:
stdout, _ = process.communicate(timeout=TIMEOUT)
except TimeoutExpired:
process.kill()
raise

self.assertEqual(process.returncode, 0)
process_stdout = stdout.strip()
environ = json.loads(process_stdout.decode("utf-8"))

self.assertEqual(environ["FunctionUrl"], "https://custom.example.com/")

@pytest.mark.flaky(reruns=3)
def test_invoke_multi_tenant_function(self):
command_list = InvokeIntegBase.get_command_list(
Expand Down
35 changes: 35 additions & 0 deletions tests/integration/testdata/invoke/template.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,17 @@ Parameters:
Type: String
Default: "2"

UseCustomFunctionUrl:
Type: String
Default: "true"

CustomFunctionUrl:
Type: String
Default: "https://custom.example.com/"

Conditions:
ShouldUseCustomFunctionUrl: !Equals [!Ref UseCustomFunctionUrl, "true"]

Mappings:
common:
LambdaFunction:
Expand Down Expand Up @@ -202,6 +213,30 @@ Resources:
MyRuntimeVersion: !Ref MyRuntimeVersion
EmptyDefaultParameter: !Ref EmptyDefaultParameter

FunctionWithUrlConfig:
Type: AWS::Serverless::Function
Properties:
Handler: main.handler
Runtime: python3.9
CodeUri: .
Timeout: 600
FunctionUrlConfig:
AuthType: NONE

EchoEnvWithFnIf:
Type: AWS::Serverless::Function
Properties:
Handler: main.env_var_echo_hanler
Runtime: python3.9
CodeUri: .
Timeout: 600
Environment:
Variables:
FunctionUrl: !If
- ShouldUseCustomFunctionUrl
- !Ref CustomFunctionUrl
- !GetAtt FunctionWithUrlConfigUrl.FunctionUrl

TimeoutFunctionWithStringParameter:
Type: AWS::Serverless::Function
Properties:
Expand Down
50 changes: 49 additions & 1 deletion tests/unit/lib/intrinsic_resolver/test_intrinsic_resolver.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
from copy import deepcopy
from pathlib import Path
from unittest import TestCase
from unittest.mock import patch
from unittest.mock import MagicMock, patch

from parameterized import parameterized

Expand Down Expand Up @@ -906,6 +906,38 @@ def test_fn_if_condition_not_bool_fail(self):
with self.assertRaises(InvalidIntrinsicException, msg="Invalid Condition"):
self.resolver.intrinsic_property_resolver({"Fn::If": ["InvalidCondition", "test", "test"]}, True)

def test_fn_if_selects_resolvable_true_branch_ignoring_unresolvable_false_branch(self):
intrinsic = {"Fn::If": ["TestCondition", "resolved-value", {"Fn::GetAtt": ["Function2Url", "FunctionUrl"]}]}

result = self.resolver.intrinsic_property_resolver(intrinsic, False)
self.assertEqual(result, "resolved-value")

def test_fn_if_selects_resolvable_false_branch_ignoring_unresolvable_true_branch(self):
intrinsic = {"Fn::If": ["NotTestCondition", {"Fn::GetAtt": ["Function2Url", "FunctionUrl"]}, "resolved-value"]}

result = self.resolver.intrinsic_property_resolver(intrinsic, False)
self.assertEqual(result, "resolved-value")

def test_fn_if_does_not_evaluate_unselected_false_branch(self):
mock_handle_fn_getatt = MagicMock()
self.resolver.intrinsic_key_function_map[IntrinsicResolver.FN_GET_ATT] = mock_handle_fn_getatt
intrinsic = {"Fn::If": ["TestCondition", "resolved-value", {"Fn::GetAtt": ["Function2Url", "FunctionUrl"]}]}

result = self.resolver.intrinsic_property_resolver(intrinsic, False)

self.assertEqual(result, "resolved-value")
mock_handle_fn_getatt.assert_not_called()

def test_fn_if_does_not_evaluate_unselected_true_branch(self):
mock_handle_fn_getatt = MagicMock()
self.resolver.intrinsic_key_function_map[IntrinsicResolver.FN_GET_ATT] = mock_handle_fn_getatt
intrinsic = {"Fn::If": ["NotTestCondition", {"Fn::GetAtt": ["Function2Url", "FunctionUrl"]}, "resolved-value"]}

result = self.resolver.intrinsic_property_resolver(intrinsic, False)

self.assertEqual(result, "resolved-value")
mock_handle_fn_getatt.assert_not_called()


class TestIntrinsicAttribteResolution(TestCase):
def setUp(self):
Expand Down Expand Up @@ -1013,6 +1045,22 @@ def test_template_ignore_errors(self):
}
self.assertEqual(expected_template, dict(result))

def test_template_ignore_errors_leaves_unresolvable_layer_getatt_as_dict(self):
resources = deepcopy(self.resources)
resources["ReferenceLambdaLayerVersionLambdaFunction"]["Properties"]["Layers"] = [
{"Fn::GetAtt": ["NestedStack", "Outputs.MyDepLayer"]}
]
template = {"Mappings": self.mappings, "Conditions": self.conditions, "Resources": resources}
symbol_resolver = IntrinsicsSymbolTable(template=template, logical_id_translator=self.logical_id_translator)
resolver = IntrinsicResolver(template=template, symbol_resolver=symbol_resolver)

result = resolver.resolve_attribute(resources, ignore_errors=True)

self.assertEqual(
result["ReferenceLambdaLayerVersionLambdaFunction"]["Properties"]["Layers"],
[{"Fn::GetAtt": ["NestedStack", "Outputs.MyDepLayer"]}],
)


class TestResolveTemplate(TestCase):
def test_parameter_not_resolved(self):
Expand Down