fix HTTP HEAD method operation detection in ASF

Co-authored-by: Thomas Rausch <thomas@thrau.at>

localstack/aws/protocol/op_router.py

@@ -115,7 +115,25 @@ def matches(self, query_args: MultiDict, headers: Headers) -> bool:
         return True
 
 
-class _RequestMatchingRule(Rule):
+class _StrictMethodRule(Rule):
+    """
+    Small extension to Werkzeug's Rule class which reverts unwanted assumptions made by Werkzeug.
+    Reverted assumptions:
+    - Werkzeug automatically matches HEAD requests to the corresponding GET request (i.e. Werkzeug's rule automatically
+      adds the HEAD HTTP method to a rule which should only match GET requests). This is implemented to simplify
+      implementing an app compliant with HTTP (where a HEAD request needs to return the headers of a corresponding GET
+      request), but it is unwanted for our strict rule matching in here.
+    """
+
+    def __init__(self, string: str, method: str, **kwargs) -> None:
+        super().__init__(string=string, methods=[method], **kwargs)
+
+        # Make sure Werkzeug's Rule does not add any other methods
+        # (f.e. the HEAD method even though the rule should only match GET)
+        self.methods = {method.upper()}
+
+
+class _RequestMatchingRule(_StrictMethodRule):
     """
     A Werkzeug Rule extension which initially acts as a normal rule (i.e. matches a path and method).
 
@@ -125,8 +143,10 @@ class _RequestMatchingRule(Rule):
     The result of `match_request` is only meaningful if this wrapping rule also matches.
     """
 
-    def __init__(self, string: str, *args, operations: List[_HttpOperation], **kwargs) -> None:
-        super().__init__(string, *args, **kwargs)
+    def __init__(
+        self, string: str, operations: List[_HttpOperation], method: str, **kwargs
+    ) -> None:
+        super().__init__(string=string, method=method, **kwargs)
         # Create a rule which checks all required arguments (not only the path and method)
         rules = [_RequiredArgsRule(op) for op in operations]
         # Sort the rules descending based on their rule score
@@ -221,11 +241,11 @@ def _create_service_map(service: ServiceModel) -> Map:
             # if there is only a single operation for a (path, method) combination,
             # the default Werkzeug rule can be used directly (this is the case for most rules)
             op = ops[0]
-            rules.append(Rule(rule_string, methods=[method], endpoint=op.operation))  # type: ignore
+            rules.append(_StrictMethodRule(string=rule_string, method=method, endpoint=op.operation))  # type: ignore
         else:
             # if there is an ambiguity with only the (path, method) combination,
             # a custom rule - which can use additional request metadata - needs to be used
-            rules.append(_RequestMatchingRule(rule_string, methods=[method], operations=ops))
+            rules.append(_RequestMatchingRule(string=rule_string, method=method, operations=ops))
 
     return Map(rules=rules, merge_slashes=False, converters={"path": GreedyPathConverter})
 

tests/unit/aws/protocol/test_op_router.py

@@ -47,3 +47,13 @@ def test_greedy_path_converter():
     assert matcher.match("/some-route//foo/bar/bar") == (None, {"p": "/foo/bar"})
     with pytest.raises(NotFound):
         matcher.match("/some-route//foo/baz")
+
+
+def test_s3_head_request():
+    router = RestServiceOperationRouter(load_service("s3"))
+
+    op, _ = router.match(Request("GET", "/my-bucket/my-key/"))
+    assert op.name == "GetObject"
+
+    op, _ = router.match(Request("HEAD", "/my-bucket/my-key/"))
+    assert op.name == "HeadObject"

tests/unit/aws/protocol/test_parser.py

@@ -889,7 +889,7 @@ def test_restjson_operation_detection_with_subpath():
         )
 
 
-def test_s3_operation_detection():
+def test_s3_get_operation_detection():
     """
     Test if the S3 operation detection works for ambiguous operations. GetObject is the worst, because it is
     overloaded with the exact same requestURI by another non-deprecated function where the only distinction is the
@@ -900,6 +900,13 @@ def test_s3_operation_detection():
     )
 
 
+def test_s3_head_operation_detection():
+    """Test if the S3 operation detection works for HEAD operations."""
+    _botocore_parser_integration_test(
+        service="s3", action="HeadObject", Bucket="test-bucket", Key="foo/bar/test.json"
+    )
+
+
 def test_s3_put_object_keys_with_slashes():
     _botocore_parser_integration_test(
         service="s3",