Fix ALB multi value headers (#189)

* copy with some slight changes from #179

* move headers conversion from `List[Tuple[bytes, bytes]]` to `List[List[bytes]]` up to where the headers are initially transformed

* no longer assert that first case mutated string is lowercase

* only pull headers from one of `multiValueHeaders` and `headers`

Author: jurasofish

mangum/handlers/aws_alb.py

@@ -1,6 +1,7 @@
 import base64
 import urllib.parse
 from typing import Any, Dict, Generator, List, Tuple
+from itertools import islice
 
 from .abstract_handler import AbstractHandler
 from .. import Response, Request
@@ -25,12 +26,25 @@ def all_casings(input_string: str) -> Generator:
                 yield first.upper() + sub_casing
 
 
+def case_mutated_headers(multi_value_headers: Dict[str, List[str]]) -> Dict[str, str]:
+    """Create str/str key/value headers, with duplicate keys case mutated."""
+    headers = {}
+    for key, values in multi_value_headers.items():
+        if len(values) > 0:
+            casings = list(islice(all_casings(key), len(values)))
+            for value, cased_key in zip(values, casings):
+                headers[cased_key] = value
+    return headers
+
+
 class AwsAlb(AbstractHandler):
     """
     Handles AWS Elastic Load Balancer, really Application Load Balancer events
     transforming them into ASGI Scope and handling responses
 
-    See: https://docs.aws.amazon.com/lambda/latest/dg/services-alb.html
+    See:
+        1. https://docs.aws.amazon.com/lambda/latest/dg/services-alb.html
+        2. https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html  # noqa: E501
     """
 
     TYPE = "AWS_ALB"
@@ -71,22 +85,40 @@ def encode_query_string(self) -> bytes:
 
         return urllib.parse.urlencode(query).encode()
 
+    def transform_headers(self) -> List[Tuple[bytes, bytes]]:
+        """Convert headers to a list of two-tuples per ASGI spec.
+
+        Only one of `multiValueHeaders` or `headers` should be defined in the
+        trigger event. However, we act as though they both might exist and pull
+        headers out of both.
+        """
+        headers = []
+        if "multiValueHeaders" in self.trigger_event:
+            for k, v in self.trigger_event["multiValueHeaders"].items():
+                for inner_v in v:
+                    headers.append((k.lower().encode(), inner_v.encode()))
+        else:
+            for k, v in self.trigger_event["headers"].items():
+                headers.append((k.lower().encode(), v.encode()))
+        return headers
+
     @property
     def request(self) -> Request:
         event = self.trigger_event
 
-        headers = {}
-        if event.get("headers"):
-            headers = {k.lower(): v for k, v in event.get("headers", {}).items()}
+        headers = self.transform_headers()
+        list_headers = [list(x) for x in headers]
+        # Unique headers. If there are duplicates, it will use the last defined.
+        uq_headers = {k.decode(): v.decode() for k, v in headers}
 
-        source_ip = headers.get("x-forwarded-for", "")
+        source_ip = uq_headers.get("x-forwarded-for", "")
         path = event["path"]
         http_method = event["httpMethod"]
         query_string = self.encode_query_string()
 
-        server_name = headers.get("host", "mangum")
+        server_name = uq_headers.get("host", "mangum")
         if ":" not in server_name:
-            server_port = headers.get("x-forwarded-port", 80)
+            server_port = uq_headers.get("x-forwarded-port", 80)
         else:
             server_name, server_port = server_name.split(":")  # pragma: no cover
         server = (server_name, int(server_port))
@@ -97,9 +129,9 @@ def request(self) -> Request:
 
         return Request(
             method=http_method,
-            headers=[[k.encode(), v.encode()] for k, v in headers.items()],
+            headers=list_headers,
             path=urllib.parse.unquote(path),
-            scheme=headers.get("x-forwarded-proto", "https"),
+            scheme=uq_headers.get("x-forwarded-proto", "https"),
             query_string=query_string,
             server=server,
             client=client,
@@ -119,36 +151,34 @@ def body(self) -> bytes:
 
         return body
 
-    def handle_headers(
-        self,
-        response_headers: List[List[bytes]],
-    ) -> Tuple[Dict[str, str], Dict[str, List[str]]]:
-        headers, multi_value_headers = self._handle_multi_value_headers(
-            response_headers
-        )
-        if "multiValueHeaders" not in self.trigger_event:
-            # If there are multiple occurrences of headers, create case-mutated
-            # variations: https://github.com/logandk/serverless-wsgi/issues/11
-            for key, values in multi_value_headers.items():
-                if len(values) > 1:
-                    for value, cased_key in zip(values, all_casings(key)):
-                        headers[cased_key] = value
-
-            multi_value_headers = {}
+    def transform_response(self, response: Response) -> Dict[str, Any]:
 
-        return headers, multi_value_headers
+        multi_value_headers: Dict[str, List[str]] = {}
+        for key, value in response.headers:
+            lower_key = key.decode().lower()
+            if lower_key not in multi_value_headers:
+                multi_value_headers[lower_key] = []
+            multi_value_headers[lower_key].append(value.decode())
 
-    def transform_response(self, response: Response) -> Dict[str, Any]:
-        headers, multi_value_headers = self.handle_headers(response.headers)
+        headers = case_mutated_headers(multi_value_headers)
 
         body, is_base64_encoded = self._handle_base64_response_body(
             response.body, headers
         )
 
-        return {
+        out = {
             "statusCode": response.status,
-            "headers": headers,
-            "multiValueHeaders": multi_value_headers,
             "body": body,
             "isBase64Encoded": is_base64_encoded,
         }
+
+        #  "You must use multiValueHeaders if you have enabled multi-value headers
+        #  and headers otherwise"
+        #  https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html
+        multi_value_headers_enabled = "multiValueHeaders" in self.trigger_event
+        if multi_value_headers_enabled:
+            out["multiValueHeaders"] = multi_value_headers
+        else:
+            out["headers"] = headers
+
+        return out