Don't rewind past already-read streams

Author: JordonPhillips

packages/smithy-http/src/smithy_http/serializers.py

@@ -129,6 +129,7 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
                 )
                 yield binding_serializer
                 content_length = payload.tell()
+                payload.seek(0)
         else:
             payload = BytesIO()
             payload_serializer = self._payload_codec.create_serializer(payload)
@@ -154,8 +155,8 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
                     binding_matcher,
                 )
                 yield binding_serializer
+            payload.seek(0)
 
-        self._seek(payload, 0)
         headers = binding_serializer.header_serializer.headers
         if content_type is not None:
             headers.append(("content-type", content_type))
@@ -165,7 +166,7 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
 
         fields = tuples_to_fields(headers)
         if content_length_required and "content-length" not in fields:
-            content_length = self._compute_content_length(payload)
+            content_length = _compute_content_length(payload)
             if content_length is None:
                 raise SerializationError(
                     "This operation requires the the content length of the input "
@@ -187,17 +188,24 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
             body=payload,
         )
 
-    def _seek(self, payload: Any, pos: int, whence: int = 0) -> None:
-        if (
-            seek := getattr(payload, "seek", None)
-        ) is not None and not iscoroutinefunction(seek):
-            seek(pos, whence)
 
-    def _compute_content_length(self, payload: Any) -> int | None:
-        content_length = self._seek(payload, 0, 2)
-        if content_length is not None:
-            self._seek(payload, 0, 0)
+def _compute_content_length(payload: Any) -> int | None:
+    if (tell := getattr(payload, "tell", None)) is not None and not iscoroutinefunction(
+        tell
+    ):
+        start: int = tell()
+        if (end := _seek(payload, 0, 2)) is not None:
+            content_length: int = end - start
+            _seek(payload, start, 0)
             return content_length
+    return None
+
+
+def _seek(payload: Any, pos: int, whence: int = 0) -> None:
+    if (seek := getattr(payload, "seek", None)) is not None and not iscoroutinefunction(
+        seek
+    ):
+        seek(pos, whence)
 
 
 class HTTPRequestBindingSerializer(InterceptingSerializer):
@@ -303,6 +311,7 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
                 )
                 yield binding_serializer
                 content_length = payload.tell()
+                payload.seek(0)
         else:
             payload = BytesIO()
             payload_serializer = self._payload_codec.create_serializer(payload)
@@ -323,8 +332,8 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
                     binding_matcher,
                 )
                 yield binding_serializer
+            payload.seek(0)
 
-        self._seek(payload, 0)
         headers = binding_serializer.header_serializer.headers
         if content_type is not None:
             headers.append(("content-type", content_type))
@@ -334,7 +343,7 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
 
         fields = tuples_to_fields(headers)
         if content_length_required and "content-length" not in fields:
-            content_length = self._compute_content_length(payload)
+            content_length = _compute_content_length(payload)
             if content_length is None:
                 raise SerializationError(
                     "This operation requires the the content length of the input "
@@ -355,18 +364,6 @@ def begin_struct(self, schema: Schema) -> Iterator[ShapeSerializer]:
             status=status,
         )
 
-    def _seek(self, payload: Any, pos: int, whence: int = 0) -> int | None:
-        if (
-            seek := getattr(payload, "seek", None)
-        ) is not None and not iscoroutinefunction(seek):
-            return seek(pos, whence)
-
-    def _compute_content_length(self, payload: Any) -> int | None:
-        content_length = self._seek(payload, 0, 2)
-        if content_length is not None:
-            self._seek(payload, 0, 0)
-            return content_length
-
 
 class HTTPResponseBindingSerializer(InterceptingSerializer):
     """Delegates HTTP response bindings to binding-location-specific serializers."""