Use bytearray for request body accumulation (#2845)

* Use bytearray for request body accumulation

Accumulating request body with bytes += creates a new bytes object on
every chunk, leading to O(n^2) allocation for fragmented bodies.
bytearray extends in-place (amortized O(1)), avoiding the quadratic cost.

* Add fragmented body benchmark for chunked body accumulation

Sends 100KB in 390 x 256-byte chunks to exercise the body += path
that triggers O(n^2) allocation with bytes concatenation.

* Revert "Add fragmented body benchmark for chunked body accumulation"

This reverts commit 47662509c1.

uvicorn/protocols/http/h11_impl.py

@@ -397,7 +397,7 @@ class RequestResponseCycle:
         self.waiting_for_100_continue = conn.they_are_waiting_for_100_continue
 
         # Request state
-        self.body = b""
+        self.body = bytearray()
         self.more_body = True
 
         # Response state
@@ -543,8 +543,8 @@ class RequestResponseCycle:
 
         message: HTTPRequestEvent = {
             "type": "http.request",
-            "body": self.body,
+            "body": bytes(self.body),
             "more_body": self.more_body,
         }
-        self.body = b""
+        self.body = bytearray()
         return message

uvicorn/protocols/http/httptools_impl.py

@@ -401,7 +401,7 @@ class RequestResponseCycle:
         self.waiting_for_100_continue = expect_100_continue
 
         # Request state
-        self.body = b""
+        self.body = bytearray()
         self.more_body = True
 
         # Response state
@@ -572,6 +572,6 @@ class RequestResponseCycle:
 
         if self.disconnected or self.response_complete:
             return {"type": "http.disconnect"}
-        message: HTTPRequestEvent = {"type": "http.request", "body": self.body, "more_body": self.more_body}
-        self.body = b""
+        message: HTTPRequestEvent = {"type": "http.request", "body": bytes(self.body), "more_body": self.more_body}
+        self.body = bytearray()
         return message