GZIP content length mismatch fix

starlette/middleware/gzip.py

@@ -58,9 +58,7 @@ async def send_with_gzip(self, message: Message) -> None:
                 await self.send(message)
             elif not more_body:
                 # Standard GZip response.
-                self.gzip_file.write(body)
-                self.gzip_file.close()
-                body = self.gzip_buffer.getvalue()
+                body = self._set_response_body(body, more_body)
 
                 headers = MutableHeaders(raw=self.initial_message["headers"])
                 headers["Content-Encoding"] = "gzip"
@@ -77,11 +75,7 @@ async def send_with_gzip(self, message: Message) -> None:
                 headers.add_vary_header("Accept-Encoding")
                 del headers["Content-Length"]
 
-                self.gzip_file.write(body)
-                message["body"] = self.gzip_buffer.getvalue()
-                self.gzip_buffer.seek(0)
-                self.gzip_buffer.truncate()
-
+                message["body"] = self._set_response_body(body, more_body)
                 await self.send(self.initial_message)
                 await self.send(message)
 
@@ -90,16 +84,30 @@ async def send_with_gzip(self, message: Message) -> None:
             body = message.get("body", b"")
             more_body = message.get("more_body", False)
 
-            self.gzip_file.write(body)
-            if not more_body:
-                self.gzip_file.close()
-
-            message["body"] = self.gzip_buffer.getvalue()
-            self.gzip_buffer.seek(0)
-            self.gzip_buffer.truncate()
+            message["body"] = self._set_response_body(body, more_body)
 
             await self.send(message)
 
+    def _set_response_body(self, body, more_body: bool = True) -> bytes:
+        """Null byte fix
+
+        If the response body is null and we try to get the buffervalue, a null byte is produced.
+        This causes a content length mismatch.
+
+        This issues exists in the upstream Gzip middleware in starlette.
+        - https://github.com/tiangolo/fastapi/issues/4050
+        - https://github.com/tiangolo/fastapi/issues/2818
+
+        """
+        if body and body not in {b"", b"null"}:
+            self.gzip_file.write(body)
+        if not more_body:
+            self.gzip_file.close()
+        value = self.gzip_buffer.getvalue()
+        self.gzip_buffer.seek(0)
+        self.gzip_buffer.truncate()
+        return value
+
 
 async def unattached_send(message: Message) -> typing.NoReturn:
     raise RuntimeError("send awaitable not set")  # pragma: no cover