feat: Optional max_file_size when parsing form

docs/requests.md

@@ -122,6 +122,13 @@ async with request.form(max_files=1000, max_fields=1000):
     ...
 ```
 
+You can configure maximum size per file uploaded with the parameter `max_file_size`:
+
+```python
+async with request.form(max_file_size=100*1024*1024):  # 100 MB limit per file
+    ...
+```
+
 !!! info
     These limits are for security reasons, allowing an unlimited number of fields or files could lead to a denial of service attack by consuming a lot of CPU and memory parsing too many empty fields.
 

starlette/formparsers.py

@@ -126,12 +126,14 @@ def __init__(
         *,
         max_files: int | float = 1000,
         max_fields: int | float = 1000,
+        max_part_file_size: int | float | None = None,
     ) -> None:
         assert multipart is not None, "The `python-multipart` library must be installed to use form parsing."
         self.headers = headers
         self.stream = stream
         self.max_files = max_files
         self.max_fields = max_fields
+        self.max_part_file_size = max_part_file_size
         self.items: list[tuple[str, str | UploadFile]] = []
         self._current_files = 0
         self._current_fields = 0
@@ -247,6 +249,12 @@ async def parse(self) -> FormData:
                 # the main thread.
                 for part, data in self._file_parts_to_write:
                     assert part.file  # for type checkers
+                    if (
+                        self.max_part_file_size is not None
+                        and part.file.size is not None
+                        and part.file.size + len(data) > self.max_part_file_size
+                    ):
+                        raise MultiPartException(f"File exceeds maximum size of {self.max_part_file_size} bytes.")
                     await part.file.write(data)
                 for part in self._file_parts_to_finish:
                     assert part.file  # for type checkers

starlette/requests.py

@@ -245,7 +245,9 @@ async def json(self) -> typing.Any:
             self._json = json.loads(body)
         return self._json
 
-    async def _get_form(self, *, max_files: int | float = 1000, max_fields: int | float = 1000) -> FormData:
+    async def _get_form(
+        self, *, max_files: int | float = 1000, max_fields: int | float = 1000, max_file_size: int | None
+    ) -> FormData:
         if self._form is None:
             assert (
                 parse_options_header is not None
@@ -260,6 +262,7 @@ async def _get_form(self, *, max_files: int | float = 1000, max_fields: int | fl
                         self.stream(),
                         max_files=max_files,
                         max_fields=max_fields,
+                        max_part_file_size=max_file_size,
                     )
                     self._form = await multipart_parser.parse()
                 except MultiPartException as exc:
@@ -274,9 +277,11 @@ async def _get_form(self, *, max_files: int | float = 1000, max_fields: int | fl
         return self._form
 
     def form(
-        self, *, max_files: int | float = 1000, max_fields: int | float = 1000
+        self, *, max_files: int | float = 1000, max_fields: int | float = 1000, max_file_size: int | None = None
     ) -> AwaitableOrContextManager[FormData]:
-        return AwaitableOrContextManagerWrapper(self._get_form(max_files=max_files, max_fields=max_fields))
+        return AwaitableOrContextManagerWrapper(
+            self._get_form(max_files=max_files, max_fields=max_fields, max_file_size=max_file_size)
+        )
 
     async def close(self) -> None:
         if self._form is not None:  # pragma: no branch

tests/test_formparsers.py

@@ -4,6 +4,7 @@
 import typing
 from contextlib import nullcontext as does_not_raise
 from pathlib import Path
+from secrets import token_bytes
 
 import pytest
 
@@ -127,6 +128,29 @@ async def app(scope: Scope, receive: Receive, send: Send) -> None:
     return app
 
 
+def make_app_max_file_size(max_file_size: int) -> ASGIApp:
+    async def app(scope: Scope, receive: Receive, send: Send) -> None:
+        request = Request(scope, receive)
+        data = await request.form(max_file_size=max_file_size)
+        output: dict[str, typing.Any] = {}
+        for key, value in data.items():
+            if isinstance(value, UploadFile):
+                content = await value.read()
+                output[key] = {
+                    "filename": value.filename,
+                    "size": value.size,
+                    "content": content.decode(),
+                    "content_type": value.content_type,
+                }
+            else:
+                output[key] = value
+        await request.close()
+        response = JSONResponse(output)
+        await response(scope, receive, send)
+
+    return app
+
+
 def test_multipart_request_data(tmpdir: Path, test_client_factory: TestClientFactory) -> None:
     client = test_client_factory(app)
     response = client.post("/", data={"some": "data"}, files=FORCE_MULTIPART)
@@ -580,6 +604,30 @@ def test_too_many_files_and_fields_raise(
         assert res.text == "Too many files. Maximum number of files is 1000."
 
 
+@pytest.mark.parametrize(
+    "app,expectation",
+    [
+        (make_app_max_file_size(1024), pytest.raises(MultiPartException)),
+        (Starlette(routes=[Mount("/", app=make_app_max_file_size(1024))]), does_not_raise()),
+    ],
+)
+def test_max_part_file_size_raise(
+    tmpdir: Path,
+    app: ASGIApp,
+    expectation: typing.ContextManager[Exception],
+    test_client_factory: TestClientFactory,
+) -> None:
+    path = os.path.join(tmpdir, "test.txt")
+    with open(path, "wb") as file:
+        file.write(token_bytes(1024 + 1))
+
+    client = test_client_factory(app)
+    with open(path, "rb") as f, expectation:
+        response = client.post("/", files={"test": f})
+        assert response.status_code == 400
+        assert response.text == "File exceeds maximum size of 1024 bytes."
+
+
 @pytest.mark.parametrize(
     "app,expectation",
     [